Unidirectional Magnetoresistance Research Articles

Large unidirectional magnetoresistance from the dual functionality of copper oxide in naturally oxidized light-metal Al/Cu bilayer films

Large unidirectional magnetoresistance from the dual functionality of copper oxide in naturally oxidized light-metal Al/Cu bilayer films

Unidirectional magnetoresistance in an insulating compensated ferrimagnet

We investigate the unidirectional magnetoresistance (UMR) in a GdIG(Gd3Fe5O12)/Pt bilayer. We find a suppression of the observed UMR as the magnetic field increases, suggesting a magnonic origin for the UMR effect. Furthermore, through the examination of the temperature dependence of UMR, we observe that the sign of the UMR remains unchanged at the compensation temperature. This observation indicates that the magnon contributing to the UMR is associated with the lowest magnon mode with a distinct handedness. Our results provide compelling evidence for the existence of magnonic UMR in compensated ferrimagnets, offering potential advantages for spintronic applications.

Magnon-mediated spin Hall magnetoresistance and unidirectional magnetoresistance in Pt/NiO/NiFe structures

Spin–orbit torque provides an efficient strategy for electric manipulation of magnetization. However, Joule heat accompanying with electron motion in the electron-mediated spin current result in unavoidable power dissipation. Moreover, the spin diffusion length in electron-mediated spin current is relatively short, preventing the transmission of spin information over long distances. Magnon-mediated spin current, without moving electrons, can be an excellent alternative to the conventional spin current. Magnon-mediated transfer torque effect has been reported in several previous works. Here, we report the magnon-mediated spin Hall magnetoresistance (SMR) and unidirectional magnetoresistance (UMR) in Pt/NiO/NiFe structures. The significant SMR and UMR were observed in the samples with the NiO thickness up to 60 nm, demonstrating the efficient transmission of magnon-mediated spin current over long distances in the NiO layer. In addition, we observed current-induced in-plane magnetization switching in the NiFe layer via the UMR measurement. These results demonstrated the possibility for developing the efficient spintronic devices operated by magnons.

Publisher’s Note: “Evidences of thermoelectrically driven unidirectional magnetoresistance from a single Weyl ferromagnet Co2MnGa” [APL Mater. 11, 121109 (2023)

Publisher’s Note: “Evidences of thermoelectrically driven unidirectional magnetoresistance from a single Weyl ferromagnet Co2MnGa” [APL Mater. 11, 121109 (2023)

Quantitative analysis of magnon characteristics with unidirectional magnetoresistance

Quantitative analysis of magnon characteristics with unidirectional magnetoresistance

Evidences of thermoelectrically driven unidirectional magnetoresistance from a single Weyl ferromagnet Co2MnGa

Weyl ferromagnets, with large anomalous Hall (and Nernst) effects, are an ideal playground to study unconventional transport phenomena. Here, we report a sizable unidirectional magnetoresistance with a ratio of up to 7.73 × 10−5 per current density of 1 MA cm−2 in single-layer epitaxial Co2MnGa films. Surprisingly, the nonlinear signal has an isotropic crystallographic axis dependence and scales almost linearly with the film thickness. Both features cannot be explained by the spin transport from an intrinsic band structure, but rather agree with the current induced transverse thermoelectric effect. By employing a 1D heat transfer model to account for the temperature gradient, we derived an analytical expression of this thermoelectrically driven unidirectional magnetoresistance, from which a upper bound of transverse thermopower Sxy = 3.70 ± 1.10 µV K−1 can be obtained. Our work provides direct evidences of thermoelectric voltages in the nonlinear transport signals that may be extended to other material systems as well.

Coexistence of Magnon-Induced and Rashba-Induced Unidirectional Magnetoresistance in Antiferromagnets.

Unidirectional magnetoresistance (UMR) has been intensively studied in ferromagnetic systems, which is mainly induced by spin-dependent and spin-flip electron scattering. Yet, UMR in antiferromagnetic (AFM) systems has not been fully understood to date. In this work, we reported UMR in a YFeO3/Pt heterostructure where YFeO3 is a typical AFM insulator. Magnetic-field dependence and temperature dependence of transport measurements indicate that magnon dynamics and interfacial Rashba splitting are two individual origins for AFM UMR, which is consistent with the UMR theory in ferromagnetic systems. We further established a comprehensive theoretical model that incorporates micromagnetic simulation, density functional theory calculation, and the tight-binding model, which explain the observed AFM UMR phenomenon well. Our work sheds light on the intrinsic transport property of the AFM system and may facilitate the development of AFM spintronic devices.

Frequency-dependent phonon-mediated unidirectional magnetoresistance in a metal on an insulator with highly nonequilibrium magnons

Frequency-dependent phonon-mediated unidirectional magnetoresistance in a metal on an insulator with highly nonequilibrium magnons

Controllable unidirectional magnetoresistance in ferromagnetic films with broken symmetry

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Large Spin-Orbit-Torque Efficiency and Room-Temperature Magnetization Switching in SrIrO3/ Co-Fe-B Heterostructures

Spin-orbit-torque (SOT) switching is a potentially energy-efficient mechanism by which electrical current can control magnetization. We demonstrate room-temperature SOT switching in nanometer-thick ${\mathrm{Co}\mathrm{Fe}\mathrm{B}/\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_{3}$ bilayers despite the amorphous nature of CoFeB and the interface. This behavior is attributed to the large spin Hall angle in ${\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_{3}$ as well as high spin transmission at the interface that contribute to the SOT efficiency ${\ensuremath{\xi}}_{\mathrm{SOT}}$. Our bilayers exhibit large ${\ensuremath{\xi}}_{\mathrm{SOT}}$ of up to 1.4 and effective spin Hall conductivity of $0.9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ ($\ensuremath{\hbar}/$2e)${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\mathrm{m}}^{\ensuremath{-}1}$. In our bilayers, we observe unidirectional magnetoresistance and current-induced magnetization switching with a low critical current density of $1.38\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.2em}{0ex}}{\mathrm{A}/\mathrm{m}}^{2}$ at room temperature. Our results are promising for heterostructures combining technologically relevant CoFeB with high spin-orbit-coupling oxides.

Room-Temperature Gate-Tunable Nonreciprocal Charge Transport in Lattice-Matched InSb/CdTe Heterostructures.

Symmetry manipulation can be used to effectively tailor the physical order in solid-state systems. With the breaking of both the inversion and time-reversal symmetries, nonreciprocal magneto-transport may arise in nonmagnetic systems to enrich spin-orbit effects. Here, the observation of unidirectional magnetoresistance (UMR) in lattice-matched InSb/CdTe films is investigated up to room temperature. Benefiting from the strong built-in electric field of 0.13 Vnm-1 in the heterojunction region, the resulting Rashba-type spin-orbit coupling and quantum confinement result in a distinct sinusoidal UMR signal with a nonreciprocal coefficient that is 1-2 orders of magnitude larger than most non-centrosymmetric materials at 298 K. Moreover, this heterostructure configuration enables highly efficient gate tuning of the rectification response, wherein the UMR amplitude is enhanced by 40%. The results of this study advocate the use of narrow-bandgap semiconductor-based hybrid systems with robust spin textures as suitable platforms for the pursuit of controllable chiral spin-orbit applications.

Origins of observational errors in field sweep DC measurements for unidirectional magnetoresistance

Understanding the mechanisms of unidirectional magnetoresistance (UMR) has become an important topic for its potential application of a two-terminal spin–orbit torque device. Field sweep DC measurements have been proposed and adopted to measure the value of UMR instead of second harmonic measurements. In this paper, potential measurement errors in conventional DC measurements are investigated. Oersted field and field-like torque usually do not influence the measurement, but a large field-like torque was found to lead to an anisotropic magnetoresistance difference when the sample is not perfectly aligned with the external field. The existence of ordinary magnetoresistance was also found to contribute to a large background. In this paper, an alternative measurement method for UMR was proposed and demonstrated to address those issues related to previous DC measurements. Our work may broaden the understanding of the error sources of UMR measurements and provide a reliable DC measurement method for the characterization of UMR.

Unidirectional orbital magnetoresistance in light-metal–ferromagnet bilayers

We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital-to-spin conversion length. Our results provide insight into orbital-to-spin momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.

Coexistence of spin-orbit torque and unidirectional magnetoresistance effect induced by spin polarization with spin rotation symmetry in Co/Cu/Co structures

The conventional spin-orbit torque (SOT) and magnetoresistance effect observed in normal-metal (NM)/ferromagnet (FM) bilayers originate from the interaction between magnetic moments and spin with in-plane transverse polarization (${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{y}$). In FM/NM/FM trilayer structures, the presence of an extra FM layer breaks the symmetry, resulting in spin polarization other than ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{y}$ and the corresponding SOT. However, the study on the unidirectional magnetoresistance (UMR) effect induced by the spin polarization with spin rotation (SR) symmetry is still missing. In this work, we investigate the SOT and UMR effect in a Co/Cu/Co structure with crossed anisotropy by utilizing the harmonic longitudinal voltage measurement. We demonstrate that, in addition to the harmonic terms originating from the fieldlike SOT of the spin polarization with spin Hall and SR symmetry, two different unidirectional harmonic signals are observed in the angular and field scan measurements. Further, combining the SOT effective fields results from the harmonic Hall voltage measurement, we successfully separate the contributions of the observed unidirectional harmonic signals, which include the dampinglike SOT contribution through the giant magnetoresistance effect, and the UMR induced by the spin current with different polarization directions through spin-dependent scattering mechanism.

A large unidirectional magnetoresistance in Fe–Sn heterostructure devices

Unidirectional magnetoresistance (UMR) is an attractive magnetic-field sensing technique as it enables us to detect the in-plane direction of the external magnetic field with a single element. However, the UMR amplitude is typically several orders of magnitude smaller than those of other directional magnetoresistances exhibited in ferromagnetic thin films, hindering sensing applications using the UMR. For a directional magnetic sensor using the UMR, an in-depth understanding of the mechanisms of the UMR and its enhancement are highly desirable. In this study, the structural dependence of the UMR in Fe–Sn heterostructure devices is investigated. We find a weak dependence of the UMR on interface configuration in the oxide cap/Fe–Sn heterostructures on various oxide substrates. In contrast, the normalized UMR amplitude is enhanced by a factor of 27 with increasing Fe–Sn layer thickness from 4.0 to 100 nm. These results suggest that the magnetothermal effect dominates the large UMR in the Fe–Sn heterostructure devices.

Unidirectional Magnetoresistance in Antiferromagnet/Heavy-Metal Bilayers

Observation of a unidirectional magnetoresistance in an antiferromagnet offers a way to monitor the direction of microscopic magnetic moments in such devices, a crucial ability for future spintronics technologies.

Large Anomalous Unidirectional Magnetoresistance in a Single Ferromagnetic Layer

Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the spin Hall effect (SHE) provides a facile means of probing in-plane magnetization to avoid complex magnetic tunnel junctions. However, the UMR signal is very weak and usually requires a lock-in amplifier for detection, even in a bilayer involving $\mathrm{Ta}$ or $\mathrm{Pt}$ with a large spin Hall angle (SHA). Here, we report a type of UMR, termed the anomalous UMR (AUMR), in a single $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ layer without any adjacent SHE layers, where the UMR signal is about 10 times larger than that in $\mathrm{Ta}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ structures and can be detected by using conventional dc multimeters in the absence of lock-in amplifiers. We further demonstrate that the extracted AUMR, by excluding thermal contributions, shows reversal signs for the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ and $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Fe}$ single layers with opposite SHAs, indicating that the AUMR may originate from the self-generated spin accumulation interacting with magnetization through the giant-magnetoresistance-like mechanism. These results suggest that the AUMR contributes UMR signals larger than the interfacial spin Hall UMR in the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$-based systems, providing a convenient and reliable approach to detect in-plane magnetization for two-terminal spintronic devices.

Giant bipolar unidirectional photomagnetoresistance

Positive magnetoresistance (PMR) and negative magnetoresistance (NMR)describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR).

Efficient room-temperature magnetization direction detection by means of the enhanced anomalous Nernst effect in a Weyl ferromagnet

The topological nature of Heusler-alloy-based Weyl ferromagnet, Co${}_{2}$MnGa, results in an abundance of intriguing physical phenomena. In this work, a colossal enhancement of unidirectional magnetoresistance induced by the large anomalous Nernst effect is realized in nano-sized wires, which enables the operation of a prototypical non-volatile all-electric two-terminal memory device at room temperature. A size scaling investigation indicates that there is further room for improving the magnetoresistance ratio. These findings can pave a new avenue for creating efficient thermo-spintronic devices using novel topological materials.

Spin anomalous-Hall unidirectional magnetoresistance

We predict a spin anomalous-Hall unidirectional magnetoresistance (AH-UMR) in conducting bilayers composed of a ferromagnetic layer and a nonmagnetic layer, which does not rely on the spin Hall effect in the normal metal layer---in stark contrast to the well-studied unidirectional spin-Hall magnetoresistance---but instead arises from the spin anomalous Hall effect in the ferromagnetic layer. Physically, it is the charge-spin conversion induced by the spin anomalous Hall effect that conspires with the structural inversion asymmetry to generate a net nonequilibrium spin density in the ferromagnetic layer, which, in turn, modulates the resistance of the bilayer when the direction of the applied current or the magnetization is reversed. The dependencies of the spin AH-UMR effect on materials and geometric parameters are analyzed and compared with other nonlinear magnetoresistances. In particular, we show that, in magnetic bilayers where spin anomalous Hall and spin Hall effects are comparable, the overall UMR may undergo a sign change when the thickness of either layer is varied, suggesting a scheme to quantify the spin Hall or spin anomalous Hall angle via a nonlinear transport measurement.