Spin Hall Magnetoresistance Signal Research Articles

Modulation of spin Hall magnetoresistance in α-Fe2O3/Pt by uniaxial tensile strain

We demonstrate the modulation of spin Hall magnetoresistance (SMR) by uniaxial tensile strain at RT in heterostructures of antiferromagnetic insulating (0001)-oriented α-Fe2O3 (hematite) and Pt. The SMR signals consistent with previous reports are observed in α-Fe2O3/Pt without strain. Under tensile strains, we found that the magnetoresistance amplitude changes almost linearly with the strain amplitude. The observed magnetoresistance change is attributed to the strain modulation of SMR owing to the change in domain distribution via magnetoelastic couplings. The results provide a useful method for controlling the spin-dependent transport effect in antiferromagnetic-insulator/metal bilayers.

Spin Hall magnetoresistance in quasi-two-dimensional antiferromagnetic-insulator/metal bilayer systems

We study the temperature dependence of spin Hall magnetoresistance (SMR) in antiferromagnetic insulator (AFI)/metal bilayer systems. We calculate the amplitude of the SMR signal by using a quantum Monte Carlo simulation and examine how the SMR depends on the amplitude of the spin, thickness of the AFI layer, and randomness of the exchange interactions. Our results for simple quantum spin models provide a useful starting point for understanding SMR measurements on atomic layers of magnetic compounds.

Perpendicular antiferromagnetic ordering and canted spins in van der Waals FePS3 probed by spin Hall magnetoresistance

Van der Waals antiferromagnets (AFMs) provide a two-dimensional (2D) platform for spintronic devices with exceptional properties. However, the electric transport features of the magnetic order of van der Waals AFM influenced by different field directions and amplitudes has not been demonstrated systematically. In this letter, we investigate the magnetic texture of the exfoliated van der Waals FePS3, a uniaxial AFM with perpendicular anisotropy, detected by the spin Hall magnetoresistance (SMR). Magnetic field- and temperature- dependent longitudinal magnetoresistance measurements in three orthogonal directions for the exfoliated FePS3/Pt nanostructures are conducted. The modulations in the SMR signal enable the separation of two contributions to the SMR, one of which corresponds to the negative signature of AFM SMR caused by in-plane field rotations, and the other of which is caused by canted spins in perpendicular AFM order. Our findings offer great guidance for further research and investigation using SMR approach of the magnetic texture in van der Waals AFMs.

Positive spin Hall magnetoresistance in single-crystalline Pt/CoO(001) bilayers

The spin Hall magnetoresistance (SMR) effect in single-crystalline Pt/CoO(001) bilayers has been systematically investigated. X-ray magnetic linear dichroism measurements prove that CoO antiferromagnetic (AFM) spins can be switched into the direction orthogonal to the applied field. We find the SMR signal is comprised of two components related to either the switching of CoO AFM N\'eel order or the applied strong field effect. Both SMR components show a ``positive'' angular dependence with ${R}_{\ensuremath{\parallel}}>{R}_{\ensuremath{\perp}}$, while ${R}_{\ensuremath{\parallel}}\phantom{\rule{4pt}{0ex}}({R}_{\ensuremath{\perp}})$ is defined as the resistance with the applied in-plane field parallel (perpendicular) to the current. The observed positive SMR is mainly attributed to the uncompensated spins at the Pt/CoO interface, instead of the CoO AFM spins. Our study may attract a great deal of interest to understand the complicated SMR effect in AFM spintronics materials.

Tunable Spin Hall Magnetoresistance in All-Antiferromagnetic Heterostructures

We investigate the spin Hall magnetoresistance (SMR) in all-antiferromagnetic heterostructures α-Fe2O3/Cr2O3 with Pt contacts. When the temperature is ultralow (< 50 K), the spin current generated in the Pt layer cannot be transmitted through Cr2O3 (t = 4 nm), and the SMR is near zero. Meanwhile, when the temperature is higher than the spin fluctuation temperature T F (≈ 50 K) of Cr2O3 and lower than its Néel temperature T N (≈ 300 K), the spin current goes through the Cr2O3 layer and is reflected at the α-Fe2O3/Cr2O3 interface; an antiferromagnetic (negative) SMR is observed. As temperature increases higher than T N, paramagnetic (positive) SMR mainly arises from the spin current reflection at the Cr2O3/Pt interface. The transition temperatures from negative to positive SMR are enhanced with increasing Cr2O3 layer thickness, accompanied by the absence of SMR signals when t = 10 nm. Such a tunable SMR builds a bridge between spin transport and structures. It also enriches antiferromagnetic spintronics.

Noncollinear spin state and unusual magnetoresistance in ferrimagnet Co-Gd

Rare-earth transition-metal ferrimagnetic materials are promising candidates for next-generation spintronic devices owing to their small magnetic moments and fast dynamics. However, only a few studies have been conducted thus far on the transport properties of noncollinear ferrimagnetic spin textures. In this study, we investigate the transverse and longitudinal resistances in a ferrimagnet/heavy-metal heterostructure composed of Co-Gd and Pt. The transition-metal sublattice arrangement and magnetic field--temperature phase diagram are obtained based on unconventional hysteresis loops observed near the compensation point. In the collinear regime, the spin Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) exhibit normal oscillation curves. In the noncollinear regime, the SMR and AMR signals are inverted for the existence of a 90\ifmmode^\circ\else\textdegree\fi{} phase shift between magnetization and magnetic fields. Owing to the conical spin textures in the spin-flop state, sharp peaks emerge when the field is perpendicular to the film plane. The abnormal magnetoresistance in our results can be used to analyze complex magnetic structures, which plays an important role in the all-electrical readout of noncollinear spintronics devices.

Antiferromagnetic Ordering and Uncoupled Spins in CaFe2O4 Thin Films Probed by Spin Hall Magnetoresistance

AbstractCaFe2O4 is a uniaxial antiferromagnet displaying two coexisting magnetic orderings, A and B, characterized by ↑↑↓↓ and ↑↓↑↓ spin modulation, respectively, and the emergence of a net magnetization in a limited temperature range, which is not yet understood. The spin Hall magnetoresistance (SMR) is probed at the interface between Pt and CaFe2O4 and the crystallographic domain structure of thin film samples is exploited to perform single‐ and multi‐domain scale measurements. The SMR response, upon rotating the magnetic field along three orthogonal planes, shows little effect of the strong magnetocrystalline and shape anisotropies. Together with the response to a varying magnetic field strength, the modulations in the SMR signal allow to extract two contributions: one corresponds to the long‐range antiferromagnetic ordering, supporting a single ground state scenario; while the second contribution originates from uncompensated, non‐interacting spins. These are expected to exist at the antiphase boundaries between antiferromagnetic domains. Here, it is shown that these are also uncoupled from the antiferromagnetic ordering. Nonetheless, the long range correlations that emerge in the proximity of the critical antiferromagnetic transition can give rise to ordering of the uncompensated spins and be responsible for the net magnetization observed in this antiferromagnet.

Theory of spin-Hall magnetoresistance in the ac terahertz regime

In bilayers consisting of a normal metal (N) with spin-orbit coupling and a ferromagnet (F), the combination of the spin-Hall effect, the spin-transfer torque, and the inverse spin-Hall effect gives a small correction to the in-plane conductivity of N, which is referred to as spin-Hall magnetoresistance (SMR). We here present a theory of the SMR and the associated off-diagonal conductivity corrections for frequencies up to the terahertz regime. We show that the SMR signal has pronounced singularities at the spin-wave frequencies of F, which identifies it as a potential tool for all-electric spectroscopy of magnon modes. A systematic change of the magnitude of the SMR at lower frequencies is associated with the onset of a longitudinal magnonic contribution to spin transport across the F-N interface.

Self-intercalated two-dimensional magnetic semiconductor V8(S1-xSex)15

The exploration of two-dimensional (2D) magnets has attracted considerable attention due to their potential applications in spintronic devices over the past few years. Recently, a variety of 2D vanadium-based (V-based) chalcogenides have been demonstrated to own fascinating magnetic properties by both the theoretical predication and experimental realization. However, ternary V-based compounds have rarely been studied. Here, we synthesized three kinds of ultrathin ternary self-intercalated V8(S1-xSex)15 sheets with different Se contents by the atmospheric pressure chemical vapor deposition method. The Se content x is 8.7%, 12.1%, and 19.7%. The Raman spectra indicate that these three kinds of self-intercalated V8(S1-xSex)15 nanosheets with a different Se content own the same crystal structure. All self-intercalated V8(S1-xSex)15 nanosheets exhibit a semiconducting behavior, and the conducting type transits from ambipolar to p-type as the Se content increases. The spin Hall magnetoresistance (SMR) signal can be detected in the Pt/V8(S1-xSex)15 bilayer structure, and SMR signals (amplitudes) gradually weaken with the increasing temperature. These results manifest that self-intercalated V8(S1-xSex)15 own both semiconducting and magnetic characteristics.

Effective strain manipulation of the antiferromagnetic state of polycrystalline NiO

As a candidate material for applications such as magnetic memory, polycrystalline antiferromagnets offer the same robustness to external magnetic fields, THz spin dynamics, and lack of stray fields as their single crystalline counterparts, but without the limitation of epitaxial growth and lattice matched substrates. Here, we first report the detection of the average Néel vector orientation in polycrystalline NiO via spin Hall magnetoresistance (SMR). Second, by applying strain through a piezo-electric substrate, we reduce the critical magnetic field required to reach a saturation of the SMR signal, indicating a change of the anisotropy. Our results are consistent with polycrystalline NiO exhibiting a positive sign of the in-plane magnetostriction. This method of anisotropy-tuning offers an energy efficient, on-chip alternative to manipulate a polycrystalline antiferromagnet's magnetic state.

All-electrical detection of skyrmion lattice state and chiral surface twists

We study the high-temperature phase diagram of the chiral magnetic insulator Cu$_2$OSeO$_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu$_2$OSeO$_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in non-collinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

Microscopic theory of spin Hall magnetoresistance

We consider a microscopic theory for the spin Hall magnetoresistance (SMR). We generally formulate a spin conductance at an interface between a normal metal and a magnetic insulator in terms of spin susceptibilities. We reveal that SMR is composed of static and dynamic parts. The static part, which is almost independent of the temperature, originates from spin flip caused by an interfacial exchange coupling. However, the dynamic part, which is induced by the creation or annihilation of magnons, has an opposite sign from the static part. By the spin-wave approximation, we predict that the latter results in a nontrivial sign change of the SMR signal at a finite temperature. In addition, we derive the Onsager relation between spin conductance and thermal spin-current noise.

A comparative study of spin Hall magnetoresistance in Fe2O3-based systems

Spin Hall magnetoresistance (SMR) provides a unique opportunity for the signal readout of magnetic ordering, especially for magnetic insulators. However, the magnitude of SMR signals is quite small, generally lower than one in a thousand, which is a persistent challenge in the field of spintronics. Here, we investigate SMR at room temperature in α-Fe2O3/Pt and γ-Fe2O3/Pt bilayers as well as the α-Fe2O3/Pt/γ-Fe2O3 trilayer, where α-Fe2O3 and γ-Fe2O3 are antiferromagnetic and ferrimagnetic materials, respectively. In contrast to the positive SMR in the γ-Fe2O3/Pt bilayer, an unprecedentedly large negative SMR with the magnitude of ∼0.35% is observed in the α-Fe2O3/Pt bilayer due to the spin-flop. The α-Fe2O3/Pt/γ-Fe2O3 trilayer shows features of a combination of positive and negative SMR. A comparative study of SMR in the Fe2O3 family not only enriches spin physics, but also makes Fe2O3 a versatile candidate in spintronic devices.

Spin structure and spin Hall magnetoresistance of epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO3

We report a combined study of imaging the antiferromagnetic (AFM) spin structure and measuring the spin Hall magnetoresistance (SMR) in epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO3. X-ray magnetic linear dichroism photoemission electron microscopy measurements reveal that the AFM spins of the SmFeO3(1 1 0) align in the plane of the film. Angularly dependent magnetoresistance measurements show that SmFeO3/Ta bilayers exhibit a positive SMR, in contrast to the negative SMR expected in previously studied collinear AFMs. The SMR amplitude increases linearly with increasing external magnetic field at higher magnetic fields, suggesting that field-induced canting of the AFM spins plays an important role. In contrast, around the coercive field, no detectable SMR signal is observed, indicating that the SMR of the AFM and canting magnetization components cancel out. Below 50 K, the SMR amplitude increases sizably by a factor of two as compared to room temperature, which likely correlates with the long-range ordering of the Sm ions. Our results show that the SMR is a sensitive technique for non-equilibrium spin systems of non-collinear AFMs.

Increase in spin-Hall effect and influence of anomalous Nernst effect on spin-Hall magnetoresistance in β-phase and α-phase W100−xTax/CoFeB systems

We studied the spin-Hall effect in W100−xTax with various Ta compositions for W-Ta alloy/CoFeB/MgO heterostructures by means of spin-Hall magnetoresistance (SMR). We synthesized W-Ta alloys with both α-phase and β-phase by controlling the sputtering condition. We found that, in both α-phase and β-phase W-Ta alloys, the amplitude in SMR signals increases by alloying W with Ta at certain x. The analysis based on the drift-diffusion model reveals that the increase in SMR originates from an enhancement of the spin-Hall angles θSH. Moreover, we observed the influence of anomalous Nernst effect on SMR for the high-resistive W-Ta/CoFeB systems.

Annealing Temperature Effects on Spin Hall Magnetoresistance in Perpendicularly Magnetized W/CoFeB Bilayers

In this paper, we studied the annealing temperature effects on the spin Hall magnetoresistance (SMR) in W/CoFeB metallic bilayer systems. The bilayers were deposited using dc magnetron sputtering. The angle-dependent magnetoresistance was performed in a field of 8 T. The sample was rotated in the yz plane to isolate the SMR signal from the anisotropic magnetoresistance present in metallic systems. For annealing temperatures above 350 °C, we found the SMR signal reduces with an increase in annealing temperature. By measuring the SMR for samples with a varying W layer thickness we were able to find that the spin Hall angle decreases as annealing temperature increases and the spin diffusion length for W remains almost constant with annealing temperature.

Full angular dependence of the spin Hall and ordinary magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films

We report the observation of the three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) in a bilayer of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt, without any ferromagnetic element. The detected angular-dependent longitudinal and transverse magnetoresistances are measured by rotating the sample in magnetic fields up to 11 T, along three orthogonal planes (xy-, yz- and xz-rotation planes, where the z-axis is orthogonal to the sample plane). The total magnetoresistance has contributions arising from both the SMR and ordinary magnetoresistance. The onset of the SMR signal occurs between 1 and 3 T and no saturation is visible up to 11 T. The three-dimensional angular dependence of the SMR can be explained by a model considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic S- and T-domains in the NiO(001), stemming from the competition between the Zeeman energy and the elastic clamping effect of the non-magnetic MgO substrate. From the observed SMR ratio, we estimate the spin mixing conductance at the NiO/Pt interface to be greater than $2\times10^{14}$ ${\Omega}^{-1}$ $m^{-2}$. Our results demonstrate the possibility to electrically detect the N\'eel vector direction in stable NiO(001) thin films, for rotations in the xy- and xz- planes. Moreover, we show that a careful subtraction of the ordinary magnetoresistance contribution is crucial to correctly estimate the amplitude of the SMR.

Negative spin Hall magnetoresistance in antiferromagnetic Cr2O3/Ta bilayer at low temperature region

We demonstrate the negative spin Hall magnetoresistance (SMR) observed in antiferromagnetic Cr2O3/Ta bilayers at low temperature. The SMR signals are changed from the positive to negative magnitude when monotonously reducing the temperature from 300 K to 50 K. The positive and negative SMR signals are expected to be associated with the two different ways for injection of the spin current, into the boundary ferromagnetic region and the bulk antiferromagnetic region of α-Cr2O3 (0001), respectively. The slopes of the abnormal Hall curves exhibit the same sign with the SMR signal. From the SMR ratio under 3 T, the spin mixing conductance at the Cr2O3/Ta interface is estimated to be 1.12 × 1014 Ω−1·m−2, which is comparable to the one observed in the Y3Fe5O12(YIG)/Pt structure and our early results of the Cr2O3/W structure.

Investigation of the difference between spin Hall magnetoresistance rectification and spin pumping from the viewpoint of magnetization dynamics

Spin pumping (SP) and spin rectification due to spin Hall magnetoresistance (SMR) can result in a dc resonant voltage signal, when magnetization in ferromagnetic insulator/nonmagnetic structures experiences ferromagnetic resonance. Since the two effects are often interrelated, quantitative identification of them is important for studying the dynamic nonlocal spin transport through an interface. In this letter, the key difference between SP and SMR rectification was investigated from the viewpoint of spin dynamics. The phase-dependent nature of SMR rectification, which is the fundamental characteristic distinguishing it from SP, was tested by a well-designed experiment. In this experiment, two identical yttrium iron garnet/Pt strips with a π phase difference in dynamic magnetization show the same SP signals and inverse SMR signals.

Spin Hall magnetoresistance in the non-collinear ferrimagnet GdIG close to the compensation temperature

We investigate the spin Hall magnetoresistance (SMR) in a gadolinium iron garnet (GdIG)/platinum (Pt) heterostructure by angular dependent magnetoresistance measurements. The magnetic structure of the ferromagnetic insulator GdIG is non-collinear near the compensation temperature, while it is collinear far from the compensation temperature. In the collinear regime, the SMR signal in GdIG is consistent with the usual relation well established in the collinear magnet yttrium iron garnet, with the angle between magnetization and spin Hall spin polarization direction. In the non-collinear regime, both an SMR signal with inverted sign and a more complex angular dependence with four maxima are observed within one sweep cycle. The number of maxima as well as the relative strength of different maxima depend strongly on temperature and field strength. Our results evidence a complex SMR behavior in the non-collinear magnetic regime that goes beyond the conventional formalism developed for collinear magnetic structures.