The field of spintronics is based on the manipulation of the spin degree of freedom. It uses the carrier spin angular momentum as a basic functional unit in addition to the charge. The first requirement of a semiconductor-based spintronic technology is the efficient generation of spin-polarized carriers into the device heterostructure made of Si or Ge (the materials of mainstream microelectronics) at room temperature. In this presentation, we focus on the generation of a sizeable spin population into Ge by spin pumping. Spin pumping corresponds to the generation of a pure spin current in the Ge film by exciting the ferromagnetic resonance of an adjacent ferromagnetic electrode with microwaves. The pure spin current is then detected using spin-orbit based effects. Our aim is to understand the basic mechanisms of spin pumping into Ge as well as the spin-to-charge conversion by inverse spin Hall effect (ISHE, bulk effect) [1-4] and Rashba-Edelstein effect (interface effect) . The influence of interface states is clearly demonstrated. Moreover, using the spin-split Rashba sub-surface states of the Ge(111) surface, we succeeded in demonstrating a giant conversion of a spin current generated by spin pumping into a charge current by the Rashba-Edelstein effect [6,7]. Our experimental findings are supported by ab-initio calculations.
1. Rojas-Sánchez, J.-C. et al. Spin pumping and inverse spin Hall effect in germanium. Phys. Rev. B 88, (2013).
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3. Valenzuela, S. O. and Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature 442, 176–179 (2006).
4. Saitoh, E., Ueda, M., Miyajima, H. and Tatara, G. Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Appl Phys Lett 88, 2509 (2006).
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7. Rojas-Sánchez, J.-C. et al. Spin-to-charge conversion using Rashba coupling at the interface between non-magnetic materials. Nat Comms 4, (2013).
Spintronics evolves along new paths involving non-magnetic materials having large spin-obit coupling, typically 5d metals, allowing for example large spin-to-charge current conversion (spin Hall and Rashba-Edelstein effects). These heavy metals have other effects: in proximity of magnetic thin films they can burst out the Dzyaloshinskii-Moriya interaction leading to the stabilization of chiral magnetic structures. Another source of recent interest relies on “non-trivial topologies”, either of the band structure of the topological insulators, or of the spin textures in magnetic thin films.
We will discuss our recent progress to control the topological textures known as skyrmions in multilayers made of heavy metals and magnetic layers. Aiming at using skyrmions as magnetic bits in “racetrack memory” structures, one of the present challenges is to efficiently move skyrmions with dimensions of a few tens of nanometers. The topology of these magnetic structures imposes peculiar dynamics, interesting both in fundamental and applied perspectives. Simulations indicate that spin-orbit torques, through the absorption of the spin current generated by a nearby layer, should be the most efficient method. The conducting surfaces of topological insulators at which the carriers’ spin and momentum are locked, can display better spin-to-charge conversion than what is found using heavy metals. However, the control of the interfaces is crucial to conserve the Dirac cone and the associated spin-momentum locking. We demonstrate by ARPES and spin pumping experiments how the properties of the α-Sn thin film topological insulator are preserved and can be used for spintronics, maybe to move skyrmions!
The antiferromagnetic order is expected to have a high potential in next-generation spintronic applications. It is resistant to perturbation by magnetic fields, produces no stray fields, displays ultrafast dynamics and may generate large magneto-transport effects. In spintronic materials, spin currents are key to unravelling spin dependent transport phenomena. Here, spin pumping results from the non-equilibrium magnetization dynamics of a ferromagnetic spin injector, which pumps a spin current into an adjacent spin sink. This spin sink absorbs the current to an extent which depends on its spin-dependent properties. The properties of the spin sink can be recorded either through the changes induced in ferromagnetic damping or through direct electrical means, such as by measuring the inverse spin Hall voltage. In this talk, we will deal with the injection of a spin current in thin antiferromagnetic sinks. Measurements of the spin penetration depths and absorption mechanisms were obtained for polycrystalline Ir20Mn80 and Fe50Mn50 films (Appl. Phys. Lett. 104, 032406 (2014)). More interestingly, spins propagate more efficiently in layers where the magnetic order is fluctuating rather than static. The experimental data were compared to some of the recently developed theories and converted into interfacial spin mixing conductance enhancements. These findings help us progress towards the development of more efficient spin sources, while also providing an alternative method to probe magnetic phase transitions (Phys. Rev. Lett. in press (2016)). This type of alternative method is particularly needed to deal with the case of thin materials with no net magnetic moments, such as thin antiferromagnets.