Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque

All-electrical perpendicular switching of chiral antiferromagnetic order

Antiferromagnets as active components in devices have been attracting attention due to their negligible stray field and ultrafast magnetic dynamics, which are promising for high-density and fast memory devices. Although the switching of antiferromagnetic (AFM) order by current-induced spin-orbit torque (SOT) has already been realized, field-free SOT-induced bidirectional switching of AFM order in perpendicular geometry has been elusive. Here we experimentally demonstrate field-free perpendicular switching of the magnetic octupole moct in chiral AFM Mn3Sn by combining in-plane and out-of-plane SOTs generated by two-dimensional van der Vaals WTe2. The out-of-plane SOT breaks the in-plane inversion symmetry and leads to deterministic bidirectional switching of moct in polycrystalline Mn3Sn even without a fixed perpendicular moct easy axis. The switching ratio reaches up to 80% compared to 20-30% in polycrystalline Mn3Sn and the critical current density is reduced by an order of magnitude to around 1 MA cm-2. Our work promotes the development of chiral AFM devices toward practical application.

Spin-torque-driven gigahertz magnetization dynamics in the non-collinear antiferromagnet Mn3Sn

Non-collinear antiferromagnets, such as Mn3Sn, stand out for their topological properties and potential in antiferromagnetic spintronics. This emerging field aims at harnessing ultrafast magnetization dynamics of antiferromagnets through spin torques. Here we report the time-resolved dynamics of Mn3Sn on a picosecond timescale, driven by an optically induced spin current pulse. Our results reveal that the magnetization of Mn3Sn tilts immediately after the spin current pulse and subsequently undergoes 70 GHz precession. This immediate tilting underscores the predominant role of damping-like torque stemming from spin current absorption by Mn3Sn. We also determine the spin coherence length of Mn3Sn to be approximately 15 nm. This value substantially exceeds that of ferromagnets, highlighting a distinct spin-dephasing process in non-collinear antiferromagnets. Our results hold promise for ultrafast applications of non-collinear antiferromagnets and enrich our understanding of their spin-transfer physics.

Spintronic devices and applications using noncollinear chiral antiferromagnets

Antiferromagnetic materials have several unique properties, such as a vanishingly small net magnetization, which generates weak dipolar fields and makes them robust against perturbation from external magnetic fields and rapid magnetization dynamics, as dictated by the geometric mean of their exchange and anisotropy energies. However, experimental and theoretical techniques to detect and manipulate the antiferromagnetic order in a fully electrical manner must be developed to enable advanced spintronic devices with antiferromagnets as their active spin-dependent elements. Among the various antiferromagnetic materials, conducting antiferromagnets offer high electrical and thermal conductivities and strong electron-spin-phonon interactions. Noncollinear metallic antiferromagnets with negative chirality, including Mn3Sn, Mn3Ge, and Mn3GaN, offer rich physics of spin momentum locking, topologically protected surface states, large spin Hall conductivity, and a magnetic spin Hall effect that arises from their topology. In this review article, we introduce the crystal structure and the physical phenomena, including the anomalous Hall and Nernst effects, spin Hall effect, and magneto-optic Kerr effect, observed in negative chirality antiferromagnets. Experimental advances related to spin-orbit torque-induced dynamics and the impact of the torque on the microscopic spin structure of Mn3Sn are also discussed. Recent experimental demonstrations of a finite room-temperature tunneling magnetoresistance in tunnel junctions with chiral antiferromagnets opens the prospect of developing spintronic devices with fully electrical readout. Applications of chiral antiferromagnets, including non-volatile memory, high-frequency signal generators/detectors, neuro-synaptic emulators, probabilistic bits, thermoelectric devices, and Josephson junctions, are highlighted. We also present analytic models that relate the performance characteristics of the device with its design parameters, thus enabling a rapid technology-device assessment. Effects of Joule heating and thermal noise on the device characteristics are briefly discussed. We close the paper by summarizing the status of research and present our outlook in this rapidly evolving research field.

Electrical mutual switching in a noncollinear-antiferromagnetic-ferromagnetic heterostructure

Spin-orbit torque (SOT) provides a promising mechanism for electrically encoding information in magnetic states. Unlike existing schemes, where the SOT is passively determined by the material and device structures, an active manipulation of the intrinsic SOT polarity would allow for flexibly programmable SOT devices. Achieving this requires electrical control of the current-induced spin polarization of the spin source. Here we demonstrate a proof-of-concept current-programmed SOT device. Using a noncollinear-antiferromagnetic/nonmagnetic/ferromagnetic Mn3Sn/Mo/CoFeB heterostructure at zero magnetic field, we show current-induced switching in the CoFeB layer due to the spin current polarized by the magnetic structure of the Mn3Sn; by properly tuning the driving current, the spin current from the CoFeB further reverses the magnetic orientation of the Mn3Sn, which determines the polarity of the subsequent switching of the CoFeB. This scheme of mutual switching can be achieved in a spin-valve-like simple protocol because each magnetic layer serves as a reversible spin source and target magnetic electrode. It yields intriguing proof-of-concept functionalities for unconventional logic and neuromorphic computing.

Antiferromagnetic spin-torque diode effect in a kagome Weyl semimetal

Spintronics based on ferromagnets has enabled the development of microwave oscillators and diodes. To achieve even faster operation, antiferromagnets hold great promise despite their challenging manipulation. So far, controlling antiferromagnetic order with microwave currents remains elusive. Here we induce the coherent rotation of antiferromagnetic spins in a Weyl antiferromagnet W/Mn3Sn epitaxial bilayer by DC spin-orbit torque. We show the efficient coupling of this spin rotation with microwave current. The coupled dynamics produce a DC anomalous Hall voltage through rectification, which we coin the antiferromagnetic spin-torque diode effect. Unlike in ferromagnetic systems, the output voltage shows minimal dependence on frequency because of the stabilization of the precession cone angle by exchange interactions. Between 10 GHz and 30 GHz, the output voltage decreases by only 10%. Numerical simulations further reveal that the rectification signals arise from the fast frequency modulation of chiral spin rotation by microwave spin-orbit torque. These results may help the development of high-speed microwave devices for next-generation telecommunication applications.

Anomalous Nernst Effect-Based Near-Field Imaging of Magnetic Nanostructures

The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially resolved ANE measurements beyond the optical diffraction limit. Such a method has been previously used to map in-plane magnetized magnetic textures. However, the spatial distribution of the out-of-plane temperature gradient, which is needed to fully interpret such ANE-based imaging, was not studied. We therefore use a well-known magnetic texture, a magnetic vortex core, to demonstrate the reliability of the ANE method for imaging of magnetic domains with nanoscale resolution. Moreover, since the ANE signal is directly proportional to the temperature gradient, we can also consider the inverse problem and deduce information about the nanoscale temperature distribution. Our results together with finite element modeling indicate that besides the out-of-plane temperature gradients there are even larger in-plane temperature gradients. Thus, we extend the ANE imaging to study the out-of-plane magnetization in a racetrack nanowire by detecting the ANE signal generated by in-plane temperature gradients. In all cases, a spatial resolution of ≈70 nm is obtained. These results are significant for the rapidly growing field of thermoelectric imaging of antiferromagnetic spintronic device structures.

Non-relativistic torque and Edelstein effect in non-collinear magnets

The Edelstein effect is the origin of the spin-orbit torque: a current-induced torque that is used for the electrical control of ferromagnetic and antiferromagnetic materials. This effect originates from the relativistic spin-orbit coupling, which necessitates utilizing materials with heavy elements. Here, we show that in magnetic materials with non-collinear magnetic order, the Edelstein effect and, consequently, a current-induced torque can exist even in the absence of the spin-orbit coupling. Using group symmetry analysis, model calculations, and realistic simulations on selected compounds, we identify large classes of non-collinear magnet candidates and demonstrate that the current-driven torque is of similar magnitude as the celebrated spin-orbit torque in conventional transition metal structures. We also show that this torque can exist in an insulating material, which could allow for highly efficient electrical control of magnetic order.

Ferromagnetic resonance measurement with frequency modulation down to 2 K

Ferromagnetic resonance (FMR) spectroscopy is a powerful technique to study the precessional dynamics of magnetization in thin film heterostructures. It provides valuable information about the mechanisms of exchange bias, spin angular momentum transfer across interfaces, and excitation of magnons. A key desirable feature of FMR spectrometers is the capability to study magnetization dynamics over a wide phase space of temperature (T), frequency (f), and magnetic field (B). The design, fabrication, and testing of such a spectrometer, which uses frequency modulation techniques for improved detection of microwave absorption, reduces heat load in the cryostat and allows simultaneous measurements of inverse spin Hall effect (ISHE) induced dc voltages, is described in this paper. The apparatus is based on a 2-port transmitted microwave signal measurement using a grounded co-planar waveguide. The input radio frequency (RF) signal, frequency modulated at a tunable f-band, excites spin precession in the sample, and the attenuated RF signal is measured phase sensitively. The sample stage, inserted in the bore of a superconducting solenoid, allows magnetic field and temperature variability of 0 to ±5 T and 2-310 K, respectively. We demonstrate the working of this Cryo-FMR and ISHE spectrometer on thin films of Ni80Fe20 and Fe60Co20B20 over a wide T, B, and f phase space.

Electrical 180° switching of Néel vector in spin-splitting antiferromagnet

Antiferromagnetic spintronics have attracted wide attention due to its great potential in constructing ultradense and ultrafast antiferromagnetic memory that suits modern high-performance information technology. The electrical 180° switching of Néel vector is a long-term goal for developing electrical-controllable antiferromagnetic memory with opposite Néel vectors as binary "0" and "1." However, the state-of-art antiferromagnetic switching mechanisms have long been limited for 90° or 120° switching of Néel vector, which unavoidably require multiple writing channels that contradict ultradense integration. Here, we propose a deterministic switching mechanism based on spin-orbit torque with asymmetric energy barrier and experimentally achieve electrical 180° switching of spin-splitting antiferromagnet Mn5Si3. Such a 180° switching is read out by the Néel vector-induced anomalous Hall effect. On the basis of our writing and readout methods, we fabricate an antiferromagnet device with electrical-controllable high- and low-resistance states that accomplishes robust write and read cycles. Besides fundamental advance, our work promotes practical spin-splitting antiferromagnetic devices based on spin-splitting antiferromagnet.

Effective electrical manipulation of a topological antiferromagnet by orbital torques

The electrical control of the non-trivial topology in Weyl antiferromagnets is of great interest for the development of next-generation spintronic devices. Recent studies suggest that the spin Hall effect can switch the topological antiferromagnetic order. However, the switching efficiency remains relatively low. Here, we demonstrate the effective manipulation of antiferromagnetic order in the Weyl semimetal Mn3Sn using orbital torques originating from either metal Mn or oxide CuOx. Although Mn3Sn can convert orbital current to spin current on its own, we find that inserting a heavy metal layer, such as Pt, of appropriate thickness can effectively reduce the critical switching current density by one order of magnitude. In addition, we show that the memristor-like switching behaviour of Mn3Sn can mimic the potentiation and depression processes of a synapse with high linearity-which may be beneficial for constructing accurate artificial neural networks. Our work paves a way for manipulating the topological antiferromagnetic order and may inspire more high-performance antiferromagnetic functional devices.

Spin-torque-driven antiferromagnetic resonance

The intrinsic fast dynamics make antiferromagnetic spintronics a promising avenue for faster data processing. Ultrafast antiferromagnetic resonance-generated spin current provides valuable access to antiferromagnetic spin dynamics. However, the inverse effect, spin-torque-driven antiferromagnetic resonance (ST-AFMR), which is attractive for practical utilization of fast devices but seriously impeded by difficulties in controlling and detecting Néel vectors, remains elusive. We observe ST-AFMR in Y3Fe5O12/α-Fe2O3/Pt at room temperature. The Néel vector oscillates and contributes to voltage signal owing to antiferromagnetic negative spin Hall magnetoresistance-induced spin rectification effect, which has the opposite sign to ferromagnets. The Néel vector in antiferromagnetic α-Fe2O3 is strongly coupled to the magnetization in Y3Fe5O12 buffer, resulting in the convenient control of Néel vectors. ST-AFMR experiment is bolstered by micromagnetic simulations, where both the Néel vector and the canted moment of α-Fe2O3 are in elliptic resonance. These findings shed light on the spin current-induced dynamics in antiferromagnets and represent a step toward electrically controlled antiferromagnetic terahertz emitters.

Efficient Noncollinear Antiferromagnetic State Switching Induced by the Orbital Hall Effect in Chromium

Recently, orbital Hall current has attracted attention as an alternative method to switch the magnetization of ferromagnets. Here we present our findings on electrical switching of the antiferromagnetic state in Mn3Sn/Cr, where despite the much smaller spin Hall angle of Cr, the switching current density is comparable to heavy metal-based heterostructures. However, the inverse process, i.e., spin-to-charge conversion in Cr-based heterostructures, is much less efficient than the Pt-based equivalents, as manifested in the 1 order of magnitude smaller terahertz emission intensity and spin current-induced magnetoresistance. These results in combination with the slow decay of terahertz emission against Cr thickness (diffusion length of ∼11 nm) suggest that the observed magnetic switching can be attributed to orbital current generation in Cr, followed by efficient conversion to spin current. Our work demonstrates the potential of light metals like Cr as efficient orbital/spin current sources for antiferromagnetic spintronics.

Anomalous spin current anisotropy in a noncollinear antiferromagnet

Cubic materials host high crystal symmetry and hence are not expected to support anisotropy in transport phenomena. In contrast to this common expectation, here we report an anomalous anisotropy of spin current can emerge in the (001) film of Mn3Pt, a noncollinear antiferromagnetic spin source with face-centered cubic structure. Such spin current anisotropy originates from the intertwined time reversal-odd ([Formula: see text]-odd) and time reversal-even ([Formula: see text]-even) spin Hall effects. Based on symmetry analyses and experimental characterizations of the current-induced spin torques in Mn3Pt-based heterostructures, we find that the spin current generated by Mn3Pt (001) exhibits exotic dependences on the current direction for all the spin components, deviating from that in conventional cubic systems. We also demonstrate that such an anisotropic spin current can be used to realize low-power spintronic applications such as the efficient field-free switching of the perpendicular magnetizations.

Handedness anomaly in a non-collinear antiferromagnet under spin-orbit torque

Non-collinear antiferromagnets are an emerging family of spintronic materials because they not only possess the general advantages of antiferromagnets but also enable more advanced functionalities. Recently, in an intriguing non-collinear antiferromagnet Mn3Sn, where the octupole moment is defined as the collective magnetic order parameter, spin-orbit torque (SOT) switching has been achieved in seemingly the same protocol as in ferromagnets. Nevertheless, it is fundamentally important to explore the unknown octupole moment dynamics and contrast it with the magnetization vector of ferromagnets. Here we report a handedness anomaly in the SOT-driven dynamics of Mn3Sn: when spin current is injected, the octupole moment rotates in the opposite direction to the individual moments, leading to a SOT switching polarity distinct from ferromagnets. By using second-harmonic and d.c. magnetometry, we track the SOT effect onto the octupole moment during its rotation and reveal that the handedness anomaly stems from the interactions between the injected spin and the unique chiral-spin structure of Mn3Sn. We further establish the torque balancing equation of the magnetic octupole moment and quantify the SOT efficiency. Our finding provides a guideline for understanding and implementing the electrical manipulation of non-collinear antiferromagnets, which in nature differs from the well-established collinear magnets.

Generation of out-of-plane polarized spin current by spin swapping

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn₃Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn₃Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn₃Sn layer and whose temperature dependence follows that of this layer's antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn₃Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.

Coherent antiferromagnetic spintronics

Antiferromagnets have attracted extensive interest as a material platform in spintronics. So far, antiferromagnet-enabled spin-orbitronics, spin-transfer electronics and spin caloritronics have formed the bases of antiferromagnetic spintronics. Spin transport and manipulation based on coherent antiferromagnetic dynamics have recently emerged, pushing the developing field of antiferromagnetic spintronics towards a new stage distinguished by the features of spin coherence. In this Review, we categorize and analyse the critical effects that harness the coherence of antiferromagnets for spintronic applications, including spin pumping from monochromatic antiferromagnetic magnons, spin transmission via phase-correlated antiferromagnetic magnons, electrically induced spin rotation and ultrafast spin-orbit effects in antiferromagnets. We also discuss future opportunities in research and applications stimulated by the principles, materials and phenomena of coherent antiferromagnetic spintronics.

All-electrical reading and writing of spin chirality

Spintronics promises potential data encoding and computing technologies. Spin chirality plays a very important role in the properties of many topological and noncollinear magnetic materials. Here, we propose the all-electrical detection and manipulation of spin chirality in insulating chiral antiferromagnets. We demonstrate that the spin chirality in insulating epitaxial films of TbMnO3 can be read electrically via the spin Seebeck effect and can be switched by electric fields via the multiferroic coupling of the spin chirality to the ferroelectric polarization. Moreover, multivalued states of the spin chirality can be realized by the combined application of electric and magnetic fields. Our results are a path toward next-generation, low-energy consumption memory and logic devices that rely on spin chirality.

Magnetization switching in polycrystalline Mn3Sn thin film induced by self-generated spin-polarized current

Electrical manipulation of spins is essential to design state-of-the-art spintronic devices and commonly relies on the spin current injected from a second heavy-metal material. The fact that chiral antiferromagnets produce spin current inspires us to explore the magnetization switching of chiral spins using self-generated spin torque. Here, we demonstrate the electric switching of noncollinear antiferromagnetic state in Mn₃Sn by observing a crossover from conventional spin-orbit torque to the self-generated spin torque when increasing the MgO thickness in Ta/MgO/Mn₃Sn polycrystalline films. The spin current injection from the Ta layer can be controlled and even blocked by varying the MgO thickness, but the switching sustains even at a large MgO thickness. Furthermore, the switching polarity reverses when the MgO thickness exceeds around 3 nm, which cannot be explained by the spin-orbit torque scenario due to spin current injection from the Ta layer. Evident current-induced switching is also observed in MgO/Mn₃Sn and Ti/Mn₃Sn bilayers, where external injection of spin Hall current to Mn₃Sn is negligible. The inter-grain spin-transfer torque induced by spin-polarized current explains the experimental observations. Our findings provide an alternative pathway for electrical manipulation of non-collinear antiferromagnetic state without resorting to the conventional bilayer structure.

Under an applied current, chiral antiferromagnets, such as Mn₃Sn, can produce a spin-polarized current. Here, by varying the thickness of a buffering layer, the authors show that this spin-polarized current can drive self-induced switching in polycrystalline Mn₃Sn.