This PDF file contains the front matter associated with SPIE Proceedings Volume 11470, including the Title Page, Copyright information, and Table of Contents.

LEDs employing perovskites and colloidal quantum dots now regularly exceed 20% EQE and – for some colors – achieve high luminances. Their performance and stability are quite dependent today on the emitted color (spanning UV to NIR), and the FWHM is excellent in some spectral ranges (green and blue especially) but needs work in others (red and IR). I will discuss the state of art in the field, and also discuss progress toward lasing.

Unlike common expectation that superconducting spintronics and desirable long-range triplet (LRT) proximity effects require complex ferromagnetic multilayers, relying on noncollinear/spiral magnetization or half-metals, we propose a new platform compatible with commercial spintronics. We experimentally demonstrate that interfacial spin-orbit coupling (SOC) and symmetry-filtering in all-epitaxial Fe/MgO/V junctions, cooled bellow Tc for vanadium, provide a thousand-fold increase in tunneling anisotropic magnetoresistance, supporting the LRT. The bistable magnetic anisotropy allows us to identify the SOC origin of the observed MR. Such MR reaches approximately 20% without an applied magnetic field and could be further increased for large magnetic fields that support vortices.

The basis for superconducting electronics is the Josephson junction, a thin insulating barrier that separates two superconducting leads and thereby behaves as a tunneling barrier. These junctions are the non-linear circuit element inside superconducting quantum interference devices, low temperature microwave electronics, and superconducting quantum computers. The width of this barrier can be as thin as two nanometers and the electronic properties of such junctions are strongly dependent on the morphology of the barrier, both at the interfaces of the superconducting leads and within the metal oxide itself. Using a range of computational techniques we analyse how the junctions are formed and how their electrical response depends on the junction microstructure.Our results provide new insights into the influence of fabrication conditions on the electrical response of metal-oxide barriers and the resulting performance of quantum technologies constructed from them.

We consider a mesoscopic superconducting island hosting multiple pairs of Majorana zero-energy modes. The Majorana island consists of multiple p-wave wires connected together by a s-wave superconducting backbone and is characterized by an overall charging energy Ec; the wires are coupled to normal-metal leads via tunnel junctions. We calculate the average charge on the island as well as non-local conductance matrix as a function of a p-wave pairing gap ΔP, charging energy Ec and dimensionless junction conductances gi. We find that the presence of a topological ground-state degeneracy in the island dramatically enhances charge fluctuations and leads to the suppression of Coulomb blockade effects in electron tunneling.

Majorana zero modes (MZMs) are expected to emerge in material heterostructures combining superconductivity, ferromagnetism, and spin-orbit coupling (SOC). Particularly, inducing superconductivity and magnetic exchange interactions in well-defined Shockley surface states (SS) of high quality ultrathin Au(111) layers, which intrinsically have strong SOC, has been predicted as an excellent platform for MBS. In this talk, I will first present our success in creating such heterostructure in epitaxially grown Au(111) heterostructures. By means of electron tunneling spectroscopy, I will show signatures of superconductivity induced in the two-dimensional SS of Au(111) thin film, as well as the behavior of such superconducting state under a planar Zeeman field. Second, I will show the evidence of a pair of MZMs in a fabricated Au(111) nanowire system. I will also demonstrate the scalability of such system and why it will enable future MZMs braiding circuit.

The Majorana zero mode in the semiconductor-superconductor nanowire is one of the promising candidates for topological quantum computing. Recently, in islands of nanowires, subgap-state energies have been experimentally observed to oscillate as a function of the magnetic field, showing a signature of overlapped Majorana bound states. However, the oscillation amplitude either dies away after an overshoot or decays, sharply opposite to the theoretically predicted enhanced oscillations for Majorana bound states. We reveal that a steplike distribution of spin-orbit coupling in realistic devices can induce the decaying Majorana oscillations, resulting from the coupling-induced energy repulsion between the quasiparticle spectra on the two sides of the step. Our work will inspire more works to reexamine effects of the nonuniform spin-orbit coupling, which is generally present in experimental devices.

Chemical pressure is an effective method to tune physical properties, particularly for diluted magnetic semiconductors (DMS) of which ferromagnetic ordering is mediated by charge carriers. Via substitution of smaller Ca for larger Sr, we introduce chemical pressure on (Sr,Na)(Cd,Mn)2As2 to fabricate a new DMS material (Ca,Na)(Cd,Mn)2As2. Carriers and spins are introduced by substitutions of (Ca,Na) and (Cd,Mn) respectively. The unit cell volume reduces by 6.2% after complete substitution of Ca for Sr, suggesting a subsistent chemical pressure. Importantly the local geometry of [Cd/MnAs4] tetrahedron is optimized via chemical compression that increases the Mn-As hybridization leading to enhanced ferromagnetic interactions. As a result, the maximum Curie temperature (TC) is increased by about 50% while the the maximum saturation moment increases by over 100% from (Ca,Na)(Cd,Mn)2As2 to (Sr,Na)(Cd,Mn)2As2. We also successful synthesized (Ca,Na)(Zn,Mn)2Sb2 as a new DMS. This feature of independently charge and spin doping allows us to separately investigate the effect of carriers and of spins on the ferromagnetic properties of this new DMS alloy, and particularly of its critical ferromagnetic behavior.[1-8] We use modified Arrott plot analysis to establish critical exponents b, g, and d for this alloy. We find that at low Mn concentrations (< 10 at.%), it is governed by short-range 3D-Ising behavior, with experimental values of b, g, and d very close to theoretical 3D-Ising values of 0.325, 1.24, and 4.815. However, as the Mn concentration increases, this DMS material exhibits a mixed-phase behavior, with g retaining its 3D-Ising characteristics, but b crossing over to longer-range mean-field behavior.

Topological excitations such as Majorana fermions provide unique pathways to fault-tolerant quantum computing. Recent progress has been enabled by proximity effects between non-superconducting materials and superconductors. Currently, Majorana devices based on semiconductors require application of an external magnetic field to open a helical gap. However, the presence of magnetic field can suppress the superconductivity and place geometric restrictions on the device. A promising path forward is to realize Majorana modes without field by integrating ferromagnets or antiferromagnets with semiconductors and superconductors. Here, we study ballistic InSb nanowire (NW) devices with ferromagnetic contacts. Magneto-transport measurements on these devices display hysteretic features spanning from the many-modes regime to few modes, which demonstrate spin-splitting transport across the NWs. Moreover, electrostatic gating can tune the transport in a regime where the device acts as a spin filter.

Superconducting spintronics is an emerging field which creates a synergy between traditional spintronics and superconductivity through the creation of equal spin-paired electrons (triplet Cooper pairs) [1]. These triplet Cooper pairs are immune to the pair breaking exchange field in a ferromagnet and can propagate over length scales which are significantly longer than the conventional singlet Cooper pair coherence lengths in ferromagnets. These dissipationless triplet currents carry a net spin which raises the intriguing possibility of ultra-low-dissipation superconducting spintronics. Traditionally these triplet pairs were generated through complex inhomogeneous magnetic textures which are often difficult to create and control [2,3]. Recently, several experimental and theoretical studies reported the creation and control of triplet Cooper pairs using spin-orbit coupling thereby simplifying the structures [4-7]. I will discuss the recent progress in this area, specifically focussing on two recent results: controlling the superconducting transition temperature using spin-orbit coupling [4] and superconductivity driven magnetisation reorientation [8].

Photovoltaic effect, e.g., solar cells, converts light into DC electric current. This phenomenon takes place in various setups such as in noncentrosymmetric crystals and semiconductor pn junctions. Recently, we proposed a theory for producing DC spin current in magnets using electromagnetic waves, i.e., the spin-current counterpart of the solar cells. Our calculation shows that the nonlinear conductivity for the spin current is nonzero in a variety of noncentrosymmetric magnets, implying that the phenomenon is ubiquitous in inversion-asymmetric materials with magnetic excitations. Intuitively, this phenomenon is a bulk photovoltaic effect of magnetic excitations, where electrons and holes, visible light, and inversion-asymmetric semiconductors are replaced with magnons or spinons, THz or GHz waves, and asymmetric magnetic insulators, respectively. We also show that the photon- driven spin current is shift current type, and as a result, the current is stable against impurity scattering. This bulk photovoltaic spin current is in sharp contrast to that of well-known spin pumping that takes place at the interface between a magnet and a metal.

The rectified non-linear response of a clean, time-reversal symmetric, undoped semiconductor to an ac electric field includes a well known intrinsic shift current. In a recent work we demonstrated that, when Kramers degeneracy is broken, a distinct second order rectified response appears due to Bloch state anomalous velocities in a system with an oscillating Fermi surface. This effect has been termed the resonant photovoltaic effect (RPE). It manifests itself as a resonant galvanic current peak at the interband absorption threshold in doped semiconductors or semimetals with approximate particle-hole symmetry. In this manuscript we present a detailed theory of this effect, and we evaluate the RPE for a model of the surface states of a magnetized topological insulator.

We have measured a helicity-dependent photocurrent at zero external magnetic field in a device based on a semiconductor quantum well embedded in a p-i-n junction. The device is excited under vertical incidence with circularly polarized light. The spin filtering effect is evidenced in the range 77–300 K at B = 0 owing to a CoFeB/MgO spin filter with out-of-plane magnetization in remanence. The helicity-dependent photocurrent is explored as a function of the temperature and bias. These characteristics are compared with those of a spin photocurrent device with in-plane magnetized CoFeB/MgO spin filter, excited under oblique incidence with circularly polarized light. In contrast to the in-plane spin filter device, the circularly polarized light asymmetry of the photocurrent in the out-of-plane device depends weakly on the external bias. This may be attributed to the large spin relaxation anisotropy in III-V zinc-blende quantum wells in the presence of a vertical electric field.

Proximity exchange coupling across an MgO tunnel barrier is observed up to room temperature. The proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation in a Fe/MgO/Si tunnel contact, modifies the spin precession of the spin accumulation at the MgO/Si interface and is detected via the inverted Hanle effect in a three-terminal configuration. It produces shifts and hysteresis of the inverted Hanle curves, and discontinuities at the coercive field of the ferromagnet, at which the magnetization and thereby the exchange field is reversed. The proximity exchange field is observed not only at low temperature, as we previously reported, but also at room temperature. We compare the magnitude of the exchange field at different temperatures, and complement this with data on the tunnel resistance and the tunnel spin polarization, which was extracted from four-terminal non-local spin transport in the same devices.

Neuromorphic computing captures the quintessential neural behaviors of the brain and is a promising candidate for the beyond-von Neumann computer architectures, featuring low power consumption and high parallelism. The neuronal lateral inhibition feature, closely associated with the biological receptive field, is crucial to neuronal competition in the nervous system as well as its neuromorphic hardware counterpart. The domain wall - magnetic tunnel junction (DW-MTJ) neuron is an emerging spintronic artificial neuron device exhibiting intrinsic lateral inhibition. This work discusses lateral inhibition mechanism of the DW-MTJ neuron and shows by micromagnetic simulation that lateral inhibition is efficiently enhanced by the Dzyaloshinskii-Moriya interaction (DMI).

Neuromorphic computing (NC) has emerged as one of the leading computation methodologies with low power consumption. Conventional CMOS based implementation of a complex computing system leads to power hungry, unscalable and inefficient designs. The present technologies are inefficient for computation intensive tasks such as neural networks. Hence, there is a need for unconventional devices and techniques. Nano-scale devices with the ability to mimic biological computation efficiently have been explored. Non-volatile memories such as resistive random access memory (RRAM) and phase change memory (PCM) are used for mapping synaptic functionality. Spintronic devices are among the suitable candidate to implement NC with low power consumption. Neuron as well as synapse functionality for neural networks are mapped using spin devices such as spin transfer torque magnetic random access memory (STT-MRAM) and spin orbit torque (SOT-MRAM). This paper compares the performance metrics of spin devices with other non-volatile memories for the implementation of neural network architecture with a single hidden layer. Spin device architecture consumes less area and much lower leakage power while to achieve the same level of accuracy as other devices. The MNIST dataset image classification achieved 1.17X reduction in leakage power, 9.42X reduction in latency and 1.08X reduced area consumption using SOT-MRAM device as compared to RRAM, PCM and conventional static random access memory (SRAM) respectively.

Manipulation of magnetic chirality is of outermost importance due to the great technological and scientific opportunities it unlocks. Over the last decade, this manipulation has been achieved through the harnessing of chiral spin interactions in non-centrosymmetric materials and thin film interfaces (DMI), allowing for the creation and tuning of topologically non-trivial chiral spin textures such as spin-spirals and Skyrmions, of great scientific and technological interest. Here we will discuss how state of the art 3D nano-printing can be employed to create 3D geometries that host complex chiral spin textures and how such geometries can be combined to create artificial interfaces where spin textures of different chirality are forced to match each other [1]. Characterization of the resulting spin states using magnetic X-ray microscopy will be presented. [1] Sanz-Hernández, D., et al. arXiv:2001.07130 [cond-mat.mes-hall]

Non-coplanar spatial or momentum-space distribution of an effective magnetic field acting on a free electron due to exchange or spin-orbit interaction couples electron spin precession with its motion. This coupling gives rise to a skew scattering underlying anomalous and topological Hall effects. We show that in a non-adiabatic interaction regime the emergent skew scattering can be expressed in terms of chirality in real or momentum space. The approach appears to be rather universal for transport phenomena in systems with broken time reversal symmetry. We show that scattering on a chiral spin texture produces a transversal current proportional to the real space spin chirality. Using the similar approach combined with tunneling Hamiltonian formalism we explain the chirality-driven skew tunneling of electrons and holes at a semiconductor interface. The theory can be relevant for unidirectional magnetoresistance in dilute magnetic semiconductors and magnetic topological insulators.

Current-induced torques enable fast and efficient control of the state of magnetic tunnel junctions (MTJ). While switching dynamics has been extensively studied in spin transfer torque (STT) driven MTJ devices, less is known about the transient dynamics and actual reversal speed of individual switching events induced by spin-orbit torques (SOT). Our real-time single-shot measurements reveal that SOT switching unfolds by a stochastic two-step process involving the nucleation and propagation of a reversed domain. Timescales and statistical distributions of these processes differ significantly when compared to switching by STT. The stochastic nature of the switching dynamics can be minimized by the combined action of SOT, STT, and the voltage control of magnetic anisotropy, resulting in reproducible sub-nanosecond switching with a distribution of switching onsets and durations approaching 0.1 ns.

We show that the current-induced torque generated can be significantly enhanced by the oxidation of metals. The oxidation allows efficient generation of the current-induced torques using Cu, a light metal with weak spin-orbit coupling [1]. We also show that the torque efficiency of ferromagnetic-metal/Cu-oxide bilayers can be maximized by a fine tuning of the oxidation level of the Cu layer [2]. The origin of the maximization is attributed to the enhancement of the interface spin-orbit coupling, showing that the bulk spin-orbit coupling of the oxidized Cu layer plays a minor role in the generation of the torque generation by naturally oxidized Cu. [1] H. An, Y. Kageyama, Y. Kanno, N. Enishi and K. Ando, Nature Communications 7, 13069 (2016). [2] Y. Kageyama, Y. Tazaki, H. An, T. Harumoto, T. Gao, J. Shi, and K. Ando, Science Advances 5, eaax4278 (2019).

We employ a finite element discretization scheme to solve numerically the coupled spin and charge transport equations in spin-transfer torque magnetoresistive random access memory cells. To adapt the drift-diffusion formalism to the case of a magnetic tunnel junction, we model the tunnel barrier as a material with a low magnetization-dependent conductivity and a large spin diffusion constant. This generalized spin and charge drift-diffusion approach is applied to determine the torques entering the Landau-Lifshitz-Gilbert equation to describe the magnetization dynamics. In particular, the switching times under a fixed voltage, a fixed current, and a fixed current density are compared.

The Hall effect occurring in a Hall bar is revisited on the basis on non-equilibrium thermodynamics principles. Following the approach developed in a previous work (Creff et al. J. Appl. Phys 2020), the stationary state is defined by the minimum power dissipation principle, and the well-known results about the charge accumulation at the lateral edges and the corresponding Hall voltage are recovered. Beyond, the effect of a sizable current leakage occurring at the edges is investigated. An analytical expression of this output current - proportional to the magnetic field and the leakage resistance - is derived.

The spin-charge conversion in ferromagnetic materials is a recent intriguing research topic. We report the large spin anomalous Hall effect (spin-AHE) in an L10-FePt ferromagnet. The spin anomalous Hall angle was evaluated to be ~ 0.25 for the L10-FePt from the linewidth modulation of ferromagnetic resonance spectra by dc current application. The evaluation of spin anomalous Hall angle at different configurations between applied electric current and the L10-FePt magnetization allowed us to examine the symmetry of spin-AHE. By utilizing the large spin anomalous Hall angle of L10-FePt, we also have successfully demonstrated spin-AHE-induced switching of the Permalloy magnetization.

Spin-orbit coupling (SOC) can convert a charge current into a spin current, enabling electrical control of magnetization, the foundation of spintronic devices. A quintessential example of SOC-induced transport in a ferromagnetic conductor (FM) is the anomalous Hall effect (AHE), in which an electric current perpendicular to the magnetization generates a transverse spin current and charge accumulations on the surface. Applying similar considerations to the configuration of a current parallel to the magnetization, SOC should also give rise to a transverse spin current with spins orthogonal to both the magnetization and spin current. The transverse spins precess rapidly about the magnetization direction and exert torque on the magnetization as they dephase, in analogy with spin transfer torque. In this talk, we will discuss this transverse spin Hall effect (TSHE) leads to anomalous spin-orbit torque (ASOT) on the top and bottom surfaces of a FM. In addition, we will also discuss the

In this talk, we explore the variety of ways in which ferromagnets electrically generate spin currents. We present first principles transport calculations giving the strength and magnetization dependence of these various mechanisms in transition metal ferromagnets. We compute full spin current tensors to probe all spin directions and show how each mechanism contributes to the spin currents allowed by symmetry. To help guide the interpretation of experiments, we also present calculations of spin current generation in magnetic heterostructures to quantify their transmission through interfaces and the spin torques they exert. Finally, we discuss experiments that have probed these novel mechanisms of spin current generation. Unlocking the full potential of ferromagnets as spin current sources will help create a powerful tool for spintronic devices such as magnetic memories.

While classical spintronics relies on the use of ferromagnetic materials to generate and detect spin currents, spin-orbitronics exploits the spin-orbit coupling (SOC) in non-magnetic systems to reach this goal. An efficient spin current detection and generation has been achieved in heavy metals such as Pt, W or Ta thanks spin to charge interconversion due to the Spin Hall Effect. However, an even larger interconversion was made possible by the use of the Direct and Inverse Edelstein Effect (EE and IEE) in systems with broken inversion symmetry at interfaces, inducing Rashba SOC. We observed the IEE in an in interfaces-engineered high-carrier-density SrTiO3 two-dimensional electron gas (2DEG) by mean of SP-FMR. This interconversion can be modulated by the application of a gate voltage, reaching very high values thanks to the enhanced Rashba splitting due to orbital mixing, and the vicinity of the 2DEG Fermi level with an of avoided band crossing with topologically non-trivial order [1]. By combining this high interconversion efficiency with induced ferroelectric properties in SrTiO3, we show that it is possible to control the sign of the spin to charge interconversion in a non-volatile fashion by manipulating the spin orbit properties of the 2DEG through an electric control of the polarization direction [2]. This electrically controlled non-volatile interconversion sign switching opens the way to ultra-low power spintronics, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

Spin-orbit (SO) field generated via spin Hall effect (SHE) or Rashba-Edelstein effect (REE) is one of the most important topics in spintronics for both fundamental physics and practical applications. Therefore a lot of SO materials such as heavy metals or topological insulators have been intensively studied so far for high conversion efficiency or functionality. But the SO torque efficiency is a material specific value in any case. It is ideal for us to synthesize the SO materials which have variable efficiency on our purposes. Here using a combination of heavy metals (HMs), we demonstrate to create extra SO fields such as damping-like (DL) and field-like (FL) fields on a Ni₈₀Fe₂₀(Py) layer via the REE at a W/Pt interface away from the Py layer through spin diffusion in the heavy metal layer. This means that we can manipulate the effective SO fields via the interface generated by a combination of materials apart from SHE contribution, and synthesize a designable SO material. Moreover we found opposite sign of the extra FL field in an inverse stacking structure like Py/Pt/W compared with in the Py/W/Pt. This implies that the extra field is coming from the HMs interface which has Rashba effect due to the change of the effective field direction at the interfaces. This leads us to a new concept of “synthetic SO materials” which paves a way for seeking high efficiency or superior functionalities in SO torque.

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research [1]. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale [2]. However, the use of ultra-fast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization [3] is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin orbit torques on the magnetization. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control.

Spin-orbit torque in metallic heterostructures arises due to multiple microscopic mechanisms, which presents a challenge for theoretical understanding and interpretation of the experimental data. First-principles calculations provide valuable insight through controlled studies of the dependence of spin-orbit torques on the relevant parameters in realistic disordered heterostructures. Recent results from such calculations and progress in understanding the mechanisms of spin-orbit torque will be discussed. It was found that the damping-like torque in ferromagnet/heavy-metal bilayers tends to have a large interfacial contribution that is comparable to the conventional spin-Hall contribution. Calculations with varying degrees of interfacial intermixing show that it does not strongly affect the damping-like torque but can strongly enhance the field-like torque. Recent results for ferromagnet/normal-metal/ferromagnet trilayers and antiferromagnet/normal-metal bilayers will also be discussed.

Writing magnetic random-access memory (MRAM) by ultrafast and energy-efficient spin-orbit torque (SOT) has been impeded by the orthogonality between spin polarization and thermally stable perpendicular magnetic anisotropy (PMA). Previously proposed approaches to break this symmetry increase the fabrication complexity, are highly sensitive to the SOT current duration and magnitude, or increase the switching energy. To overcome these challenges, we exploit the precessional nature of the field-like SOT to propose a toggle PMA SOT-MRAM with simple structure that is controlled by a unidirectional SOT current. The proposed MRAM achieves field-free and energy-efficient switching that is robust to variations in the SOT current magnitude and duration with greater than 50% tolerance demonstrated through micromagnetic simulation. The deformation-free structure provides efficient data read-out and can be leveraged for directional writing through a simple XOR between the stored and incoming bits.

Ultrashort optical pulses applied to ferromagnets excite spin polarized electronic distributions far from equilibrium (which are often referred to as "hot electrons"). Such an ultrashort-pulse excitation can lead to demagnetization, but also to a loss of electronic spin polarization due to hot-electron transport in and out of ferromagnetic layers. I will present some of our work on both topics starting with a microscopic study of the of electron-phonon and exchange scattering dynamics in a two-sublattice model with itinerant electrons. I will also present results on spin-dependent transport [3] of optically excited hot electrons in ferromagnet-metal heterostructures which have gained interest as THz emitters in recent years. The numerical solution is achieved using a Particle-In-Cell approach to treat both transport and scattering effects in a numerically efficient way that is based on ab-initio input and can be easily adapted to different structures. If time permits, I will briefly discuss a microscopic model of the inverse Faraday effect. In the framework of a ferromagnetic Rashba system with a band gap, one can compute the complete switching dynamics including spin-orbit coupling, mean-field ferromagnetism and the effect of off-resonant optical fields/pulses.

When a material is in a strongly out-of-equilibrium condition an enormous number of degrees of freedom are unconstrained. This allows for the emergence of a number of qualitatively new phenomena. One famous example is the ultrafast spin transport triggered by femtosecond laser excitations. Few years ago I proposed a method to inject giants spin currents in semiconductors. We have recently experimentally confirmed the feasibility of the injection and broke by several orders of magnitude the record for peak amplitude of spin current injection in semiconductors. Another examples of the critical impact of out-of-equilibrium dynamics we have measured a large and anomalous THz emission from carbon nanotubes (CNTs). By using an unprecedentedly precise theoretical description of the thermalisation process, we have unveiled the importance of the role of excitons and in particular exciton impact ionisation to the THz emission in CNTs.

Diamond nanoprobes with single engineered NV centers allow enhancing scanning probe microscopy with quantum metrology. In this talk, I will introduce the basic concepts and technology of diamond nanoprobes, and give illustrative examples of applications in nanoscale imaging of magnetism and currents.

Nitrogen-vacancy centers in diamond have been long used for spin-based optical sensing and considered viable candidates for implementing spin-based quantum information protocols. Their unique advantages include their nanoscale size and the optical readout of the electron spin state. These features make them particularly fit for the use in integrated nanophotonic and quantum optical devices. We will discuss how the optical Purcell effect in plasmonic systems affects the NV spin readout signal and present the demonstration of this readout through optical plasmons in an integrated nanophotonic interface. We will show that spin relaxometry is a powerful tool that allows to probe the magnonic density of states and its electrical tuning with sub-um spatial resolution. Furthermore, we will discuss how spin signal readout sensitivity can be enhanced with the use of plasmonic nanostructures and Bayesian measurement methods.

Here, we predict a strong coherent coupling in the quantum regime between V[TCNE]x magnons [1,2] and spin coherent NV center levels. First, we calculate analytically the magnon dispersion and fringe field and obtain a NV spin-magnon coupling strength of 8.7 kHz for a disk of diameter 1 μm. Secondly, we derive an analytical expression for the cooperativity as a function of NV center position for a general magnetic disk on top of the diamond substrate. Using realistic coherence times of both NV centers and V[TCNE]x magnons, we predict cooperativity C ≈ 6 for NV centers placed within a wide spatial region. Thus our proposal provides an avenue for coupling between NV centers separated by ≈1 μm. The material is based on work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0019250. [1] Na Zhu, et. al., Applied Physics Letters 109, 082402 (2016). [2] A. Franson, et. al., APL Materials 7, 121113 (2019).

We present the ab initio calculations of impurities in several systems with varying band gaps. First, we investigate rare-earth erbium in wide-gap semiconductor yttrium-oxide and silicon, and we report the defect formation energies for different charged states. We also derive parameters for the crystal field Hamiltonian to describe the energy levels. Next, we study the stability of the silicon and iron impurities in GaAs host in neutral and negatively charged states. We show that lowering the crystal symmetry by moving the impurity from the substitutional site to an interstitial site results in a second minimum in the defect formation energy, which is also verified by STM images. The capability of calculating and manipulating electronic and spintronic properties of rare earths and transition metal dopants leads to the design of novel quantum materials. We acknowledge support from the Center for Emergent Materials, an NSF MRSEC under Award No. DMR-1420451.

The field of THz spintronics is a novel direction in the research field of spintronics that combines magnetism with optical physics and ultrafast photonics. The experimental scheme of the field involves the use of femtosecond laser pulses to trigger ultrafast spin and charge dynamics in bilayers composed of ferromagnetic (FM) and nonmagnetic (NM) thin films where the NM layer features a strong spin-orbit coupling. The key technological and scientific challenges of THz spintronic emitters is to increase their intensity and frequency bandwidth. To achieve this the control of the source of the radiation, namely the transport of the ultrafast spin current is required. However, the transfer of a spin current from a FM to a NM layer is a highly interface-sensitive effect. In this work we study the properties of the spin current transport through the interface measuring the strength of the THz emission and compare it to the effective spin mixing conductance, one of the key concepts in the spin current transport through interfaces. The results show an enhancement of the spin mixing conductance for interfaces with higher degree of epitaxy similarly to the improvement of the THz emission. The proportionality between spin mixing conductance and THz emission can define new directions in engineering the emission of spintronic THz emitters.

We report on terahertz (THz) emission from a single layer ferromagnet which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect (AHE), thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed by introducing a magnetization gradient in the thickness direction of the ferromagnet. The emission can also be enhanced using a synthetic antiferromagnet (SAF) via the same AHE mechanism. In addition, we demonstrate by both simulation and experiment that the THz emission provides a powerful tool to probe the magnetization reversal process of individual ferromagnetic layers in the SAF structure – an important building block for all types of spintronic devices.

Terahertz (THz) emission spectroscopy in spin systems has become a very powerful method to generate THz radiation and to investigate the properties of Rashba or Topological Insulator surface states. The THz emission can be generated in heavy metallic or in more general Rashba systems. In 3d/5d transient metal bilayers THz emission in via the Inverse Spin Hall effect. Beyond heavy metal structures, Rashba states are strong candidates for THz-spintronics owing to their high spin to charge conversion properties. Here we present 2D electron gas with strong Rashba spin-orbit coupling and demonstrate THz emission via the Inverse Edelstein Effect.

Search for Non-Abelions - particles whose exchange transforms the quantum state of a system noncommutatively- is driven both by the quest for deeper understanding of nature and prospects for universal topological quantum computation. Examples of non-Abelions are Majorana and parafermion zero modes. However, physical systems that can host these exotic excitations are rare and hard to realize in experiments. To demonstrate new examples of such experimentally feasible systems, we will describe the domain walls formed in spin transitions in the integer and fractional quantum Hall effects between spin- polarized and spin-unpolarized phases. We show that superconducting proximity coupling to domain walls between two topologically distinct fractional quantum Hall liquids leads to emergence of parafermions. While Majorana zero modes form a double degenerate state, parafermions emerge as a six-fold degenerate state. Emergence of parafermions in electron and hole systems is discussed.

Floquet states are photon-dressed electronic states that emerge when driven by a time-periodic optical excitation. The presence of these states changes the equilibrium picture of electronic transitions into one where transitions between Floquet states are allowed. We have recently investigated the indirect exchange (RKKY) interaction mediated by itinerant electrons that are driven strongly out of equilibrium by circularly polarized light in two-dimensional electron systems with parabolic and linear dispersion (graphene). Our results indicate that emergence of non-equilibrium Floquet states leads to new and qualitatively different regimes of RKKY coupling, in which the sign (ferromagnetic or antiferromagnetic) and the period of the RKKY oscillation become tunable through the frequency and amplitude of the laser. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Early Career Award #DE-SC0019326.

Landauer's principle states that erasure of each bit of information in a system requires at least a unit of energy kBTln2 to be dissipated. In return, the blank bit may possibly be utilized to extract usable work of the amount kBTln2, in keeping with the second law of thermodynamics. In this work, we build on our earlier work on spin Hall devices and focus on heat and charge transport in generic spintronics devices in the presence of a spin bath. We show how a properly initialized nuclear spin subsystem can be used as a memory resource for a Maxwell's Demon to harvest available heat energy from the reservoirs to induce charge current that can power an external electrical load. We also show how to initialize the nuclear spin subsystem using charge currents which necessarily dissipate energy. This opens up a new avenue towards energy storage applications using spintronics devices.

Quantum-classical spin hybrids composed of physical system with complimentary characteristics have enabled novel capabilities and functionalities within the realm of existing technology. One half of such hybrid systems is the quantum impurity spin with small spin quantum number such that its description is governed by the counter-intuitive laws of quantum mechanics. The other is a classical magnet with large spin quantum number such that its dynamics can be captured within the framework of classical physics. Such hybrids give rise to possibilities where controlling the degrees of freedom in one system can be leveraged to control dynamics in the other. Leveraging the demonstrated spintronic tools of classical magnet dynamics, we demonstrate two significant steps towards realizing a quantum network for information processing applications. One, a theoretically designed regime where electrical control of non-linear magnetization dynamics of a nanomagnet provides a local, coherent, and low-power drive to manipulate a coupled quantum impurity spin without introducing additional decoherence. Another, where we demonstrate via a joint theoretical and experimental effort, the electrical tuning of interaction between electrically-controlled propagating magnons in an extended magnet and a quantum impurity spin. The merits of such a hybrid system provide pathways to overcome the bottlenecks associated with local controllability of individual quantum spins in a quantum network and modulate the interaction mediating the two subsystems.

When graphene is in proximity to a transition metal dichalcogenide (TMDC), it acquires an enhanced spin-orbit interaction (SOI). Here, I will discuss the relevant consequences of this unique type of SOI for spin manipulation in graphene-TMDC heterostructures. Firstly, I will present our recent experiments which demonstrate how the spin dynamics in graphene is strongly modified by the proximity to a TMDC. The results show that the spin lifetime depends on the spin orientation, and it is larger for spins pointing out of the graphene’s plane compared with the spins pointing in-plane. I will further present an unprecedented electric-field tunability of such anisotropic features in graphene-WS2 heterostructures. Finally, I will demonstrate that the enhanced SOI leads to spin-charge interconversion in graphene. By using spin precession measurements, we can separate the contributions of the spin Hall and the spin galvanic effects.

Magnetism, one of the most fundamental physical properties, has revolutionized significant technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emerging materials and reduced dimensions. The recently discovered magnetic 2D van der Waals materials (hereafter abbreviated as “2D magnets”) provide ideal platforms to enable the atomically-thin, flexible, lightweight magneto-optic and magnetoelectric devices. The seamless integration of 2D magnets with dissimilar electronic and photonic materials further opens up exciting possibilities for unprecedented properties and functionalities. In this talk, I will speak on our experimental observation of the 2D ferromagnet, analyze the current progress and the existing challenges in this emerging field, and show how we push the boundary by exploring the potential of 2D antiferromagnets for spintronics.

When the Coulomb interaction becomes strong in semiconductors, the electrons and holes can interact to form excitonic states, which govern the materials’ optical properties. In atomically thin transition metal dichalcogenide (TMD) devices encapsulated by boron nitride, we observe various excitonic states with distinctive spin and valley configurations, including the bright and dark excitons, and intra- and inter-valley excitons. These excitons exhibit complex interactions with photons and phonons, giving rise to a panoply of emission lines. We have tailored the excitonic properties by using the charge density, magnetic field, and moiré potential. These novel excitonic states with adjustable properties hold promises for the development of next-generation excitonic devices.

We extend our layer-by-layer model of photonic eigenmodes (resonance, polarization) of semiconductor laser and VCSELs (Fordos PRA 2017) by considering local optical properties in order to describe the above-threshold lasing even in the multimode regime. Such generalization, consistent with the semi-classical description based on the optical Bloch equations, allows to describe the optical gain and the wave propagation within the cavity in a more realistic way. The formalism is suitable to study the mode competition, since the important physical effects such as spatial-hole burning or cross-saturation mechanisms are included self-consistently. The only input parameters are those describing geometrical and local material properties of the cavity and the gain media, without using any mean-field approximations. We will present our generalized approach for modelling real devices and will show how it can serve in determining the main optical and physical properties of such devices.

Since todays Internet traffic is more and more concentrated in hyperscale datacenters,¹ new concepts for shortrange optical communication systems with high modulation bandwidth, high temperature stability, and low energy consumption are urgently needed. Birefringent spin-lasers, in particular spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs), are a novel type of ultrafast laser devices which promise to serve as ultrafast transmitters for the next generation of optical communication systems. While current-driven intensitymodulated VCSELs are state-of-the-art laser devices for short-range communication, their modulation bandwidth is limited to values below 40 GHz.^{2, 3} Recently, we were able to demonstrate that modulating carrier spin and light polarization in spin-VCSELs instead of carrier density and light intensity in conventional devices enables ultrafast polarization dynamics and a modulation bandwidth of more than 200 GHz.⁴ This high modulation bandwidth was achieved by increasing the resonance frequency of the coupled carrier spin-photon system by implementing high values of birefringence to the cavity of 850 nm GaAs/GaAlAs VCSELs. Here, we show experimental results for the intensity and polarization dynamics in highly birefringent spin-VCSELs as a function of bias current, birefringence, and temperature and demonstrate the capability of spin-VCSELs for ultralow energy consumption and high temperature stability. Furthermore, we present first results on polarization dynamics in 1.3 μm VCSELs for potential long-range communication systems and discuss novel concepts for future integrated and electrically pumped devices.

We propose and numerically investigate a method for direct in-phase quadrature (IQ) modulation of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for coherent communications. In particular, an injection-locked spin-VCSEL is investigated by using spin-flip rate equations for generating quadrature amplitude modulation (QAM) signals. A polarizer 45° tilted to the polarization axis of the injected external light is used to extract the in-phase and quadrature-phase components of the output light from the injection-locked spin-VCSEL. It is confirmed that QAM signals can be generated with the proposed method successfully, and long electron spin relaxation time and small linear birefringence in the spin-VCSEL are suitable for efficiently generating clear QAM constellations with a low error vector magnitude. A detuning condition for the injection locking of the spin-VCSEL is found to be important for obtaining wide modulation bandwidth of the direct IQ modulation.

Data centers play an important role in the ongoing demand for higher data rates. Here intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) are the emitters of choice. The intensity dynamics resonance is currently limited to around 30 GHz. Using the much faster polarization dynamics in VCSELs can be a promising alternative. The polarization dynamics resonance is mainly determined by the frequency difference of the two orthogonal linearly polarized modes, which is defined by the birefringence. We have experimentally investigated mechanisms for birefringence manipulation from large-scale down to on-chip solutions. Polarization oscillations with frequencies in excess of 200 GHz have been observed.

Ordinary materials react to an electric field by generating an electrical polarization, and a magnetic field normally generates a magnetization. In magnetoelectric media, this conventional scheme is turned upside-down: an electric polarization is induced by a magnetic field and a magnetization results from an electric field. We develop a detailed theory for how this effect comes about in materials nanostructured into ultra-thin conducting sheets. Such quantum wells are found to exhibit a sizable magnetoelectric effect that could enable novel device applications. Antiferromagnetic order plays a central role in our theory. We reveal magnetic order in quantum-well structures to arise from dissipationless equilibrium currents. In the magnetoelectric effect, an electric field manipulates the antiferromagnetic currents such that they give rise to a macroscopic net magnetization. Our theory provides new avenues for pursuing antiferromagnetic spintronics as a post-silico electronics paradigm.

One of the main tasks in the investigation of 2-dimensional transition metal dichalcogenides is the determination of valley lifetimes. In this work, we combine time-resolved Kerr rotation with electrical transport measurements to explore the gate-dependent valley lifetimes of free conduction band electrons of monolayer WSe₂. When tuning the Fermi energy into the conduction band we observe a strong decrease of the respective valley lifetimes which is consistent with both spin-orbit and electron-phonon scattering. We explain the formation of a valley polarization by the scattering of optically excited valley polarized bright trions into dark states by intervalley scattering. Furthermore, we show that the conventional time-resolved Kerr rotation measurement scheme has to be modified to account for photo-induced gate screening effects. Disregarding this adaptation can lead to erroneous conclusions drawn from gate-dependent optical measurements and can completely mask the true gatedependent valley dynamics.

Advances in controlling the correlated behavior of transition metal dichalcogenides have opened a new frontier of many-body physics in two dimensions. A field where these materials have yet to make a deep impact is antiferromagnetic spintronics – a relatively new research direction promising technologies with fast switching times, insensitivity to magnetic perturbations, and reduced crosstalk. Here, we present measurements on the intercalated TMD Fe1/3NbS2 which exhibits antiferromagnetic ordering below 42K. We find that remarkably low current densities of order 10KA/cm2 can reorient the magnetic order, which can be detected through changes in the sample resistance, demonstrating its use as an electronically-accessible antiferromagnetic switch. Fe1/3NbS2 is part of a larger family of magnetically intercalated TMDs, some of which may exhibit switching at room temperature, forming a platform from which to build tunable antiferromagnetic spintronic devices.

Broken inversion symmetry and time-reversal symmetry along with large spin-orbit interactions in monolayer transition metal dichalcogenides (TMDs) make them ideal candidates for novel valleytronic applications. Although successful spin transport and detection are very crucial for spintronic/valleytronic devices, electrical spin transport and spin detection due to spin-valley polarization in TMDs is still lacking. An electrical realization of spin transport and detection in TMDs demand perpendicular magnetic anisotropic (PMA) electrodes with very small Schottky barrier height (SBH). Furthermore, formation of large SBH at the metal/TMDs interfaces limits the exploitation and integration of TMDs in spintronic/valleytronic devices. In this work, we develop ferromagnetic electrodes and integrate them in MoS2 field-effect transistors. We studied the transfer characteristics of these devices and estimated SBH. SBHs extracted in these devices were found to be very small.

(110)-oriented GaAs-based quantum wells (QWs) are promising active layers of spin lasers because of long spin relaxation time at room temperature. So far, (110)GaAs/AlGaAs QWs have been investigated intensively and long carrier lifetime of several ns has been reported. In contrast, there have been few reports on the crystal growth of (110)InGaAs/(Al)GaAs QWs despite its great potential to the active layer, resulting in rather poor crystal quality with short carrier lifetime. In this study, we tried to grow high-quality (110)InGaAs/(Al)GaAs QWs by optimizing MBE growth conditions and obtained long carrier and spin lifetime (>1 ns) in the QWs grown with relatively high temperature and As/Ga flux ratio. This reveals that the (110)InGaAs/(Al)GaAs QWs are applicable to the active layer of spin lasers. In addition, we propose a novel structure for a spin transport layer between the ferromagnetic electrode (spin injector) and the active layer of spin laser.

Germanium is a very good candidate to host a versatile spintronics platform thanks to its unique spin and optical properties. Recently we focused on two approaches in order to tune the spin-orbit interaction (SOI) in this Ge-based platform. The first one relies on growing high quality epitaxial topological insulators (TIs) on a Ge (111) substrate, we developed an original method to probe the spin-to-charge conversion at the TI/Ge(111) interface by taking advantage of the Ge optical properties. The latter approach is to exploit the intrinsic SOI of Ge (111). By investigating the electrical properties of a thin Ge(111), we found a large unidirectional Rashba magnetoresistance, which we ascribe to the interplay between the externally applied magnetic field and the current-induced pseudo-magnetic field applied in the spin-splitted subsurface states of Ge (111). Both studies open a door towards spin manipulation with electric fields in an all-semiconductor technology platform.

In this talk, we will present our general multiband k. p approach to describe the properties of spin-polarized transport and spin-dependent tunneling within semiconductors interfaces and related heterostructures. Hereafter, we will apply our formalism and platform to describe the properties of spin-dependent tunneling along the particular [110]-direction. This represents a nontrivial case for the description of the spin-current because of the presence of order terms in the wave vector k higher than two in the effective Hamiltonian. We demonstrate here that the multiband techniques employed and developed here beyond the actual state of the art also appear to be a perfect numerical tool to check and validate some analytical developments e.g. based on perturbation technique approaches, also allowing to alleviate some fundamental analytical issues.

Septuple layers of MnBi₂Te₄ type are important building blocks of magnetic topological insulators and semimetals but also examples of two-dimensional magnetic materials with critical temperatures up to a few tens of Kelvin. Using density functional theory (DFT) we study the exchange interactions in MnBi₂Te₄, MnBi₂Se₄, and MnSb₂Te₄ films to highlight the importance of higher-order terms and the contributions of relativistic anisotropic interactions. We also discuss the possibility to induce Dzyaloshinskii-Moriya interaction by gating with an external electric field.

An intriguing property of a three-dimensional topological insulator (TI) is the existence of surface states with spin-momentum locking. We report the discovery of a new type of Hall effect in a TI Bi₂Se₃ film [1]. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall and anisotropic magnetoresistance. At variance with the nonlinear Hall effect due to Berry curvature dipole in time-reversal invariant materials, this novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry-breaking, which also exists in other non-centrosymmetric materials [e.g., WTe₂ and the 2DEG on the SrTiO₃(001) surface] with a large span of magnitude.

The antiferromagnetic coupling in [Fe/MgO]N multilayers is found to be strongly depending on the number of bilayer repeats (N). The results support the presence of long range interlayer exchange interactions as well as profound quantum well states defined by the total extension of the Fe/MgO(001) samples.

Nanoscale magnets are the building blocks of many existing and emergent spintronic applications. Controlling magnetic damping in nanomagnets holds the key to improving the performance of future technologies. Here, we demonstrate that a ferromagnetic nanoparticle can exhibit spin dynamics qualitatively different from those predicted by the harmonic oscillator model. Nonlinear contributions to the damping can be unusually strong, and the effective damping parameter itself can exhibit resonant dependence on field/frequency. We observe a resonant magnon scattering process that drastically alters the magnetization dynamics of a nanomagnet driven by spin torques. It reverses the effect of the spin torque on magnetic damping and turns an anti-damping torque into a dissipation-enhancing torque. The discovery of this counter-intuitive effect advances our understanding of spin dynamics in nanoscale magnetic systems and has far-reaching implications for spintronic applications.

An interplay of conservation and breaking of local and global symmetries in topological phases of matter leads to the emergence of topological phenomena including quantum anomalous (QAH) Hall effect, topological superconductivity, and non-Abelian quantum statistics. Among the members of the family of topological materials, magnetically doped topological crystalline insulators (TCI) were foreseen to host topologically protected QAH states generating multiple dissipationless edge and surface conduction channels with Chern number C ≥1. The symmetry protected topological phase of the SnTe class of TCI is characterized by a mirror symmetry resulting in topological surface states. Theoretical and experimental studies demonstrated that four Dirac points are located at the time-reversed-invariant-momentum (TRIM) points for the (111) surface of the SnTe compounds. In this work, via low temperature magnetotransport studies, the opening of the gaps at the TRIMs is demonstrated in ferromagnetic 30 nm thick Sn_1-xMn_xTe (111) thin epitaxial layers grown onto BaF₂(111) substrates by molecular beam epitaxy. While a spin mediated magnetoconductance is observed in layers with 𝑥 ≥ 0.03, the appearance of hysteretic magnetoconductance behaviour and square-like anomalous Hall effect indicate the onset of a hole mediated ferromagnetic ordering in epitaxial Sn_1-xMn_xTe (111) layers for 𝑥 ≥ 0.06. The anomalous Hall angle, 𝜃_AH~0.3 estimated for Sn_0.92Mn_0.08Te is one of the highest recorded for magnetic topological quantum materials. The tuning of the global band topology by magnetic doping opens wide perspectives for topology driven quantum spintronic technology.

Novel spin-related phenomena originating from the topological surface states in topological insulator (TIs) have become a very exciting topic in spintronics [1-2]. Exploiting those phenomena in TI/ ferromagnet heterostructures not only opens the pathway to novel spintronic devices but also provides new insights into the large spin-orbit interaction in TIs. Here, we report on various giant spin-related phenomena, including the giant spin Hall effect (SHE), the giant interfacial Dzyaloshinskii–Moriya interaction (DMI), and the giant unidirectional spin Hall magnetoresistance (USMR) in BiSb topological insulator-ferromagnet bilayers. We found that BiSb can generate not only a large spin current for ultra-low spin-orbit-torque switching [3], but also a large interfacial DMI for generation of free-field ground-state skyrmions in BiSb/MnGa bilayers even at room temperature [4]. Furthermore, we observed a giant USMR effect up to 1.1% in a BiSb/GaMnAs bilayer [5]. Those giant phenomena can be utilized to significantly improve the performance of SOT-MRAM and skyrmion-based racetrack memories.

The boundary of topological insulators exhibits a pure spin current owing to the topological edge states. To exploit this edge spin current for spintronic purposes, a promising design is to make the topological insulator in contact with a metallic ferromagnetic thin film, such that the edge spin current can be used to manipulate the magnetization or vice versa. We demonstrate that in a 2D version of the topological insulator/ferromagnet junction, the topological edge state percolates into the ferromagnet, and the detail of the percolation depends significantly on the band structure and the magnetization orientation. Moreover, we uncover novel effects such as the existence of a room temperature persistent charge current, and elaborate how the real wave function of the percolated edge state makes the current-induced spin torque entirely field-like. This work is supported by FAPESP grant number 2016/08468-0.

The possibility to manipulate spin textures is of central importance for their utilisation in functional materials. Here it will be shown how the resulting changes in the spin texture of the electronic structure can directly be imaged by spin- and angle-resolved photoemission spectroscopy (SARPES) by the examples of a multiferroic and a polar material. By using operando SARPES it will be shown that it is possible to reverse both the Rashbaand Zeeman-type spin textures in multiferroic Ge_1-xMn_xTe thin films by applying an external electric field in a capacitor type set-up. Furthermore, by growing homoepitaxial thin films of SrTiO₃(001) the filling of the 2D electron gas found in this system can be altered to achieve a single spin-polarised Fermi surface. Combined with the superconducting properties found in SrTiO₃ this Fermi surface topology can form the starting point for the search of Majorana-type excitations.

Magnetic skyrmions are quasiparticle configurations in a magnetic film that can act as information carrying bits for ultrasmall, low power nonvolatile memory. Skyrmions can be nucleated and driven by spin-orbit torque from a current driven in a heavy metal underlying a ferromagnetic layer, the configuration commonly called a racetrack. Recently it has been shown that by hybridizing the skyrmion between Neel and Bloch types the magnus effect on skyrmion motion, which makes it veer from a straight path down a racetrack, can be effectively canceled which provides them a self-focused naturally converging lane" to travel through. This is achieved by exploiting the voltage controlled magnetic anisotropy effect whereby the magnetic anisotropy of the ferromagnetic racetrack can be positionally modulated by a gate voltage. In this work we show, using detailed micromagnetic simulations, that by using hybrid skyrmions we can obtain demultiplexer functionality out of a racetrack. We further propose a hybrid skyrmionic reconfigureable computing fabric. In conventional CMOS based field programmable gate arrays, SRAM cells are used to build a LUTs storing pre-computed truth-table of a Boolean function and a multiplexer selects one of the storage cells as the output. We show that non-volatile hybrid skyrmions can also act as the memory element and the gateable self-focused nature of the hybrid skyrmions can be exploited to program the proposed CMOS-skyrmion hybrid design to perform different logic operations. The low driving energy and non-volatility of magnetic skyrmion in a racetrack promises the development of energy efficient programmable architecture for future system-on-a-Chip (SoC) designs.

Skyrmion manipulation utilizing Voltage Control of Magnetic Anisotropy (VCMA) can lead to small footprint nanomagnetic memory devices [1-2]. This talk will focus on using intermediate skyrmion states [3] to enable robust and energy efficient magnetization reversal with VCMA in the presence of thermal noise and defects and its scaling to lateral dimensions below 50 nm. We further discuss experimental demonstration of VCMA induced nonvolatile creation and annihilation of skyrmions in an antiferromagnet/ferromagnet/oxide heterostructure film [4] by applying a few volts. This could provide a pathway to control single skyrmion in an MTJ device using VCMA.

Skyrmions are topologically protected spin textures, which may be used in spintronic devices for information storage and processing. However, skyrmions in ferromagnets have some intrinsic difficulties for their usage in nanodevices, such as the inability to move strictly along electric current due to skyrmion Hall effect. We worked out several routes to solve this problem by using instead of skyrmions novel anisotropic topological objects, such as antiskyrmions and bimerons. Yet as another solution to eliminate the skyrmion Hall effect, topological spin solitons in antiferromagnets are proposed. It will demonstrate how they can be stabilized and manipulated at finite temperatures. Remarkably, we also show how the Magnus force acting on (anti)skyrmions, and thus in turn skyrmion Hall effect, can be efficiently tuned microscopically via modulation of spin-orbit interaction.

Tuning the Dzyaloshinskii-Moriya interaction (DMI) using electric (E)-fields in magnetic devices has opened up new perspectives for controlling the stabilization of chiral spin structures. Recent efforts have used voltage-induced charge redistribution at magnetic/oxides interfaces to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of Pt/Co/HfO2 ultra thin films. The E-field effect on the DMI is linked to the migration of oxygen species from the HfO2 layer into the Co and Pt layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E-fields can be used not only for local gating in devices but also as a material design tool for post growth tuning of the DMI.

The integrity and functionality of spin-crossover complexes, which exhibit an intrinsic magnetic bistability, is generally lost on metal surfaces, with only rare exceptions. In contrast, robust metal-organic complexes such as metal porphyrins rely on external ligands to induce spin switching. We introduce a new class of spin switches utilising the mechanical movement of an axial pyridine ligand strapped to a Ni porphyrin ring. A particular design of the strap ensures a coupling between the shape of the porphyrin, the coordination number and the spin of the Ni center. The relation between the coordination and the spin state is evidenced in the crystalline state and on a Ag(111) surface using near-edge x-ray absorption fine structure spectroscopy. Reversible switching of individual complexes on Ag(111) is demonstrated using low-temperature scanning microscopy. Besides the promises in molecular spintronics, this class of systems may turn useful to control the catalytic reactivity of surfaces.

The study of quantum coherent magnonic interactions relies on the ability to excite and exploit long lived spin wave excitations. That requirement has for many years placed yittrium iron garnet (YIG) as the leader in high-Q, low-loss magnetic resonance and coherent magnetic phenomena. High-quality YIG, however, is difficult to incorporate on-chip. The organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x) has recently emerged as an alternative to YIG due to its extremely low-loss (α = 3.98 × 10^-5). Here we fill the low-loss on-chip material gap by presenting the semiconductor-compatible deposition, patterning, and characterization of V[TCNE]x thin films with lateral dimensions from microns to millimeters as well as a strong magnetic response to temperature and strain in V[TCNE]x. This work establishes the versatility of V[TCNE]x for on-chip applications requiring highly coherent magnetic excitations ranging from microwave communication to quantum information.

Magnetism is enjoying ever-increasing use in biomedical research. Although such fields may seem remote to spintronics, sometimes there can be surprisingly close connection, particularly relating to the magnetic microstructures used. This talk aims to introduce magnetics and spintronics researchers to some of the exciting new uses and potential opportunities of engineered magnetic microstructures in biomedical research. As one particular example, the talk will introduce how magnetic microstructures can enhance the functionality of magnetic resonance imaging, providing new forms of image contrast and multiplexing as well as new types of remote magnetic sensing of physiological and environmental conditions.

Antiferromagnets are magnetically ordered materials without a macroscopic magnetization. As a result, they are of interest in the development of memory devices where data cannot be erased by external magnetic fields. However, the absence of a macroscopic moment also makes it difficult to electrically control their magnetic order (Néel vector). Here we show that pillars of antiferromagnetic PtMn, which are grown on heavy metal layers of Pt and Ta, can be reversibly switched between different magnetic states by electric currents [1]. The devices are based on materials that are typically used in the magnetic memory industry, and we observe switching with very low current densities. Furthermore, by varying the amplitude of the writing current, multi-level memory characteristics can be achieved.

Magnetic domain-wall devices, modulated by the spin-transfer torque or the spin-orbit torque effect, can implement logical operations in a manner that is inherently compact and cascadable. Using circuit simulations with micromagnetics-validated compact models, we evaluate the device requirements for domain-wall logic that has low latency, outperforms scaled CMOS logic in energy efficiency, and remains robust to process variations. We further show how the inherent non-volatility of these devices can be leveraged to construct stateful logic circuits that save energy and area relative to their CMOS counterparts and propose novel logic architectures that exploit the unique advantages of domain-wall devices.

Combining optical, electronic, and magnetic functionality in a single material is a major challenge in spintronics. The discovery of optically triggered magnetization in transition-metal doped semiconductor nanocrystals at room temperature more than a decade ago brought chemically prepared nanoparticles into play. Here, we demonstrate how the exchange interaction between magnetic dopants and charge carriers in the host matrix can be tailored by engineering size, shape, and composition of nanocrystals. Incorporation of single magnetic impurities into colloidal quantum dots yield unique discoveries like huge zero-field exchange splittings or digital doping effects. Doping nanocrystals with an ensemble of magnetic impurities, in contrast, leads to collective spin effects like magnetic polaron formation. Embedding these materials into an electronic device, current-driven polaron formation is shown, while single nanocrystal studies reveal a pathway of directed magnetic polaron formation.

We propose a dual-spin-bit Physically Unclonable Function for securing Internet-of-Things(IoT) from geometry variations of magnetic thin-films embedded inside IoT devices. Such structure-dependent device-specific keys serve for authentication,identification and privacy preservation of users against Identity theft, side-channel attacks and remote hacking in IoT networks. In our design,electrically tunable magnetic domain wall motion of the thin-films produces device-geometry sensitive spin-transfer-torque response, providing path for device-specific memory-based Spintronic Physically Unclonable key generations.An array of such magnetic films with each film containing memristive spin-bit act as the building block of spin-torque driven cryptographic logic circuits for the IoT devices.

Using the standard formalism of Lorentz transformation of the special theory of relativity, we derive the exact expression of the Thomas precession rate for an electron in a classical circular orbit around the nucleus of a hydrogen-like atom.

This paper considers spin - j representation of the Lie group SU(2) and its applications to contemporary physics from Observer's Mathematics point of view (see www.mathrelativity.com). This mathematics was introduced by authors based on denial of infinity idea. We proved here the following. Theorem. In Observers Mathematics the probability of spin - j transformation is a homomorphism (representation) of SU(2) to the set of matrix transformations of a linear space of polynomial functions is less than 1. Theorem. The probability of elementary fermions have half-integer spin in Observer's Mathematics is less than 1. The probability of elementary bosons have integer spin in Observer's Mathematics is less than 1.

Random number generators play an essential role in stochastic circuits, neuromorphic computing and cryptographic systems. Existing CMOS based true random number generators (TRNGs) incur a significant performance, power and area overheads. The systems based on magnetic tunnel junctions (MTJs) offer favorable features such as high resilience, low energy consumption, good performance, and less complexity. Compared with the conventional MTJ switching approaches, voltage controlled magnetic anisotropy switching (VCMA) features low switching current, minimal control circuitry and calibration process, and insensitivity to the write pulse duration. In this paper an efficient design for TRNG based on VCMA using MTJ is presented. The proposed magnetic TRNG consists of a standard two-terminal MTJ connected to the drain terminal of a PMOS transistor which controls the pulse width of the applied bias voltage. MTJ is used as a noise source device and its precessional switching characteristics are exploited to design the TRNG. The resistance of the device fluctuates while operating in precessional switching regime and the magnetization state is selected randomly via thermal fluctuations. The voltage developed across the MTJ corresponding to its random state results into a random bit. A 64-bit parallel random number generator has been implemented and evaluated using Spice simulating tool (with 45 nm CMOS technology node). The proposed circuit has an advantage of less circuit complexity and high energy efficiency. It achieves a high throughput of 64Gbps with an energy consumption of 5.6 fJ/bit.

In recent years, nano-oscillators have gained enormous interest in the research domain of spintronics. These nanooscillators generate microwave signals using spin-transfer torque (STT) mechanism and demonstrate key features in terms of scalability, frequency tunability, wide operating temperature, and high integrability with CMOS technology. Spin devices such as STT, domain wall (DW), and spin-orbit torque (SOT) devices can be interchangeably used to build these nano-oscillators. The magnetic domain wall in nanowires shows stable chirality induced by interfacial Dzyaloshinskii–Moriya Interaction (DMI) and exhibit ultra-high-speed DW motion. A spin current injecting into the nanowire create magnetic oscillations by moving the DW between the ends of DMI confined region. In this manuscript, current driven domain wall motion (DWM)-based nano-oscillator has been presented to achieve a high tuneable frequency. Furthermore, a comparative study of STT, SOT, and current induced DWM based nano-oscillators has been carried out. The potential applications of these nano-oscillators include frequency signal generation, signal modulation, neuromorphic computing, and microwave signal detection/generation. The capability of spintronic oscillators to communicate together, their long lifetime, and low energy consumption unfolds the path for the fast and parallel computation systems.

Magnetic skyrmions are chiral spin textures which are topologically stable entities. These are the promising candidates for data storage and logic devices owing to nanoscale size and remarkably low current density required to drive them in contrast to the magnetic domain walls. Antiferromagnetic (AFM) skyrmions are spin textures with equal and opposite topological charge in each sub-lattice leading to cancellation of Magnus force in contrary to ferromagnetic (FM) skyrmions that exhibits skyrmion (SkHE) Hall effect. FM skyrmions also lacks behind the AFM skyrmions by virtue of their sensitivity towards the stray fields. The formation of skyrmions in magnetic materials possessing broken inversion symmetry is subject to material parameters. Therefore, in this paper, we have demonstrated the nucleation of AFM skyrmions by injection of spin polarized current and effect of tuning the magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI) constant on skyrmion nucleation. The variation of velocity w.r.t. the applied current density in a nanotrack is also investigated.

We have explored the use of a magneto-optical diffraction grating, in the form of a YIG (Y₃Fe₅O₁₂) continuous film, as a mean of controlling light with mesoscale magnetic textures. The existence of a periodicity for the out-of-plane magnetic component opens up the possibility of using the film as a magneto-optical diffraction grating. For incident light linearly-polarized along the stripe direction, the Faraday rotation yields a polarization profile of the transmitted light characterized by a uniform distribution along the direction parallel to the stripes and a periodic phase-reversal texture along the corresponding orthogonal direction. The interference pattern resulting from the latter gives rise to magnetic Bragg peaks, in good agreement with the Fourier transform of the real-space magnetic domain texture recorded with Kerr microscopy. These results highlight the potential of controlling the scattering of light by employing and tailoring meso-scale magnetic textures.

Nanomagnetic Logic (NML) based computation started emerging as a key alternative towards the Beyond CMOS based Rebooting Computing paradigm. Arithmetic computation using this NML can be performed using majority (based on the principles of Magnetic Quantum-dot Cellular Automata) and non-majority logic gates. However there have been extensive studies on the majority logic gates based NML arithmetic architectures, its counterpart needs extensive analysis. Kurtz et al., and Varga et al., have performed the pioneering work in designing non- majority based `AND', `OR' operations. Here in this study, we introduce the non-majority voting gate based half-subtractor and adder architecture design methodology. To the best of author's knowledge this is the first time demonstration of arithmetic nanomagnetic implementation using non-majority gate. Subsequently, it can be inferred from the micromagnetic simulations and modeling that an asymmetrically shaped nanomagnet can be used as a compute nanomagnet in performing computation. The results show that only 4 nanomagnets are required to perform half subtractor/adder leading to area efficient design. This reduction is achieved by exploiting the principles of SP hybrid anisotropy and Boolean optimizations. Thus it can be observed that the non-majority voting based computation can lead to area efficient nanomagnetic arithmetic architecture design. Consequently, the thermal bit-stability analysis has also been carried out for the proposed sub-5o nm design.