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Spin-related magnetoresistance (MR) from versatile condensed matter systems can be employed as the main mechanism to detect the orientation of magnetization in next generation magnetoresistive random-access memories (MRAM). Recently, novel MR effects such as spin Hall MR (SMR) [1,2] and spin-dependent unidirectional MR (SD-UMR) [3,4] are extensively studied due to the correlation between MR and spin-orbit interactions in both classical and emergent materials systems.&lt;br/&gt;In our work, we disentangle the underlying mechanisms of the UMR in W/CoFeB magnetic heterostructures by systematic current-dependent and field-dependent MR measurements at room temperature. As shown in Fig.1, the current-dependent UMR of W/CoFeB heterostructures can be roughly divided into three regimes. In the first and second regimes, UMR increases linearly and then nonlinearly to the applied current density &lt;i&gt;J&lt;/i&gt; as &lt;i&gt;J&lt;/i&gt; reaches ~5×10&lt;sup&gt;10 &lt;/sup&gt; A/m&lt;sup&gt;2&lt;/sup&gt;, which originates from the SD-UMR [Fig.2(a)] and spin-flip UMR (SF-UMR) [Fig.2 (b)] [4]. In the third regime, as &lt;i&gt;J&lt;/i&gt; reaches ~ 10&lt;sup&gt;11 &lt;/sup&gt; A/m&lt;sup&gt;2&lt;/sup&gt;, the UMR ratio will further saturate at 0.36%, which is larger than the typical UMR magnitudes reported previously in metallic system. With the aid of numerical simulations, this large UMR can be attributed to the extra contribution from SMR, which stems from the tilting of magnetization M caused by the competition between the spin Hall effect induced spin torques (STs) from W layer and the external field &lt;i&gt;H&lt;sub&gt;y&lt;/sub&gt;&lt;/i&gt;. This tilting behavior is illustratively shown in Fig.2 (c). Consequently, we define this UMR as the spin-transfer torque UMR (STT-UMR). The damping like-ST (DL-ST) efficiency of W/CoFeB heterostructures can be also roughly estimated by using both the experimental UMR data and the simulation results. Our finding therefore verifies the correlation between the STs and this large room-temperature UMR, thereby providing an alternative approach to determine DL-ST efficiency.

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Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

2022 Joint MMM-INTERMAG

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

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Spin-related magnetoresistance (MR) from versatile condensed matter systems can be employed as the main mechanism to detect the orientation of magnetization in next generation magnetoresistive random-access memories (MRAM). Recently, novel MR effects such as spin Hall MR (SMR) [1,2] and spin-dependent unidirectional MR (SD-UMR) [3,4] are extensively studied due to the correlation between MR and spin-orbit interactions in both classical and emergent materials systems. In our work, we disentangle the underlying mechanisms of the UMR in W/CoFeB magnetic heterostructures by systematic current-dependent and field-dependent MR measurements at room temperature. As shown in Fig.1, the current-dependent UMR of W/CoFeB heterostructures can be roughly divided into three regimes. In the first and second regimes, UMR increases linearly and then nonlinearly to the applied current density J as J reaches ~5×1010 A/m2, which originates from the SD-UMR [Fig.2(a)] and spin-flip UMR (SF-UMR) [Fig.2 (b)] [4]. In the third regime, as J reaches ~ 1011 A/m2, the UMR ratio will further saturate at 0.36%, which is larger than the typical UMR magnitudes reported previously in metallic system. With the aid of numerical simulations, this large UMR can be attributed to the extra contribution from SMR, which stems from the tilting of magnetization M caused by the competition between the spin Hall effect induced spin torques (STs) from W layer and the external field Hy. This tilting behavior is illustratively shown in Fig.2 (c). Consequently, we define this UMR as the spin-transfer torque UMR (STT-UMR). The damping like-ST (DL-ST) efficiency of W/CoFeB heterostructures can be also roughly estimated by using both the experimental UMR data and the simulation results. Our finding therefore verifies the correlation between the STs and this large room-temperature UMR, thereby providing an alternative approach to determine DL-ST efficiency.

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Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

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The seach for ferromagnetic insulating materials with multifunctional properties is currently a highly sought after objective in spintronics. In current spintronics, functional devices are typically made of a bilayer composed of a material with large spin-orbit coupling (NM) and a ferromagnet (FM). These type of devices allow functionalities like the manipulation of the FM magnetization by the spin Hall effect (SHE)1 in the NM or energy harvesting by means of its inverse counterpart the inverse spin Hall effect.2 Insulating ferrimagnets are preferred for this purpose to pave the way towards low dissipation spintronics devices.3 Additional functionalities like the possibility of the electric field control of the magnetic properties of such systems could be given to these heterostructures through the introduction of multifunctional ferromagnets opening the possibility of having more efficient and versatile devices.4 In this work we have studied the thermo-spin current generation in bilayers composed of Pt and the multifunctional magnetoelectric Ga0.6Fe1.4O3 (GFO).5 We compare the performance of the system with the widely used yttrium iron garnet obtaining a similar value of the spin Seebeck effect, likewise to what was observed previously in spin Hall magnetoresistance. We also rule out the possibility of a dominant proximity effect in the thermo-spin voltage by exploring the different magnetoresistive effects in the bilayer. In addition, by fabrication of thermo-spin devices with controlled dimensions we are able to accurately quantify the relevant parameters of the thermal effects, obtaining more accurate and comparable values for the spin Seebeck coefficient. These results pave the way for the use of the magnetoelectric multiferroic GFO with a view to control the spin current production of NM/FM heterostructures by an electric field.

![](https://assets.underline.io/uploads/markdown_image/1/image/13d9b9f0af0c5a8976a8a2c5fc7f452e.jpg)

Fig. 1. Thermo-spin current generation in YIG and GFO. The thermo-spin voltage is comparable in both systems.

Thermal spin current generation in the multifunctional ferrimagnet Ga0.6Fe1.4O3

Charge-spin interconversion using large Spin Orbit (SO) material has been intensively studies thanks to potential spintronic application in information and communication technology. It has been demonstrated that the interface between heavy metal and both metallic and insulating ferromagnetic layers could lead to spin current absorption and manipulation of the magnetization driven by electrical pulse[1][2]. More recently Rashba surface states and Topological insulator (TI) have been reported like a good candidate for the spin to charge interconversion[3]. Nevertheless, current shunting by metallic ferromagnet at the interface with TI makes hard to address unambiguously the charge to spin conversion from surface states. As a supplementary effect, the role of proximity effect and exchange interaction in between ferromagnetic material and TI needs to be clarified. To comprehend this issue, we first grow a thin film of magnetic insulator Tm3Fe5O12(TmIG) by off-axis sputtering and demonstrated the high quality of the film in the thickness range of 5 to 15 nm (Figure. 1(a)). We achieved to demonstrate, as a reference experiment, spin current absorption at the interface in between Pt and TmIG by 2nd harmonic experiment to extract torque and magnetization reversal driven by pulse current experiment. The high quality of the film makes possible to growth by MBE Bi2Se3 TI on top of it as illustrated in Figure 1(b). The TmIG films exhibit perpendicular magnetic anisotropy (Figure 1 (c)) with nearly bulk magnetization. We will address a comparative study of the interfacial effect when using SHE material (Pt) or TI (Bi2Se3) with ferromagnetic insulator TmIG. This work has been supported by the Defense Advanced Research ProjectsAgency (DARPA) Topological Excitations in Electronics (TEE) program, through Grant MIPR No. HR0011831554.

![](https://assets.underline.io/uploads/markdown_image/1/image/d2384c7e071cf59f2e36c9d864aa2f2a.jpg)

Figure 1 (a) X-Ray (θ-2θ) diffraction patterns recorded on TmIG(5-15nm) thin films grown on GGG substrate. (b) RHEED patterns recorded before (on TmIG surface) and after Bi2Se3 growth. (c) Magnetization hysteresis recorded in out-of-plane field on TmIG(15nm) sample.

Towards the development of Tm3Fe5O12 magnetic insulator and Bi2Se3 topological insulator for charge-spin interconversion

The combination of spin-driven thermoelectric (STE) devices based on spin Seebeck effect (SSE), and radioactive isotopes as heat sources, has potential as a next-generation method of power generation in applications such as power supplies for space probes. However, there has been very limited knowledge available [1] indicating the irradiation tolerance of spin thermoelectric devices. Through analysis using a heavy ion-beam accelerator and the hard X-ray photoemission spectroscopy (HAXPES) measurements, we show that a prototypical STE device based on Y3Fe5O12/Pt heterostructures has tolerance to irradiation of high-energy heavy-ion beams [2]. We used 320 MeV gold ion beams modeling cumulative damages due to fission products emitted from the surface of spent nuclear fuels. By varying the dose level, we confirmed that the thermoelectric and magnetic properties of the SSE elements are not affected by the ion-irradiation dose up to 1010 ions/cm2 fluence and that the SSE signal is extinguished around 1012 ions/cm2, in which the ion tracks almost fully cover the sample surface. In addition, the HAXPES measurements were performed to understand the effects at the interface of Y3Fe5O12/Pt. The HAXPES measurements suggest that the chemical reaction that diminishes the SSE signals [3] is enhanced with the increase of the irradiation dose. We share the current understandings of the damage analysis in Y3Fe5O12/Pt for developing better STE devices applicable to harsh environmental usages. This work was supported by MEXT Innovative Nuclear Research and Development Program Grant Number JPMXD0212345678.

The damage analysis for irradiation tolerant spin-driven thermoelectric device based on Y3Fe5O12/Pt heterostructures

Recent discovery of “unexpected” spin-orbit torques (SOTs) within magnetic single layers has attracted attention. However, it has remained elusive as to how to understand and how to tune the SOTs. In this talk, we will first show a strong bulk dampinglike SOT and its characteristics in composition-uniform ferromagnetic CoPt that is polycrystalline and chemically disordered. We find that the bulk dampinglike SOT arises from the interplay of a strong bulk spin Hall effect of CoPt (an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation) with a “hidden” inversion symmetry-breaking (not a gradient in composition, magnetism, electron scattering, or grain shapes). In the second part, utilizing the single layers of chemically disordered FexPt1-x, we unveil the mechanism of the bulk SOTs by studying their dependence on the introduction of a controlled vertical composition gradient and on temperature. We find that the associated “hidden” inversion symmetry-breaking that allows for a non-zero net torque is most likely a strain non-uniformity that can arise from a composition gradient and/or the substrate. The findings point to the possibility of developing low-power single-layer SOT devices by strain engineering.

Bulk spin-orbit torques in centrosymmetric magnetic single layers

The spin Hall effect, and its inverse, are important spin-charge conversion mechanisms widely investigated due to their fundamental importance in the development of spintronics devices. Its measurement has been mostly related to electrical detection schemes involving an interface with another magnetic material and thus, a combination of the properties of both materials as well as the interface are measured. The large scattering of reported results from these methods for the same material and temperature call out for a more direct and interface-free approach. Optical detection schemes have been successfully used for the determination of SHE in semiconductor. However, this approach has proven challenging for metallic systems, due to their considerably shorter spin diffusion lengths. Only recently, optical measurements for Pt and W have been reported [1]. Considering that magnetic circular dichroism (XMCD) has become a reference tool for precision measurements of small or diluted magnetic signals, We propose the use of XMCD-PEEM microscopy for direct, interface-free determination of SHE in metals. In particular, we report the observation of spin separation due to SHE in a single layer of Bi-doped Cu (Cu95Bi5), a material in which giant SHE has been already reported [2,3]. We have performed interface free x-ray spectroscopy measurements at the Cu L3,2 absorption edges while applying electrical current to the sample. The sign of spin accumulation depends on the direction of the current and the amplitude of the X-ray magnetic circular dichroism (XMCD) signal scales with the density current and have different sign when measuring in the L2 or L3 absorption edges, as expected for SHE. We have measured an induced magnetic moment of (2.7 ± 0.5) x 10-12μB A-1 cm2 per Cu atom averaged over the probing depth, which is of the same order as for Pt measured by magneto-optics. Our results constitute the proof of concept for the direct, interface free and element-selective measurement of the SHE in a single material by means of X-ray spectromicroscopy and highlight the potential of CuBi for spin-charge conversion applications [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/0f5bce09fb325fa1a3fcfbf281b8cd98.jpg)

(a) Visualization of the spin accumulation in a Cu95Bi5 electrode. (b) XMCD signal.

Direct X-ray detection of the spin Hall effect in CuBi

Precise control of magnetic dynamics can enable many novel computing applications, including magnetic random access memory and spin torque oscillator-based neuromorphic computing. Spin torques stand out as an excellent tool for efficiently manipulating magnetic states, and can be easily produced via spin-orbit effects in a spin source layer. Unfortunately, underlying crystal symmetries in the spin source material often restrict the geometry of the spin torques, making them poorly suited to practical applications. Magnetic ordering can break the symmetries of a spin source material and allows the generation of versatile spin torques with controllable geometries tailored to applications. Antiferromagnetic materials in particular have robust magnetic ordering that makes them ideal as spin sources. We present spin torque ferromagnetic resonance measurements (Fig. 1) and second harmonic Hall measurements characterizing the geometry and efficiency of spin torques generated in antiferromagnetic iron rhodium alloy. We find highly temperature-dependent spin torques with spin torque efficiencies above 90% at room temperature and above 300% at 170K, much higher than most competing materials. Further, these torques do not show the angular dependence expected for ordinary spin Hall torques. Instead, it appears that the torque derives from a spin current with spin polarization parallel to the FeRh magnetic order, which makes these torques useful for exciting dynamics in a variety of samples. This makes iron rhodium a compelling option for the development of spin torque technology. This work was supported as part of Quantum Materials for Energy Efficient Neuromorphic Computing, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science.

![](https://assets.underline.io/uploads/markdown_image/1/image/2acaeb111f16139961cbc9f870795133.jpg)

Fig. 1 DC Biased Spin Torque Ferromagnetic Resonance. (a) Diagram of the measurement setup. (b) Characteristic resonance scans. (c) Linewidth over a range of DC currents. Increasing current increases the spin current, modifying the effective damping linearly. Shown at 300K, 275K, 260K, and 170K. (d) Spin torque efficiency is huge and grows by a factor of 3.5 between 300K and 170K.

Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

While classical electronics focuses on the control of the charge current, spintronics aims at using a spin current, e.g. a flow of angular momentum for logical and memory applications. One of the main use of such a spin current is magnetization switching used for writing in memory devices. Indeed, when a spin current enters into a ferromagnet, its angular momentum (or polarization) is transmitted to the magnetization, and possibly changes its orientation. The transfer of angular momentum from the spin current to the magnetization can be done in two ways depending on the relative orientation between the polarization and the magnetization direction. If they are collinear, the polarization is absorbed by the ferromagnet through diffusive processes originating from the spin switchings during scattering on defects. However, when the magnetization and polarization are transverse, ballistic processes originating from the conduction electron’s spin rotation around the exchange field of the ferromagnet drive the polarization absorption. Understanding the mechanisms of the spin current relaxation into a ferromagnet is therefore of crucial interest for spintronics applications, and remain a seldom explored fundamental question. One of the main difficulty in studying spin current relaxation in a ferromagnetic material is the fact that a charge current generates parasitic effects through the anisotropic magnetoresistance and planar Hall effect. We consequently used lateral spin valves, allowing the measurement of the spin relaxation in a ferromagnet with different relative magnetization direction using a pure spin current, e.g. a flow of angular momentum in the absence of charge current. This measurement allowed us to observe the spin-current relaxation anisotropy in a ferromagnet and to extract the spin mixing conductance, an elusive but fundamental parameter of the spin-dependent transport [1]. Finally, these results allows us to move toward the study of magnetization dependent Spin Hall Effect in ferromagnetic materials.

Spin absorption anisotropy in lateral spin valves

Materials with helical crystal structures have been suggested to be a condensed matter analogue of solenoids, where a current flow produces inductive magnetic fields [1]. At the microscopic scale, a finite spin accumulation can be generated in strong spin-orbit coupling and low symmetry systems via the Edelstein effect [2,3]. Recently, chiral non-magnetic materials have received a lot of attention due to their low symmetries and complex radial spin textures, that could be potentially exploited for spintronic device applications [4,5,6]. However, a straightforward magnetotransport study on the relationship between chirality, spin texture and current-induced spin polarization is still missing. Here, we link one-to-one the direction of the spin polarization, induced by a chirality-dependent Edelstein effect, to the handedness of chiral tellurium (Te) nanowires (NWs), through simple angle-dependent magnetotransport experiments. Transmission electron microscopy (TEM) analysis and theoretical calculations corroborate the transport data. Te NWs show a large, chirality-dependent non-reciprocal charge transport when the electrical current and the magnetic field applied are collinear to the chiral axis of the crystal. By applying a gate voltage, we can also manipulate the magnetotransport response, acting on the carrier density and modifying the spin texture at the Fermi surface. The efficiency of the charge-to-spin interconversion process is one order of magnitude larger than what has been reported so far. Our results are a step forward towards the development of chiral spintronics.

![](https://assets.underline.io/uploads/markdown_image/1/image/cb46e659c3ceca2ac8d3e113ec8dbb71.jpg)

Figure 1: Sketch of the chiral structure of right- and left-handed Te NWs where the current-induced spin polarization depends on the crystal handedness

![](https://assets.underline.io/uploads/markdown_image/1/image/6de41c2cd5dcdb4672e53a8c0641cd6e.jpg)

Figure 2: On the leftt, sketch representing the Edelstein effect mechanism. The shift in kz of the Fermi contours lead to the formation of spin accumulations oriented in opposite directions according to the chirality-dependent radial spin texture of Te. On the right, current-dependent angular dependences of the magnetoresistance Rdiff= [R(Iz) - R(-Iz)]/2 measured at 9 T and 10 K for two Te NWs with opposites handedness. The signals show specular features between left- and right-handed NWs.




Chirality-dependent Edelstein effect in elemental Tellurium nanowires

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. Our study was initially motivated by the question of whether the elemental antiferromagnet Cr acts as a spin transmitter (akin to electrically insulating NiO [1]) or a spin absorber (akin to metallic IrMn [2]). To this end, we have performed ferromagnetic resonance (FMR) spin pumping measurements on spin-valve-like stacks of NiFe(10)/Cu(5)/Cr(tCr)/(Co)Fe(4), where the FMR-driven NiFe layer pumps an AC pure spin current. Any spin current transmitted through the Cu/Cr spacer would be absorbed by the (Co)Fe sink, generating additional Gilbert damping, △α, for the NiFe source. Thus, △α is proportional to how much a spin current is transmitted or absorbed across the Cr interlayer. In Figure 1, the sharp decline of △α with increasing Cr thickness indicates a remarkable trend: The introduction of Cr suppresses spin transmission and absorption – i.e., the spin current is largely reflected by the Cu/Cr spacer. However, our complementary element-selective X-ray FMR measurements confirm that the spin current is transmitted through the Cr spacer in NiFe/Cr/CoFe. We therefore identify the Cu/Cr interface, rather than the Cr bulk, as the source of the strong spin reflection. Figure 2 shows that △α varies only weakly with decreasing temperature (increasing antiferromagnetic order in Cr), which suggests that antiferromagnetism in Cr is not responsible for the spin-reflective behavior. Our finding points to the possibility that other electrically conductive metals can form effective spin reflectors, thereby offering new avenues in the fundamental studies and engineering of pure spin currents across interfaces.


![](https://assets.underline.io/uploads/markdown_image/1/image/e7884bfa3e9da4b8fbfbd64553b2d860.jpg)


[Fig. 1] △α vs. Cr layer thickness tCr in NiFe/Cu/Cr(tCr)/(Co)Fe measured at room temperature. Both the epitaxial and polycrystalline series show decaying with tCr, due to spin reflection at the Cu/Cr interface.

![](https://assets.underline.io/uploads/markdown_image/1/image/dc5e03a64fe9cbcdaa9a866583d3cb89.jpg)


[Fig. 2] △α vs. temperature, showing that spin transport is affected only weakly even as Cr develops stronger antiferromagnetic order at lower temperatures.

Spin-Reflective Metallic Cu/Cr Interface

Asymmetric charge transport at the interface of two materials with dissimilar electrical properties, such as metal/semiconductor and p-n junctions, is the fundamental aspect behind modern diode and transistor technology. Therefore, it is interesting to study pure-spin-current transport at the interface of two materials with different spin transport properties. Here we demonstrate asymmetric transport of pure-spin-currents across an interface of dissimilar non-magnetic materials, Au/Pt. In this work we present a study of spin transport by spin-pumping from either Py (Ni80Fe20) or Co in Py/Au/Pt/Co and Py/Au/Pt structures. The thickness of Pt was varied from 0 to 3 nm and the magnetic damping was measured by ferromagnetic resonance. The magnetic damping due to spin-pumping of Py in Py/Au/Pt follows the characteristic thickness dependence of ferromagnet/normal-metal. The typical ferromagnet/normal-metal/spin-sink thickness dependence is reproduced by spin-pumping from Py in the Py/Au/Pt/Co structure. Here the Co layer acts as a spin-sink. However, pumping from Co while Py is the spin-sink in the Py/Au/Pt/Co structure is very uncharacteristic of a spin-sink structure. Instead, this data suggests that very little spin-current makes it to the Py layer, i.e. a reflection of spin-current at the Au/Pt interface. The data could not be self consistently fit with the continuity of spin-accumulation and continuity of spin-current boundary conditions at the Au/Pt interface [1,2]. Including spin-memory-loss also failed to fit the data [3]. We proposed a new boundary condition that assumes that the reflected and transmitted spin-currents at the Au/Pt interface are proportional to the established spin chemical potentials on either side of the interface. This model can self consistently fit all the measured data. It suggests that spin-pumping from Py results in a ~30% reflection and ~70 transmissions of the spin-current across the Au/Pt interface. However, spin pumping from Co results in ~70% reflection and 30% transmission of spin-current at the Au/Pt interface.

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