The ability to generate pure-spin currents using the non-local spin valve (NLSV) has established it as an opportune device for testing spin transport on nanoscopic dimensions, with potential technological applications. The geometry has been used extensively to probe relaxation in a host of materials including non-magnetic metals. Nevertheless, interpretation of the non local ‘spin signal’ obtained in such a device relies on a precise ability to separate out pure spin transport and background effects.
In this talk I will present insights obtained from all-metallic NLSVs. Recent results, systematically investigating the spin signal generated in such devices has allowed us to isolate several effects which limit the generation and detection of spin signals in NLSVs. I will present findings on the interface, impurity scattering and thermoelectric contributions in such devices, including the background signals they generate.
I will also discuss the concept of thermal nanoscale conversion: using a heated scanning probe tip to locally modify the properties of magnetic thin films or devices. I will show how this offers new ways to design spintronic and non-local devices.
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.
A distinguishing feature of spin accumulation in ferromagnet-semiconductor devices is its precession in a magnetic field. This is the basis for detection techniques such as the Hanle effect, but these approaches become ineffective as the spin lifetime in the semiconductor decreases. For this reason, no electrical Hanle measurement has been demonstrated in GaAs at room temperature. We show here that by forcing the magnetization in the ferromagnet to precess at resonance instead of relying only on the Larmor precession of the spin accumulation in the semiconductor, an electrically generated spin accumulation can be detected up to 300~K. The injection bias and temperature dependence of the measured spin signal agree with those obtained using traditional methods. We further show that this new approach enables a measurement of short spin lifetimes (< 100~psec), a regime that is not accessible in semiconductors using traditional Hanle techniques.
The measurements were carried out on epitaxial Heusler alloy (Co2FeSi or Co2MnSi)/n-GaAs heterostructures. Lateral spin valve devices were fabricated by electron beam and photolithography. We compare measurements carried out by the new FMR-based technique with traditional non-local and three-terminal Hanle measurements. A full model appropriate for the measurements will be introduced, and a broader discussion in the context of spin pumping experimenments will be included in the talk. The new technique provides a simple and powerful means for detecting spin accumulation at high temperatures.
Reference: C. Liu, S. J. Patel, T. A. Peterson, C. C. Geppert, K. D. Christie, C. J. Palmstrøm, and P. A. Crowell, “Dynamic detection of electron spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance,” Nature Communications 7, 10296 (2016). http://dx.doi.org/10.1038/ncomms10296
Ultrafast terahertz spectroscopy can be used to probe charge and spin dynamics in semiconductors. We have studied THz emission from bulk InAs and GaAs and from GaAs/AlGaAs quantum wells as a function of magnetic field. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-Sapphire laser, and we recorded the THz emission spectrum and the integrated THz power as a function of magnetic field and temperature. In bulk samples the emitted radiation is produced by coupled cyclotron-plasma oscillations: we model THz emission from n-GaAs as magneto-plasma oscillations in a 3-D electron gas. THz emission from a modulation-doped parabolic quantum well is described in terms of coupled intersubband-cyclotron motion. A model including both 3-D plasma oscillations and a 2-D electron gas in a surface accumulation layer is required to describe THz emission from InAs in a magnetic field.
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