We report the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale Pt/$α$-Fe$_2$O$_3$ Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching, with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Further, analysis of images taken with different x-ray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that -in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching-changes in AFM order can be induced by purely thermal effects.
We demonstrate efficient current induced spin orbit torque switching in perpendicularly magnetized epitaxial MgO(001)//Pd/Co2FeAl/MgO heterostructures grown by magnetron sputtering. The advantage of such heterostructures for spin orbit torque MRAM devices is that they allow a unique combination of critical ingredients: i) low resistivity required for reduced electric energy consumption during writing, ii) high TMR ratio required for efficient reading and iii) strong perpendicular magnetic anisotropy for increased thermal stability.
While the electrical current manipulation of antiferromagnets (AFMs) has been demonstrated, the extent of the studied AFM materials has been limited with few systematic experiments and a poor understanding. We compare the electrical current switching of the exchange-bias field ($H_{ex}$) in AFM-Mn$_3A$N/ferromagnet-Co$_3$FeN bilayers. An applied pulse current can manipulate $H_{ex}$ with respect to the current density and FM layer magnetization, which shifts exponentially as a function of the current density. We found that the saturation current density and exponential decay constant $τ$ incre…
We demonstrate stable and reversible current induced switching of large-area ($> 100\;μm^2$) antiferromagnetic domains in NiO/Pt by performing concurrent transport and magneto-optical imaging measurements in an adapted Kerr microscope. By correlating the magnetic images of the antiferromagnetic domain changes and magneto-transport signal response in these current-induced switching experiments, we disentangle magnetic and non-magnetic contributions to the transport signal. Our table-top approach establishes a robust procedure to subtract the non-magnetic contributions in the transport signal an…
The spin-orbit torques (SOTs) generated from topological insulators (TIs) have gained increasing attention in recent years. These TIs, which are typically formed by epitaxially grown chalcogenides, possess extremely high SOT efficiencies and have great potential to be employed in the next-generation spintronics devices. However, epitaxy of these chalcogenides is required to ensure the existence of topologically-protected surface state (TSS), which limits the feasibility of using these materials in industry. In this work, we show that non-epitaxial Bi$_{x}$Te$_{1-x}$/ferromagnet heterostructure…
Reverse and stable bipolar resistive switching effect was observed in planar NdCeCuO - NdCeO - Ag heterostructures. It was shown that the current voltage charactereriscs of the BRSE observed has a diode character. Simulations were used to consider the influence of the nonuniform distribution of an electric field at the interface of a heterojunction on the effect of bipolar resistive switching in investigated structures. The inhomogeneous distribution of the electric field near the contact edge creates regions of higher electric field strength which, in turn, stimulates motion and redistributio…
We report direct imaging of boundary magnetization associated with antiferromagnetic domains in magnetoelectric epitaxial Cr2O3 thin films using diamond nitrogen vacancy microscopy. We found a correlation between magnetic domain size and structural grain size which we associate with the domain formation process. We performed field cooling, i.e., cooling from above to below the Néel temperature in the presence of magnetic field, which resulted in the selection of one of the two otherwise degenerate 180 degree domains. Lifting of such a degeneracy is achievable with a magnetic field alone due to…
Antiferromagnetic materials are of great interest for spintronics. Here we present a comprehensive study of the growth, structural characterization, and resulting magnetic properties of thin films of the non-collinear antiferromagnet Mn$_{3}$Ir. Using epitaxial engineering on MgO (001) and Al$_{2}$O$_{3}$ (0001) single crystal substrates, we control the growth of cubic $γ$-Mn$_{3}$Ir in both (001) and (111) crystal orientations, and discuss the optimization of growth conditions to achieve high-quality crystal structures with low surface roughness. Exchange bias is studied in bilayers, with exc…
We investigate the current-induced switching of the Neel order in NiO(001)/Pt heterostructures,which is manifested electrically via the spin Hall magnetoresistance. Significant reversible changes in the longitudinal and transverse resistances are found at room temperature for a current threshold lying in the range of 10^7 A/cm^2. The order-parameter switching is ascribed to the antiferromagnetic dynamics triggered by the (current-induced) antidamping torque, which orients the Neel order towards the direction of the writing current. This is in stark contrast to the case of antiferromagnets such…
We show that Co spins in Co/FeRh epitaxial bilayers grown on W(110) switch reversibly between the two orthogonal in-plane directions as the FeRh layer undergoes temperature driven antiferromagnetic-ferromagnetic (AFM-FM) phase transition. Switching of Co magnetization is characterized by a hysteretic behavior owing to a temperature hysteresis of the AFM-FM transition in FeRh. The spin reorientation of Co is driven by the evolution interfacial exchange coupling to FeRh system across the AFM-FM process. Our results provide a new method of writing information purely by a local temperature change.
The ability to manipulate antiferromagnetic (AF) moments is a key requirement for the emerging field of antiferromagnetic spintronics. Electrical switching of bi-state AF moments has been demonstrated in metallic AFs, CuMnAs and Mn$_2$Au. Recently, current-induced "saw-tooth" shaped Hall resistance was reported in Pt/NiO bilayers, while its mechanism is under debate. Here, we report the first demonstration of convincing, non-decaying, step-like electrical switching of tri-state Néel order in Pt/$α$-Fe$_2$O$_3$ bilayers. Our experimental data, together with Monte-Carlo simulations, reveal the c…
The magnetic order in antiferromagnetic (AF) materials is hard to control with external magnetic fields. However, recent advances in detecting and manipulating AF order electrically have opened up new prospects for these materials in basic and applied spintronics research. Using x-ray magnetic linear dichroism microscopy, we show here that staggered effective fields generated by electrical current can induce reproducible and reversible modification of the antiferromagnetic domain structure in microdevices fabricated from a tetragonal CuMnAs thin film. The current-induced domain switching is in…
We demonstrate reproducible voltage induced non-volatile switching of the magnetization in an epitaxial thin Fe81Ga19 film. Switching is induced at room temperature and without the aid of an external magnetic field. This is achieved by the modification of the magnetic anisotropy by mechanical strain induced by a piezoelectric transducer attached to the layer. Epitaxial Fe81Ga19 is shown to possess the favourable combination of cubic magnetic anisotropy and large magnetostriction necessary to achieve this functionality with experimentally accessible levels of strain. The switching of the magnet…
The processional switching mechanism governs magnetic switching in magnetic tunnel junctions (MTJs) in the sub-nanosecond range, which limits the application of spin transfer torque magnetic random access memory (STT-MRAM) in the ultrafast region. In this paper, we propose a new picosecond magnetic switching mechanism in a synthetic antiferromagnetic (SAF) structure using the adjustable Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction controlled by an external electric field (E-field). It is shown that along with the sign change of the RKKY interaction in the SAF structure with an external E-f…
Recent demonstrations of the electrical switching of antiferromagnets (AFs) have given an enormous impulse to the field of AF spintronics. Many of these observations are plagued by non-magnetic effects that are very difficult to distinguish from the actual magnetic ones. Here, we study the electrical switching of thin (5 nm) NiO films in Pt/NiO devices using magnetic fields up to 15 T to quantitatively disentangle these magnetic and non-magnetic effects. We demonstrate that these fields suppress the magnetic components of the electrical switching of NiO, but leave the non-magnetic components i…
Multi-rotational switching in a noncollinear antiferromagnet by spin-orbit torque
Tohoku University（东北大学）；National Institute for Materials Science（物质材料研究机构）；National Institutes for Quantum Science and Technology（量子科学技术研究开发机构）；Inamori Research Institute for Science（稻盛科学研究所）
We unravel the origin of current-induced magnetic switching of insulating antiferromagnet/heavy metal systems. We utilize concurrent transport and magneto-optical measurements to image the switching of antiferromagnetic domains in specially engineered devices of NiO/Pt bilayers. Different electrical pulsing and device geometries reveal different final states of the switching with respect to the current direction. We can explain these through simulations of the temperature induced strain and we identify the thermomagnetoelastic switching mechanism combined with thermal excitations as the origin…
We proposed a novel and simple multilayer structure to realize the deterministic switching of perpendicularly magnetized layers (Co in our work) by spin orbital torque from the spin Hall Effect in this paper. A pinned layer is introduced, antiferromagnetically coupled to the magnetic Co layer. We have confirmed the deterministic switching of perpendicularly magnetized layers from perspectives of switching loops, trajectories and switching properties under periodic spin currents through micromagnetic simulation. The antiferromagnetic interaction accounts for the deterministic switching exhibite…
We analyze the complex impact of the local magnetic spin texture on the transverse Hall-type voltage in device structures utilized to measure magnetoresistance effects. We find a highly localized and asymmetric magnetic sensitivity in the eight-terminal geometries that are frequently used in current-induced switching experiments, for instance to probe antiferromagnetic materials. Using current-induced switching of antiferromagnetic NiO/Pt as an example, we estimate the change in the spin Hall magnetoresistance signal associated with switching events based on the domain switching patterns obser…
We demonstrate tunnel magnetoresistance (TMR) junctions based on a tri layer system consisting of an epitaxial NiMnSb, aluminum oxide and CoFe tri layer. The junctions show a tunnelling magnetoresistance of Delta R/R of 8.7% at room temperature which increases to 14.7% at 4.2K. The layers show clear separate switching and a small ferromagnetic coupling. A uniaxial in plane anisotropy in the NiMnSb layer leads to different switching characteristics depending on the direction in which the magnetic field is applied, an effect which can be used for sensor applications.
FeSn is a topological semimetal (TSM) and kagome antiferromagnet (AFM) composed of alternating Fe3Sn kagome planes and honeycomb Sn planes. This unique structure gives rise to exotic features in the band structures such as the coexistence of Dirac cones and flatbands near the Fermi level, fully spin-polarized 2D surface Dirac fermions, and the ability to open a large gap in the Dirac cone by reorienting the Néel vector. In this work, we report the synthesis of high quality epitaxial (0001) FeSn films by magnetron sputtering. Using FeSn/Py heterostructures, we show a large exchange bias effect …
Antiferromagnetic materials hold promising prospects in novel types of spintronics applications. Assessing the stability of antiferromagnetic nanostructures against thermal excitations is a crucial aspect of designing devices with a high information density. Here we use theoretical calculations and numerical simulations to determine the mean switching time of antiferromagnetic nanoparticles in the superparamagnetic limit. It is demonstrated that the thermal stability is drastically reduced compared to ferromagnetic particles in the limit of low Gilbert damping, attributed to the exchange enhan…
Switching kinetics in ultrathin epitaxial BaTiO3 films confirms the existence of the homogeneous switching (without domains) in ultrathin ferroelectric films. We suppose that the homogeneous switching in ultrathin films is a common phenomenon for all ferroelectric materials.
The manipulation of antiferromagnetic order by means of spin-orbit torques opens unprecedented opportunities to exploit the dynamics of antiferromagnets in spintronic devices. In this work, we investigate the current-induced switching of the magnetic octupole vector in the Weyl antiferromagnet Mn$_3$Sn as a function of pulse shape, field, temperature, and time. We find that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements reveal that Mn$_3$Sn switching proceeds via a two-step demagnetiza…
A spin-torque ferromagnetic resonance study is performed in epitaxial $\mathrm{Fe / Ir_{15}Mn_{85}}$ bilayers with different Fe thicknesses. We measure a negative spin-Hall angle of a few percent in the antiferromagnetic IrMn in contrast to previously reported positive values. A large spin-orbit field with Rashba symmetry opposing the Oersted field is also present. Magnitudes of measured spin-orbit torques depend on the crystallographic direction of current and are correlated with the exchange bias direction set during growth. We suggest that the uncompensated moments at the Fe / IrMn interfac…
Gate-controlled supercurrent (GCS) in superconductor nanobridges has recently attracted attention as a means to create superconducting field effect transistors. Despite the clear advantage for applications with low power consumption and high switching speeds, the microscopic mechanism of the field effect is still under debate. In this work, we realize GCS for the first time in an epitaxial superconductor, which is created as a shell on an InAs nanowire. We show that the supercurrent in the epitaxial Al layer can be switched to the normal state by applying $\simeq\pm$ 23$\,$V on a bottom gate i…
There is accelerating interest in developing memory devices using antiferromagnetic (AFM) materials, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects. Recent studies have shown, however, that high current densities create non-magnetic contributions to resistive switching signals in AFM/heavy metal (AFM/HM) bilayers, complicating their interpretation. Here we introduce an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures, and d…
Electrical and optical pulsing allow for manipulating the order parameter and magnetoresistance of antiferromagnets, opening novel prospects for digital and analog data storage in spintronic devices. Recent experiments in CuMnAs have demonstrated giant resistive switching signals in single-layer antiferromagnetic films together with analog switching and relaxation characteristics relevant for neuromorphic computing. Here we report simultaneous electrical pulsing and scanning NV magnetometry of antiferromagnetic domains in CuMnAs performed using a pump-probe scheme. We observe a nano-scale frag…
We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic CuMnAs thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the time scale of the suppression. As we do not observe a direct impact of the optical pulse on the state of CuMnAs, all observed effects are primarily mediated by the substrate. The sample region of …
We present theoretical description of the precessional switching processes induced by simultaneous application of spin-polarized current and external magnetic field to antiferromagnetic component of the "pinned" layer. We found stability ranges of different static and dynamic regimes. We showed the possibility of steady current-induced precession of antiferromagnetic vector with frequency that linearly depends on the bias current. Furthermore, we found an optimal duration of current pulse required for switching between different orientations of antiferromagnetic vector and current and field de…
We investigate magnetic anisotropy and magnetization reversal mechanism in chromium telluride thin films grown by molecular beam epitaxy. We report existence of strong perpendicular anisotropy in these thin films, along with a relatively strong second order anisotropy contribution. The angular variation of the switching field observed from the magnetoresistance measurement is explained quantitatively using a one-dimensional defect model. The model reveals the relative roles of nucleation and pinning in the magnetization reversal, depending on the applied field orientation. Micromagnetic simula…
We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ~10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ~0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of GNRs, the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature, and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostat…
Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. Harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect in heavy-metal/ferromagnet systems. However, the harmonic measurement technique has never been verified in antiferromagnetic heterostructures. Here, we report for the first time harmonic m…
The theory behind the electrical switching of antiferromagnets is premised on the existence of a well defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. In this study, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet Fe$_{1/3+δ}$NbS$_2$…
Electrical manipulation of spin textures inside antiferromagnets represents a new opportunity for developing spintronics with superior speed and high device density. Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque-induced switching is however still challenging due to the complicated interactions from different sublattices. Meanwhile, because of the diminishing magnetic susceptibility, the nature and the magnitude of current-induced magnetic dynamics remain poorly characterized in antiferromagnets, whereas spurious effects further complicate experimental …
Picosecond switching of the staggered antiferromagnetic order is shown to be realizable through spin-transfer torques from a short current pulse. The coupled dynamics of sublattice magnetization is mapped onto a classical pendulum subject to gravity and a driving pulse, where switching occurs if the pendulum acquires sufficient kinetic energy during the pulse to overcome the maximum of the effective gravity potential. The optimal switching scheme is explored through the dependence of switch angle and magnetic loss on the duration and strength of the current pulse. The physics discussed here pr…
We report on the enhancement of antiferromagnetic coupling in epitaxial Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using the ballistic electron magnetic microscopy we established that the hot-electron collector current reflects magnetization alignment and the magnetocurrent exceeds 200% at room temperature. The saturation magnetic field for collector current corresponding to parallel alignment of magnetizations rises up with the tunneling current, thus demonstrating stabilization of the antiparallel alignment and increasing antiferromagnetic coupling. We connect t…
Antiferromagnets (AFMs) are widely believed to be superior than ferromagnets in spintronics because of their high stability due to the vanishingly small stray field. It is thus expected that the order parameter of AFM should always align along the easy-axis of the crystalline anisotropy. In contrast to this conventional wisdom, we find that the AFM order parameter switches away from the easy-axis below a critical anisotropy strength when an AFM is properly tailored into a nano-structure. The switching time first decreases and then increases with the damping. Above the critical anisotropy, the …
Exchange bias is one of the most extensively studied phenomena in magnetism, since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled to an antiferromagnet (AFM) and the control of the exchange bias is therefore very important for technological applications, such as magnetic random access memory and giant magnetoresistance sensors. In this letter, we report the crystal structure manipulation of the exchange bias in epitaxial hcp Cr2O3 films. By epitaxially growing twined (10-10) oriented Cr2O3 thin films, of which the c axis and spins of the Cr atoms lie in the film plane…
We report time-resolved measurements of current-induced reversal of a free magnetic layer in Py/Cu/Py elliptical nanopillars at temperatures T = 4.2 K to 160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations of the free layer switching yields numerical values for the spin torque and the Gilbert damping parameters as functions of T. The damping is strongly T-dependent, which we attribute to the antiferromagnetic pinning behavior of a thin permalloy oxide layer around the perimeter of the free layer. This adventitious antiferromagnetic pinning layer can have a major impa…
Recent advances in tuning the correlated behavior of graphene and transition-metal dichalcogenides (TMDs) have opened a new frontier in the study of many-body physics in two dimensions and promise exciting possibilities for new quantum technologies. An emerging field where these materials have yet to make a deep impact is the study of antiferromagnetic (AFM) spintronics - a relatively new research direction that promises technologies that are insensitive to external magnetic fields, fast switching times, and reduced crosstalk. In this study we present measurements on the intercalated TMD Fe1/3…
We study current-induced switching of the Néel vector in CoO/Pt bilayers to understand the underlaying antiferromagnetic switching mechanism. Surprisingly, we find that for ultra-thin CoO/Pt bilayers electrical pulses along the same path can lead to an increase or decrease of the spin Hall magnetoresistance signal, depending on the current density of the pulse. By comparing the results of these electrical measurements to XMLD-PEEM imaging of the antiferromagnetic domain structure before and after the application of current pulses, we reveal the reorientation of the Néel vector in ultra-thin Co…
Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step …
We demonstrate non-volatile, n-type, back-gated, MoS$_{2}$ transistors, placed directly on an epitaxial grown, single crystalline, PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) ferroelectric. The transistors show decent ON current (19 $μA/μ$m), high on-off ratio (10$^{7}$), and a subthreshold swing of (SS ~ 92 mV/dec) with a 100 nm thick PZT layer as the back gate oxide. Importantly, the ferroelectric polarization can directly control the channel charge, showing a clear anti-clockwise hysteresis. We have selfconsistently confirmed the switching of the ferroelectric and corresponding change in channel cu…
Symmetry Enhanced Unconventional Spin Current Anisotropy in a Collinear Antiferromagnet
University of Gothenburg（哥德堡大学）；Research Institute of Electrical Communication, Tohoku University（东北大学电气通信研究所）；Indian Institute of Technology Delhi（德里印度理工学院）
