In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Spin Hall effects

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator, we use either yttrium iron garnet, nickel ferrite, or magnetite and for the nonferromagnet, copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin-mixing conductance of 4×1014 ??1m?2, we obtain a spin Hall angle of 0.11±0.08 and a spin diffusion length of (1.5±0.5) nm for Pt in our thin-film samples

/pdf/quantitative-study-of-the-spin-hall-magnetoresistance-in-2ukri59g3o.pdf

Quantitative study of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids

The spin Seebeck effect refers to the generation of a spin voltage caused by a temperature gradient in a ferromagnet, which enables the thermal injection of spin currents from the ferromagnet into an attached nonmagnetic metal over a macroscopic scale of several millimeters. The inverse spin Hall effect converts the injected spin current into a transverse charge voltage, thereby producing electromotive force as in the conventional charge Seebeck device. Recent theoretical and experimental efforts have shown that the magnon and phonon degrees of freedom play crucial roles in the spin Seebeck effect. In this article, we present the theoretical basis for understanding the spin Seebeck effect and briefly discuss other thermal spin effects.

Theory of the Spin Seebeck Effect

A unidirectional magnetoresistance observed in bilayer metal films could be used to add directional sensitivity to conventional magnetic sensors based on anisotropic magnetoresistance.

/pdf/unidirectional-spin-hall-magnetoresistance-in-ferromagnet-3rlyshgifv.pdf

Unidirectional spin Hall magnetoresistance in ferromagnet/normal metal bilayers

Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find a nearly tenfold increase of SMR in $\mathrm{W}/\mathrm{CoFeB}$ compared to previously studied HM/ferromagnetic insulator systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the $\mathrm{HM}/\mathrm{FM}$ interface.

Spin Hall Magnetoresistance in Metallic Bilayers

A scannable laser beam is used to generate local thermal gradients in metallic (Co2FeAl) or insulating (Y3Fe5O12) ferromagnetic thin films. We study the resulting local charge and spin currents that arise due to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), respectively. In the local ANE experiments, we detect the voltage in the Co2FeAl thin film plane as a function of the laser-spot position and external magnetic field magnitude and orientation. The local SSE effect is detected in a similar fashion by exploiting the inverse spin Hall effect in a Pt layer deposited on top of the Y3Fe5O12. Our findings establish local thermal spin and charge current generation as well as spin caloritronic domain imaging.

pdf/local-charge-and-spin-currents-in-magnetothermal-landscapes-2gemxk39s7.pdf

Local charge and spin currents in magnetothermal landscapes.

F. Fesquet,1, 2, ∗ F. Kronowetter,1, 2, 3 M. Renger,1, 2 Q. Chen,1, 2 K. Honasoge,1, 2 O. Gargiulo,1, 2 Y. Nojiri,1, 2 A. Marx,1 F. Deppe,1, 2, 4 R. Gross,1, 2, 4 and K. G. Fedorov1, 2, † Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany Physik-Department, Technische Universität München, 85748 Garching, Germany Rohde & Schwarz GmbH & Co. KG, Mühldorfstraße 15, 81671 Munich, Germany Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany

pdf/perspectives-of-microwave-quantum-key-distribution-in-open-1gocjxca.pdf

Perspectives of microwave quantum key distribution in open-air


 Lithium niobate (LNO) is a well established material for surface acoustic wave (SAW) devices including resonators, delay lines and filters. Recently, multi-layer substrates based on LNO thin films have become commercially available. Here, we present a systematic low-temperature study of the performance of SAW devices fabricated on LNO-on-insulator and LNO-on-Silicon substrates and compare them to bulk LNO devices. Our study aims at assessing the performance of these substrates for quantum acoustics, i.e. the integration with superconducting circuits operating in the quantum regime. To this end, we design SAW resonators with a target frequency of 
 
 
 
 5
 
  
 
 GHz
 
 
 
 and perform experiments at millikelvin temperatures and microwave power levels corresponding to single photons or phonons. The devices are investigated regarding their internal quality factors as a function of the excitation power and temperature, which allows us to characterize and quantify losses and identify the dominating loss mechanism. For the measured devices, fitting the experimental data shows that the quality factors are limited by the coupling of the resonator to a bath of two-level-systems. Our results suggest that SAW devices on thin film LNO on silicon have comparable performance to devices on bulk LNO and are viable for use in SAW-based quantum acoustic devices.

Surface acoustic wave resonators on thin film piezoelectric substrates in the quantum regime

Temperature-Dependent Spin Transport and Current-Induced Torques in Superconductor-Ferromagnet Heterostructures.

We investigate the coupled magnetization dynamics in heterostructures of a single crystal of the chiral magnet [Formula: see text] (CSO) and a polycrystalline ferromagnet [Formula: see text] (Py) thin film using broadband ferromagnetic resonance (FMR) at cryogenic temperatures. We observe the excitation of a hybrid mode (HM) below the helimagnetic transition temperature of CSO. This HM is attributed to the spin dynamics at the CSO/Py interface. We study the HM by measuring its resonance frequencies for in plane rotations of the external magnetic field. We find that the HM exhibits dominantly fourfold anisotropy in contrast to the FMR of CSO and Py.

Hybrid magnetization dynamics in Cu2OSeO3/NiFe heterostructures

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