Session D53: Spin Transport by Electrons and Magnons

Harnessing magnetic switching and dynamics using electron and magnon spin currents

The manipulation, transmission, and detection of spin information require effective control of the magnetic orientation and dynamics in magnetic materials, which can be realized by the interactions between magnetic moments and spin current. In this talk, I will present our progress in using spin currents carried by conduction electrons and magnons to realize efficient magnetic switching and transmission of dynamical spin signals.

Using electron spin current, we firstly demonstrated current-induced magnetic switching via the strong spin-orbit torque from topological insulators at room temperature. We then studied the spin-orbit torque switching of a ferromagnetic Weyl semimetal, where the topological properties are manipulated by the magnetic switching. Beside electron spin current, spin angular momentum can also be transmitted by spin wave via magnon spin current. In the third work, we used magnetic domain walls to manipulate the phase and magnitude of a coherent spin wave, and in turn, investigated the domain wall motion driven by a spin wave. This mutual control can pave the way towards all-magnon computing devices. We then studied the nonreciprocal transmission of incoherent magnon currents in a coupled magnetic bilayer, which is useful for designing signal isolation devices. Finally, we extended the material system from ferromagnets to antiferromagnets. We demonstrated long-distance spin transport in antiferromagnetic insulators with easy-plane anisotropy, where the magnon eigenmodes are linearly polarized and propagate in a birefringence-like manner.

To conclude, multiple opportunities for magnetic switching and spin transport have been explored by the usage of electron and magnon spin currents, which represents an advancement towards energy-efficient and highly tunable memory and computing technologies.   

Withdrawn

The transport of spin information within magnetic insulators has become an area of intense interest, particularly due to new methods of injecting pure spin currents in insulators via the spin Hall effect, e.g. through nonlocal spin injection/detection geometries. However, so far these geometries have predominantly been extended to insulators with relatively simple ferromagnetic/antiferromagnetic order. Here, we use similar methods to inject spins into less conventional magnetic materials, including a material with prominent frustrated interactions as well as a multiferroic material. We hope these measurements will help in understanding how these measurement techniques extend beyond simple ferro- and antiferromagnetic systems.   

Spontaneous ferroelectric polarization tuned magnon transport in multiferroic BiFeO3

Magnon current induced spin torque, due to its minimized energy dissipation, has opened up a new arena for energy efficient magnetization switching, where spin-magnon current interconversion and pioneering work on magnon mediated spin torque switching of ferromagnet have been reported. Multiferroic material BiFeO₃, with a magneto-electric coupling between ferroelectric and antiferromagnetic orders, is a prefect candidate for studying magnon current assisted spin torque as well as spontaneous ferroelectric polarization dependent magnon transport. A study has been carried out on FM/BiFeO₃/SOC trilayer, showing that BiFeO₃ has a strong tunability on the spin current transmission efficiency across the BiFeO₃ mediator. An insertion of 1.5-unit cell of SrRuO₃, which has proven to be able to change the spontaneous polarization of BiFeO₃, could turn off the spin current transmission across BiFeO₃ via magnon by 30%.   

Pure-spin-current diode-like effect

Asymmetric charge transport at the contact of two materials with dissimilar electrical properties, such as metal/semiconductor and p-n junctions, is the fundamental feature behind modern diode and transistor technology. Motivated by this fact, we present a study of spin transport across the interface of two metals with very dissimilar spin-transport properties, Au and Pt. Spin transport is studied via spin pumping experiments in double magnetic layer Py/Au/Pt/Co structures. As the Py and Co layers have different ferromagnetic resonance fields, pure-spin-current can be generated by spin pumping from either Py or Co independently. The experiments reveal diode-like asymmetric pure-spin-current transport across the Au/Pt interface, where transmission from Au into Pt is more than twice as efficient as from Pt into Au. Our experimental results are well explained by extending conventional spin-pumping and spin-diffusion theory with boundary conditions for the spin chemical potential on either side of the Au/Pt interface.    

Nonreciprocal transmission of incoherent magnons with asymmetric diffusion length

Unequal transmissions of spin waves along opposite directions provide useful functions for signal processing. So far, the realization of such nonreciprocal spin waves has been mostly limited at gigahertz frequency, in the coherent regime via microwave excitation. Here we show that in a magnetic bilayer stack with chiral coupling, tunable nonreciprocal propagation can be realized in spin Hall effect-excited incoherent magnons, whose frequencies cover the spectrum from a few gigahertz up to terahertz. The sign of nonreciprocity is controlled by the magnetic orientations of the bilayer in a nonvolatile manner. The nonreciprocity is further verified by measurements on the magnon diffusion length, which is unequal along opposite transmission directions. Our findings enrich the knowledge on magnetic relaxation and diffusive transport, and can lead to a design of passive directional signal isolation device in the diffusive regime.   

Spin-Reflective Metallic Cu/Cr Interface

Electrically conductive metals are typically good transmitters or absorbers of pure spin currents. Here, we report evidence that an interface of two conductive elemental metals – namely, Cu and Cr– constitutes a good spin-current reflector. Using frequency-dependent and element-sensitive spin pumping methods, we have measured spin-current transport normal to NiFe/Cu/Cr(t_Cr)/(Co)Fe stacks with various Cr thicknesses t_Cr. With t_Cr just a few nm, our results demonstrate that both transmission and absorption of spin-current across Cu/Cr are suppressed – that is, the spin current is largely reflected by the Cu/Cr spacer. Complementary measurements show sizable spin transmission through a Cr spacer not interfaced with Cu, thus indicating the Cu/Cr interface – rather than the Cr bulk – to be the source of the strong spin reflection. Further, we find that this peculiar spin-reflective behavior does not originate from the antiferromagnetism of Cr. Our findings point to the possibility that a broad variety of metallic bilayers can form effective spin reflectors, thereby offering new avenues in the fundamental studies and engineering of pure spin currents in multilayers.   

Non-linear Hall effect due to asymmetric magnon scattering

The discovery of the unidirectional magnetoresistances (UMR) in magnetic multilayers has recently attracted experimental [1,2,3] and theoretical [4] interest. However, only the longitudinal contribution of these effects has been extensively studied, whereas little is known about its transverse contribution: the nonlinear Hall effect (NLHE). Its observation is for now limited to model systems: magnetic topological insulators [5] and ferromagnetic films with bulk inversion asymmetry [6]. The anomalous Nernst effect and the nonlinear contributions of the spin orbit torques to the Hall resistance make it difficult to evidence the NLHE without additional thermoelectric measurements and numerical simulations.

By using NiCu single layers with a large spin Hall angle [7] we observed UMR due to asymmetric magnon scattering in both the paramagnetic and ferromagnetic state. Close to the Curie temperature, where the magnon fluctuations are enhanced, we identify a NLHE signal with a field dependence similar to the UMR and different from other effects. This NLHE is attributed to a Rashba-like spin splitting at the NiCu interfaces in line with the observation of a large field like torque in the ferromagnetic state.   

Interplay of Hanle effect and superparamagnetism in Pt/hexagonal lutetium ferrite bilayers

Hanle magnetoresistance (MR), which is caused by dephasing of spin accumulation at boundaries due to spin precession and diffusion in a magnetic field, has been observed in heavy metal thin films with strong spin-orbit coupling, and provides a simple way to derive spin transport properties. By forming an interface between platinum (Pt) and hexagonal lutetium ferrite and measuring field-dependent and angle-dependent MR in Pt, we have observed pronounced longitudinal Hanle MR and corresponding transverse Hall effect at room temperature. Negative MR and anomalous Hall effect emerge and dominate at lower temperatures, which is reminiscent of the giant MR in granular magnetic solids. The results indicate that superparamagnetic clusters have formed at the interface and modified the spin transport properties of the Pt layer.   

Magnetic proximity-mediated Anomalous Hall effect in Pt/WS2/Fe3O4 heterostructures

When a two-dimensional (2D) transition metal dichalcogenide (TMD) is placed in proximity of a magnetic substrate, magnetism in the 2D TMD layer can be induced by the magnetic proximity effect. As a result, the charge and spin transport properties of this heterostructure can be altered by the induced magnetism. In this work, we have systematically investigated the magnetic proximity effect on the temperature dependence of transverse resistivity in Pt(5nm)/WS₂/Fe₃O₄(20nm)/Si heterostructures. Fe₃O₄ has been chosen as the magnetic substrate because its electrical conduction is drastically changed at the Verwey transition (~120 K), at which the system transforms from the high temperature metallic to low temperature insulating state. Chemical vapor deposition (CVD) grown WS₂ monolayers were first wet transferred on the top of the 20nm Fe₃O₄ film and Pt(5nm) was then deposited on the WS₂/Fe₃O₄(20nm)/Si heterostructure by magnetron sputtering. Anomalous Hall effect (AHE) has been observed in both Pt(5nm)/WS₂/Fe₃O₄(20nm)/Si and Pt(5nm)/Fe₃O₄(20nm)/Si heterostructures over a temperature range of 10-300 K. The background-corrected AHE voltage in Pt(5nm)/WS₂/Fe₃O₄(20nm)/Si exhibits a significant change around the Verwey transition, relative to Pt(5nm)/Fe₃O₄(20nm)/Si. The incorporation of the WS₂ monolayers is also found to give rise to the spin transport properties due to the interplay between the induced magnetism and electronic band structure close to the WS₂/Fe₃O₄ interface.   

Unidirectional anomalous Hall magnetoresistance

We predict a unidirectional anomalous Hall magnetoresistance (UAHMR) effect in bilayer systems composed of a ferromagnetic metal and a nonmagnetic metal, whereby the longitudinal resistance varies when the electric field direction is reversed or the magnetization is rotated. In contrast to the unidirectional spin Hall magnetoresistance (USHMR) [1], the UAHMR does not rely on the spin Hall effect in the nonmagnetic metal layer but instead arises from the anomalous Hall effect in the ferromagnetic layer. An analytical expression of the UAHMR is obtained by solving a set of drift-diffusion equations with proper boundary conditions, which allows us to analyze the dependence of the UAHMR on materials and geometric parameters of the bilayer system. We will also discuss how to distinguish the UAHMR from the USHMR experimentally when both effects are present as well as a new scheme to measure the spin/anomalous Hall angle based on the nonlinear transport effect.

[1] C. O. Avci, K. Garello, A. Ghosh, M. Gabureac, S. F. Alvarado, and P. Gambardella, Nat. Phys., 11, 570 (2015).   

Withdrawn

Heusler compounds have huge potential for applications in spintronics due to a high tunability of their electronic properties. Mn_1.4PtSn is a Heusler magnet with tetragonal, non-centrosymmetric crystal structure that hosts ferro- and antiferromagnetic, as well as strong Dzyaloshinskii-Moriya interactions. These are the key-ingredients for the presence of antiskyrmions (ASKs). Recently, ASKs were identified, even above room temperature, by Lorentz transmission microscopy (LTEM) in thin electron-transparent samples. Magnetic force microscopy revealed an almost linear dependence between the sample thickness and size of the spin textures. Owing to their topological nature, ASKs are expected to cause a topological Hall effect (THE). To date a direct detection of the THE from ASKs has been missing. We therefore investigated electrical transport in mesoscale structures fabricated by the application of focused ion beams (FIB) from high-quality single crystals of Mn_1.4PtSn. We combined magnetosensitive microscopy and electrical transport measurements. We were able to directly detect the THE as the ASK lattice emerges and demonstrate how it is effected by means of sample dimensions, temperature and field orientation.   

Coupling between electronic transport and magnetism in Mn2-xZnxSb

We successfully grew Mn_2-xZn_xSb () single crystals and investigated their electronic and magnetic properties. Despite of a similar crystal structure characterized by a Cu₂Sb type tetragonal lattice, varying Zn content leads to a systematic evolution of magnetism, which further couples with transport properties and causes a change from metallic to non-metallic electron transport near x = 0.6. Our discoveries establish a new highly tunable platform with strong coupling between magnetism and electron transport, which possibly enables opportunities for exotic phenomena and new spin-devices.   

Highly tunable spin Hall magnetoresistance in magnetoelectric multiferroic Z-type hexaferrite, Sr3Co2Fe24O41|Pt hybrids

We present spin transport studies on a low-field, room-temperature magnetoelectric multiferroic polycrystalline Sr₃Co₂Fe₂₄O₄₁|Pt heterostructure wherein, a transverse conical magnetic phase is responsible for static and dynamic magnetoelectric coupling. We measured angular dependence of spin Hall magnetoresistance (SMR) at various constant magnetic fields (H) in the range, 50 Oe to 100 kOe. Fields below a critical value of 2 kOe, yielded a negative SMR and SMR amplitude vs. H exhibited a negative gradient. Further, increase in the H resulted in the positive slope of SMR amplitude vs. H and near H ≈ 14 kOe, a crossover observed from negative to positive SMR. We employed a simple model to compute and understand SMR in Sr₃Co₂Fe₂₄O₄₁. We argue that the cone-tilting is dominant and in turn responsible for the observed nature of SMR below 2 kOe while, closing of the cone-angle is pronounced at higher H causing a reversal in sign of the SMR from negative to positive. Notably, SMR studies revealed that a change in the helicity with a reversal of H has no influence on the observed SMR. Our detailed spin transport studies on Sr₃Co₂Fe₂₄O₄₁ demonstrate high tunability of the amplitude and the sign of the SMR highlighting its potential for magnetoresistance-based spintronic devices.