Spin Hall Magnetoresistance Effect Research Articles

Interplay of spin current and magnetization in a topological-insulator/magnetic-insulator bilayer structure

The interaction between accumulated spins on the surface of a heavy metal (HM) and the magnetization of an adjacent magnetic material leads to various spin phenomena, such as spin-orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low charge-to-spin conversion efficiency of the HM. Authors of recent studies have suggested that topological insulators (TIs) are promising candidates for device applications due to their potentially higher charge-to-spin conversion efficiency. Here, we report a multifaceted study of a bilayer structure consisting of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}\mathrm{O}$ (YIG) and demonstrate an approach based on angle-dependent magnetoresistance (ADMR) measurements to determine the effective charge-to-spin conversion efficiency in TIs. Our ferromagnetic resonance measurements demonstrate efficient spin pumping from YIG to ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which is further confirmed by detection of an electromotive force generated in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ via spin-to-charge conversion. Our ADMR measurements show that the interfacial spin diffusion can significantly affect the charge transport in a way like the SHMR effect and provide an estimate of the charge-to-spin conversion efficiency in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ of $\ensuremath{\sim}0.1--0.4$. Neglecting to account for the large out-of-plane magnetoresistance of the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ results in a fivefold overestimate of the charge-to-spin conversion efficiency.

Spin Hall Magnetoresistance Effect from a Disordered Interface.

The spin Hall magnetoresistance (SMR) emerged as a reference tool to investigate the magnetic properties of materials with an all-electrical setup. Its sensitivity to the magnetization of thin films and surfaces may turn it into a valuable technique to characterize van der Waals magnetic materials, which support long-range magnetic order in atomically thin layers. However, realistic surfaces can be affected by defects and disorder, which may result in unexpected artifacts in the SMR, rather than the sole appearance of electrical noise. Here, we study the SMR response of heterostructures combining a platinum (Pt) thin film with the van der Waals antiferromagnet MnPSe3 and observe a robust SMR-like signal, which turns out to originate from the presence of strong interfacial disorder in the system. We use transmission electron microscopy (TEM) to characterize the interface between MnPSe3 and Pt, revealing the formation of a few nanometer-thick platinum-chalcogen amorphous layer. The analysis of the transport and TEM measurements suggests that the signal arises from a disordered magnetic system formed at the Pt/MnPSe3 interface, washing out the interaction between the spins of the Pt electrons and the MnPSe3 magnetic lattice. Our results show that the damaged interfaces can yield an important contribution to SMR, questioning a widespread assumption on the role of disorder in such measurements.

Breaking the symmetry of spin-sublattices in antiferromagnet by interfacial tailoring in the L1-MnPt/NaCl/Fe junction

The detection of magnetization states of the antiferromagnets is a precondition to read the binary bits (0 and 1) for the magnetic recording. The magnetoresistance effect has been utilized to detect the magnetization states in commercially ferromagnetic spintronic devices. However, reading the magnetization states of antiferromagnets is extremely hard because of the absence of net magnetic moment in the antiferromagnetic materials. Here, to generate the tunneling magnetoresistance effect in an antiferromagnetic device with the compensated surface, a method of interlayer selection and interfacial tailoring was proposed to break the symmetry of spin-sublattices in antiferromagnets. The stacking structure of the device is L10-MnPt/NaCl/Fe. The C-type antiferromagnet L10-MnPt was used as the antiferromagnetic electrode, and Fe was the other electrode. NaCl was selected as the barrier layer to break the symmetry of spin-sublattices in L10-MnPt by considering the lattice constants of different materials. It is confirmed that the device with the Mn terminal shows the C4v symmetry in which the spin-sublattices symmetry of L10-MnPt was broken. As a result, the tunneling magnetoresistance ratio of the present device can be up to 200%, which is much larger than the reported ratios of both anisotropic magnetoresistance and the spin Hall magnetoresistance effect in the antiferromagnetic devices, showing a great potential application to read the magnetization states of the antiferromagnetic devices. The spin-dependent transport mechanism of the present device was analyzed in detail by the kǁ-resolved transmission and the local density of states. This study would be beneficial to promote the development of antiferromagnetic spintronics.

Theory of Spin Hall Magnetoresistance from a Microscopic Perspective.

We present a theory of the spin Hall magnetoresistance of metals in contact with magnetic insulators. We express the spin mixing conductances, which govern the phenomenology of the effect, in terms of the microscopic parameters of the interface and the spin-spin correlation functions of the local moments on the surface of the magnetic insulator. The magnetic-field and temperature dependence of the spin mixing conductances leads to a rich behavior of the resistance due to an interplay between the Hanle effect and the spin mixing at the interface. We describe an unusual negative magnetoresistance originating from a nonlocal Hanle effect. Our theory provides a useful tool for understanding the experiments on heavy metals in contact with magnetic insulators of different kinds, and it enables the spin Hall magnetoresistance effect to be used as a technique to study magnetism at interfaces.

Influence of the two boundaries of the Pt layer on spin current transportation by spin Hall magnetoresistance

The spin Hall magnetoresistance effects (SMR) in four heterostructures consist of different boundaries of Pt layer has been investigated. The result in this work shows that the two boundaries of Pt layer in all heterostructures both influence the spin current absorption and reflection. The Pt/air interface would weaken the spin current absorption at the Co2FeSi/Pt interface on the other side of the Pt layer. On the contrary, the Pt/MgO(0 0 1) interface could boost the spin current absorption strongly at the Co2FeSi/Pt interface on the other side of the Pt layer. And this promotion effect is much stronger than the Co2FeSi/Pt interface on the spin current absorption at the other Co2FeSi/Pt interface. The MgO capping layer may avoid anisotropic magnetoresistance (AMR) induced SMR ratio decreasing at low temperature. This study provides a new way for modulating spin current absorption at the FM/HM interface.

Synchronization of Spin Torque Oscillators through Spin Hall Magnetoresistance

Spin torque oscillators placed onto a nonmagnetic heavy metal show synchronized auto-oscillations due to the coupling originating from spin Hall magnetoresistance effect. Here, we study a system having two spin torque oscillators under the effect of the spin Hall torque, and show that switching the external current direction enables us to control the phase difference of the synchronization between in-phase and antiphase.

Dynamic coupling of ferromagnets via spin Hall magnetoresistance

The synchronized magnetization dynamics in ferromagnets on a nonmagnetic heavy metal caused by the spin Hall effect is investigated theoretically. The direct and inverse spin Hall effects near the ferromagnetic/nonmagnetic interface generate longitudinal and transverse electric currents. The phenomenon is known as the spin Hall magnetoresistance effect, whose magnitude depends on the magnetization direction in the ferromagnet due to the spin transfer effect. When another ferromagnet is placed onto the same nonmagnet, these currents are again converted to the spin current by the spin Hall effect and excite the spin torque to this additional ferromagnet, resulting in the excitation of the coupled motions of the magnetizations. The in-phase or antiphase synchronization of the magnetization oscillations, depending on the value of the Gilbert damping constant and the field-like torque strength, is found in the transverse geometry by solving the Landau-Lifshitz-Gilbert equation numerically. On the other hand, in addition to these synchronizations, the synchronization having a phase difference of a quarter of a period is also found in the longitudinal geometry. The analytical theory clarifying the relation among the current, frequency, and phase difference is also developed, where it is shown that the phase differences observed in the numerical simulations correspond to that giving the fixed points of the energy supplied by the coupling torque.

Spin Hall magnetoresistance in a canted ferrimagnet

We study the spin Hall magnetoresistance effect in ferrimagnet/normal metal bilayers, comparing the response in collinear and canted magnetic phases. In the collinear magnetic phase, in which the sublattice magnetic moments are all aligned along the same axis, we observe the conventional spin Hall magnetoresistance. In contrast, in the canted phase, the magnetoresistance changes sign. Using atomistic spin model calculations of the magnetic configuration, we show that the electric transport for the different magnetic phases can be rationalized considering the individual sublattice moment orientations. This enables a magneto-transport based investigation of non-collinear magnetic textures.

Spin Orbit Coupling Controlled Spin Pumping and Spin Hall Magnetoresistance Effects

Effective spin mixing conductance (ESMC) across the nonmagnetic metal (NM)/ferromagnet interface, spin Hall conductivity (SHC), and spin diffusion length (SDL) in the NM layer govern the functionality and performance of pure spin current devices. It is shown that all three parameters can be tuned significantly via the spin orbit coupling (SOC) strength of the NM layer by virtue of the unique Pd1‐xPtx/Y3Fe5O12 system. Surprisingly, the ESMC is observed to increase significantly with x changing from 0 to 1.0, due to the enhanced local density of states for Pt‐rich alloys. The SHC in PdPt alloys turns out to be dominated by the skew scattering term. In particular, the skew scattering parameter has for the first time been rigorously demonstrated to increase with increasing SOC strength. Meanwhile, the SDL is found to decrease when Pd atoms are replaced by heavier Pt atoms, validating the SOC induced spin flip scattering model in polyvalent PdPt alloys. A thorough grasp of the dependence of these parameters on the SOC strength in the present work can help to develop a better theoretical graph on the physics of SOC and spin orbit torque switching, relevant to next generation spintronic devices.

Spin Hall magnetoimpedance

The recently discovered spin Hall magnetoresistance effect electrically probes pure spin current flow across a ferrimagnetic insulator/normal metal bilayer interface. While usually the DC electrical resistance of the bilayer is measured as a function of the magnetization orientation in the magnetic insulator, we here present magnetoimpedance measurements using bias currents with frequencies up to several GHz. We find that the spin Hall magnetoresistance effect persists up to frequencies of at least 4 GHz, enabling a fast readout of the magnetization direction in magnetic insulator/normal metal bilayers. Our data furthermore show that all interaction time constants relevant for the spin Hall magnetoresistance effect are shorter than 40 ps.

Realization of the meminductor.

The meminductor was proposed to be a fundamental circuit memdevice parallel with the memristor, linking magnetic flux and current. However, a clear material model or experimental realization of a meminductor has been challenging. Here we demonstrate pinched hysteretic magnetic flux-current signals at room temperature based on the spin Hall magnetoresistance effect in several-nanometer-thick thin films, exhibiting the nonvolatile memorizing property and magnetic energy storage ability of the meminductor. Similar to the parameters of the capacitor, resistor, and inductor, meminductance, LM, is introduced to characterize the capability of the prepared meminductor. Our findings present an indispensable element of memdevices and open an avenue for nanoscale meminductor design and manufacture, which might contribute to low-power electronic circuits, information storage, and artificial intelligence.

Experimental Test of the Spin Mixing Interface Conductivity Concept

We perform a quantitative, comparative study of the spin pumping, spin Seebeck, and spin Hall magnetoresistance effects, all detected via the inverse spin Hall effect in a series of over 20 yttrium iron garnet/Pt samples. Our experimental results fully support present, exclusively spin current-based, theoretical models using a single set of plausible parameters for spin mixing conductance, spin Hall angle, and spin diffusion length. Our findings establish the purely spintronic nature of the aforementioned effects and provide a quantitative description, in particular, of the spin Seebeck effect.

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

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 $4\times10^{14}\;\mathrm{\Omega^{-1}m^{-2}}$ we obtain a spin Hall angle of $0.11\pm0.08$ and a spin diffusion length of $(1.5\pm0.5)\;\mathrm{nm}$ for Pt in our thin film samples.