United States

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 Fe&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Pt&lt;sub&gt;1-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;, 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.

2022 Joint MMM-INTERMAG

Bulk spin-orbit torques in centrosymmetric magnetic single layers

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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.

technical paper

Pure spin currents produced by the spin Hall effect(SHE) in a nonmagnetic heavy metal has the potential to drive ultra-fast, energy-efficient spintronic devices such as spin Hall nano-oscillators(SHNOs)[1-3], spin-orbit torque based magnetic random access memory(SOT-MRAM), and spin logic devices[4,5]. One key challenge in such emerging devices remains to reduce their high current densities(Jc) and energy consumption to drive magnetization switching and auto oscillations in the adjacent ferromagnets. Efforts have been made to increase the SOT efficiency(ζSOT) by choosing suitable material combinations and/or use oxygen incorporation[6]. However, the increased ζSOT is observed at the expense of an equivalent increase in the longitudinal resistivity(ρ), which essentially cancels out any reduction in the power consumption. A more successful direction is instead to alloy elements, which are in themselves already good SOT material[7]. Here, we demonstrate such alloying of W with Ta to obtain a simultaneous increase of both the ζSOT and the spin Hall conductivity(σSH), which in turn substantially reduces the Jc in W100-xTax(5nm)/CoFeB(tCFB=1.4 to 2 nm)/MgO(2nm) SHNOs. First, we performed STFMR measurements on microbars (6 ×18 µm2, 6 ×12 µm2) to determine their magnetodynamical properties such as current induced ζSOT and σSH. The ζSOT varies from maximum ~ -0.57(x=10%) to ~-0.18(x = 50%) as compared to pure β-W, ~-0.36(x=0). Thanks to a reduced resistivity of W100-xTax, the increased ζSOT is accompanied with a significant increase of the σSH of about ~140% as compared to pure β-W. Finally, we investigated the alloying effect on the auto-oscillation threshold current(Ith) densities by fabricating SHNOs of different widths varying from 10nm to 150nm on a 4 × 14 µm2 mesa[1] and observed about 40% reduction in Ith (at x=12%) as compared to pure β-W measured. The trade-off between ζSOT, ρ, and σSH demonstrates the promising aspects of a W-Ta alloying approach for energy-efficient operation of emerging spintronic devices

![](https://assets.underline.io/uploads/markdown_image/1/image/5e590e6a057e20864ec79b7e0c117dad.jpg)

Fig.1: (a) Variation of ρ(blue) and σSH(red) vs. W100-xTax alloy composition. (b-c) Power spectral densities showing a 40% reduction in the SHNO Ith for two SHNOs with identical constriction width of 50nm.

Alloying effect on auto-oscillation properties of W100-xTax/CoFeB/MgO spin Hall nano-oscillators

Rare earth-transition metal (RETM)-based ferrimagnets are known to exhibit strong bulk perpendicular magnetic anisotropy and bulk Dzyaloshinskii-Moriya interaction [1]. These properties make RETM ferrimagnets a model system to study exciting magnetic phenomena such as magnetic skyrmion [2,3], chiral domain wall [3], all-optical switching [4], spin-orbit torque switching [5] and canted magnetic moments. Here, we demonstrate a field-free spin-orbit torque-driven multistate switching of a nearly compensated GdCo ferrimagnet sandwiched between Ta and Pt. We show that our GdCo film exhibits a strong canting of magnetic moments, and the canted moments can be deterministically switched among three stable states using a current-driven spin-orbit torque, even in the absence of a magnetic field. We further show that the three states persist in the presence of an in-plane magnetic field and the levels of the three states can be tuned with the magnitude of the magnetic field. We consider various possible origins of the canting, including concentration gradient, Dzyaloshinskii - Moriya interaction, and balanced pair-correlations of unlike atoms versus like atoms in the growth direction, and discuss various methods to differentiate among these mechanisms. Furthermore, we discuss the possibility of domain formations to explain the observed multistate. Our results provide a new perspective to study the competing magnetic interaction in RETM-based ferrimagnetic alloys at the near-compensation point, and enable a new pathway to achieve multistate spin-orbit torque devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/df0a749c720a6befff1af56d0b3f65bb.jpg)

Multistate switching of canted GdCo moments via spin-orbit torque with zero in-plane symmetry breaking field.


Field-free Spin-orbit Torque-driven Multistate Switching of Canted GdCo Moments

BiSb topological insulator is a promising candidate for spin-orbit-torque (SOT) magnetoresistive random-access memory (MRAM). A giant spin Hall angle θSH up to 52 and 10.7 has been observed for BiSb deposited by MBE on GaAs(111) [1] and by sputtering on c-plane Al2O3 [2] substrates, respectively. In realistic MRAM, however, magnetic tunnel junctions whose core structure includes an MgO barrier layer and a free ferromagnetic layer on top have to be deposited on Si/SiOx substrates. In this work, we demonstrate the large θSH = 2.4 of BiSb in all-sputtered BiSb/CoPt/MgO multilayers on Si/SiOx substrates, and investigate the SOT magnetization switching behaviors. Low power and fast switching is attained, demonstrating the robustness and reliability of BiSb as a spin current source. We deposited BiSb(10)/Pt(0.8)/Co(0.6)/Pt(0.8)/MgO(10) (Fig. 1(a)), from top to bottom on Si/SiOx substrates at room temperature (thickness in nm). The Pt/Co/Pt trilayers are perpendicularly magnetized with a large anisotropy field of 4.5 kOe (Fig. 1 (b)(c)). Fig. 1(d) shows 2nd harmonic Hall resistance R2ω - field Hx characteristics at different current density JBiSb, and a representative fitting curve (dashed) for evaluation of the antidamping-like field HAD. Fig. 1(e) shows the estimated HAD at various JBiSb, which yields θSH = 2.4. Fig. 2(a) shows the pulse width τ dependence SOT switching loop. The magnetization of CoPt trilayers can be efficiently switched by a small threshold current density JthBiSb = 2.3 MAcm-2 at pulse width of 50 ms. The JthBiSb - τ characteristics yields the thermal stability factor Δ = 32 (Fig. 2(b)). We also demonstrate robust switching by consecutive pulses (Fig. 2(c)) and fast switching down to 100 ns (Fig. 2(d)). Our work shows that it is possible to implement ultralow power SOT-MRAM using BiSb on Si/SiOx substrates.

![](https://assets.underline.io/uploads/markdown_image/1/image/377dc47146e78726a8fbab3cddd96e54.jpg)

Fig. 1 (a) Schematic structure. (b)(c) Hall resistance measured with magnetic field perpendicular to the film plane and in-plane along the current direction. (d) R2ω as a function of Hx at different JBiSb. (e) HAD as a function of JBiSb.

![](https://assets.underline.io/uploads/markdown_image/1/image/0dfa12a8b5145229ef559ecc86b500d1.jpg)

Fig. 2 (a) SOT switching loops at different τ. (b) JthBiSb as a function of τ. (c) Robust switching by 100 μs pulse currents. (d) Switching loops by short pulses.

Low power spin-orbit torque magnetization switching in all-sputtered BiSb topological insulator / perpendicularly magnetized CoPt / MgO multilayers on Si substrate

Efficient modulation of electrically-injected spin signals that is suitable for modern-day transistor functionality is yet to be established. In this work, we demonstrate in detail the fabrication of Fe/n-GaAs spin injection device and the experimental setup for an optical gating of the non-local spin transport signal. Three- and four-terminal Hanle measurements confirm successful spin injection into n-GaAs, with strong interfacial spin dephasing at high magnetic fields. Tunable-wavelength, time-resolved pump-probe Kerr rotation [1] was used to verify the strongest photoexcitation signal with 822 nm and the long spin relaxation time of ~ 2.4 ns in the channel n-GaAs layer. As seen in Figure 1, changes in the non-local spin transport signal, V3-4, were measured with the modulation light with opposite helicity illuminated in the region of pure spin current. The helicity-dependent optical modulation efficiency, calculated as the ratio between the difference and the sum of the signals at the opposite helicity, of (0.4 ± 0.3)% was obtained at 30 K at the maximum available laser power of 7 mW. Furthermore, in-situ scanning electron microscope interface imaging, developed by authors [2], was used to reveal the correlation between the contrast inhomogeneity of the Fe/n-GaAs injector interface and the injection current, as displayed in Figure 2. The larger contrast inhomogeneity observed at lower injection currents suggest “pin-hole” type charge transport at bias voltage lower than the Schottky barrier height.

![](https://assets.underline.io/uploads/markdown_image/1/image/1fb23f3b9f343fc75dd41d8812ec204e.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/d955ce302deb8d7f30dd574b818b0b9c.jpg)

Development of an Optically-Gated Fe/n-GaAs Spin-Polarised Transistor

Development of an Optically-Gated Fe/n-GaAs Spin-Polarised Transistor

Nonreciprocal electrical transport responses arise in materials with broken inversion and time-reversal symmetries [1]. Although these nonlinear magnetoresistance or Hall effects are typically weak compared with their linear analogs, they can be used to probe exotic properties like spin-orbit interaction, spin textures and superconductivity. At the surface of the 3D topological insulator (TI) Bi2Se3, a nonlinear planar Hall effect (NPHE) was reported to exist due to the hexagonally warped spin structures [2]. This NPHE is expected to be small at low carrier densities because the hexagonal warping effect is negligible at the Dirac point. In this study, we observed a similar NPHE in Bi2Se3/CoFeB and magnetic insulator/Bi2Se3 heterostructures. Though with a lower carrier density, the NPHE in these systems is two orders of magnitude larger than the previously reported one. Moreover, the strength of this NPHE scales inversely with carrier density. This suggests that the intrinsic hexagonal warping is not the dominant mechanism for the observed NPHE. Instead, we attribute this large NPHE to the recently proposed extrinsic mechanism based on the interplay between current-induced spin-polarization and scattering from spin-momentum locking inhomogeneities [3]. Our results provide experimental evidence for this new mechanism and show that the NPHE can be enhanced by disorder engineering.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5ea6152d1cf679c7030c2ca6f2fb62d.jpg)

Fig. 1 Nonlinear planar Hall effect in a YIG(2.46m)/Bi2Se3(8nm) sample.

![](https://assets.underline.io/uploads/markdown_image/1/image/895b64111427635788c6e4f435603dcd.jpg)

Fig. 2 Carrier density dependence of the nonlinear planar Hall effect.

Nonlinear planar Hall effect from spin-momentum locking inhomogeneities in topological insulator Bi2Se3

Diodes are a key component in electronics with their nature of asymmetric conduction enabling rectification in circuits. Non-linear response is one criterion to have such effects and spin waves, or the quanta magnons are known to be highly nonlinear with respect to input power or spin transfer torques (STT), which leads to microwave rectification in spin torque nano oscillators[1,2]. While non linear dynamics of magnons due to STT in confined geometry has been well studied thanks to the macrospin nature, its nonlinear nature has been so far elusive in extended films. We measure non-linear spin transport behavior of magnetic garnet thin films by spin orbit torques (SOT) in non-local geometry (Fig.1) [3] and report spin diode effect marked by a strong asymmetric conductance at large current density. Namely higher spin conductance is obtained for the polarity increasing spin fluctuations than for the opposite polarity. We demonstrate that spin diode effects are determined by the collective dynamics of magnetostatic magnons and emergence of the asymmetry requires to work in the long-range regime (large separation between the 2 electrodes &gt; 2μm) to efficiently filter out the spurious symmetric contribution of the rapidly decaying thermal magnons[3]. As the intensity of the spin current increases, the transport properties of magnetostatic magnons crosses 3 different regimes. i) At low currents, it is ballistic set by the film thickness. ii) Close to the damping compensation threshold, it switches to a highly correlated dynamical regime limited by spin-spin relaxation and shows a sharp increase of the non-local conductance. iii) As the temperature of the magnons approaches the Curie temperature, it switches to a diffusive regime, where the propagation length is reduced by collisions with thermal magnons, showing negative differential spin conductance (Fig.2).

![](https://assets.underline.io/uploads/markdown_image/1/image/e9026ed56a884d6b8ad6be08af8ce715.jpg)


Fig.1. Schematics of the four-terminal diode device which consists of two Pt electrodes (Pt1 and Pt2) separated by distance d on a Yttrium Iron Garnet thin film.


![](https://assets.underline.io/uploads/markdown_image/1/image/22ad15451ed7354affa41a5500699ba1.jpg)

Fig.2. Renormalized SOT contribution of the spin conduction i2 detected in Pt2 as a function of I1, the current injected in Pt1. Here T1 is the temperature of the Pt1 and Tc is the Curie temperature.

Spin diode effect in magnetic garnets

The longitudinal spin Seebeck effect (LSSE) is an efficient method for generating incoherent magnon excitations by applying a vertical temperature gradient across a magnetic material. To avoid the contamination of the thermally generated “pure” spin current by the electronic degrees of freedom, magnetic insulators are exclusively preferred for LSSE. Although the ferrimagnetic insulator Y3Fe5O12 (YIG) is known as the hallmark system for LSSE because of its ultra-low Gilbert damping,[1] other members of the insulating iron garnet family, including the compensated ferrimagnet Gd3Fe5O12 (GdIG), have recently attracted considerable interest due to their unique compensated spin configurations.[2] In particular, the fascinating observation of the change in sign of the LSSE signal in a GdIG/Pt bilayer around the magnetic compensation temperature due to reorientation of sublattice magnetizations has motived us to address to an emerging fundamental question: “Is there a general trend for the temperature dependence of LSSE in compensated ferrimagnets?” Therefore, we have performed a comprehensive investigation of LSSE in GGG/GdIG(t)/Pt(5nm) bilayers with 5 different thicknesses ranging between 31 and 272 nm. We find that all the GdIG films possess an in-plane magnetic easy axis, and the compensation temperature decreases from 270 to 220 K with decreasing film-thickness from 272 to 31 nm. Using a proposed rescaling method, we have demonstrated, for the first time, a “universal scaling” behavior for the temperature dependence of LSSE signal for our GdIG films with different compensation temperatures, which, we believe, holds true for other compensated ferrimagnetic systems as well. In addition, we have performed the LSSE study on a 31 nm GdIG film grown on lattice-mismatched GSGG substrate that exhibits an out-of-plane magnetic easy axis and observed a clear distinction in the magnetic field dependent LSSE signal, relative to the 31 nm GGG/GdIG film showing an in-plane magnetic easy axis. This highlights a strong correlation between the LSSE signal and the magnetic anisotropy in compensated ferrimagnets. Our findings will pave the way to explore LSSE in other novel compensated ferrimagnets.

Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

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.

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