Abstract
In this review article, we summarize some recent key results in the development of antiferromagnetic spintronics. Currentinduced switching of the Néel vector orientation has now been established in a wide range of antiferromagnetic films and antiferromagnet / heavy metal bilayers, as well as currentdriven motion of antiferromagnetic spin textures. The latter are particularly promising due to their small size and topological stability, but reading their magnetic state presents challenges. We also focus on materials whose compensated spin arrangements (either collinear or noncollinear) are coexistent with a spinsplit band structure, enabling firstorder spintronic phenomena including giant and tunneling magnetoresistance, and the anomalous Hall effect. The resulting combination of efficient electrical readout mechanisms with the advantages of a nearzero net magnetization has potential to be transformative for spintronic applications.
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Introduction
Spintronics, which utilizes the quantum mechanical spin of electrons alongside their charge, introduces new paradigms for electronic devices. Ferromagnetic materials are the building blocks of spintronic technologies as their uncompensated magnetization can be easily manipulated using magnetic fields, and can be sensitively detected using giant or tunneling magnetoresistance. An antiferromagnet is characterized by a compensated spin arrangement, whose orientation is described by a Néel vector, L. The discovery of currentinduced magnetic torques which act on antiferromagnetically coupled spins  the socalled Néelorder spinorbit torques^{1}  has enabled the emergence of spintronic devices with antiferromagnetic (AF) materials as their primary components, offering distinct characteristics to their ferromagnetic counterparts. Their insensitivity to external magnetic fields can be beneficial in applications where stability in the presence of external perturbations is crucial, while their intrinsic dynamics can be orders of magnitude faster than in ferromagnets. The latter can result in terahertz frequency spin rotation and switching, and domain wall speeds that are not limited by Walker breakdown^{1,2,3}.Though recent predictions and experimental evidence suggests domain wall speeds in ferromagnets can in some situations approach that of antiferromagnets^{4,5}. Furthermore, AF materials are abundant and diverse, including insulators, semiconductors, metals and superconductors.
This review summarizes some key recent results in the development of antiferromagnetic spintronics. It is not intended to be an exhaustive summary. We focus on four overlapping areas. In Section “Currentinduced switching of antiferromagnetic devices” we evaluate the stateoftheart and current understanding of electrical switching of AF domains. Section “Topological antiferromagnetic spin textures” reviews the emergence of topological spin textures in AF materials and their electrical control, which show promise as stable, nanoscale units of magnetic information. We also discuss two classes of materials which exhibit physical phenomena more commonly associated with ferromagnets, regardless of a nearperfect compensation of their magnetic moments. Section “Noncollinear antiferromagnets” discusses spintronic phenomena observed in noncollinear antiferromagnets of the form Mn_{3}X, with X = Sn, Ge or Pt, where the topology of the spin structure results in unique magnetic properties. For these systems, the Néel vector is not an appropriate order parameter, and higher order multipole moments must be considered^{6}. Section “Altermagnetism” reviews the recently denominated field of altermagnetism, where firstorder magnetic effects are observed in collinear systems which meet certain symmetry conditions.
Currentinduced switching of antiferromagnetic devices
Stable orientations of local magnetic moments in both ferromagnets and antiferromagnets are separated by a magnetic anisotropy energy barrier. To produce a rotation between stable orientations, a torque must be applied to overcome this barrier. The torque can result from interaction with an either locally or globally spinpolarized current, and can have a fieldlike or dampinglike character. The spinpolarized carriers may be injected from a ferromagnetic polarizer layer, or they may result from spinorbit coupling. The latter presents the opportunity for magnetic memory devices which do not contain any ferromagnetic materials. For efficient currentinduced switching of an antiferromagnet, the effective field driving the torque must be staggered, i.e. alternating in sign between opposite spin sublattices.
Concepts of spinorbit torque driven switching in AF layers were first developed in around 2014^{1}, and were experimentally realized soon after^{7}. Investigated AF devices for currentdriven switching fall into two categories:

Metallic antiferromagnets with broken parity (P) and timereversal (T) symmetries, but combined PT symmetry, shown schematically in Fig. 1a. Key examples include CuMnAs^{7,8,9,10,11,12,13,14,15} and Mn_{2}Au^{16,17,18,19}. In these systems, due to the crystal symmetry the current induces a fieldlike torque of the same sign on each magnetic sublattice, i.e. a staggered effective field^{1}.

Bilayers consisting of an insulating or metallic antiferromagnet plus a heavy metal, such as NiO/Pt, CoO/Pt, Fe_{2}O_{3}/Pt and MnPt/Pt (Fig. 1b)^{20,21,22,23,24,25,26,27,28,29}. Here the spin Hall effect in the heavy metal layer generates spin accumulation at the interface, resulting in a staggered dampinglike torque on the AF layer.
The current pulses are typically applied in a 4way cross geometry, with the aim of rotating the Néel vector between orthogonal directions (Fig. 1c). Electrical effects which scale with the square of the magnetization, including anisotropic magnetoresistance^{7,30} and spin Hall magnetoresistance^{31}, can then be used to read the resulting magnetic domain state (Fig. 1d). Simpler 2way bar geometries have also been successfully employed^{11,13,19}, as well as second harmonic measurements which in principle allow 90^{∘} and 180^{∘} Néel vector switching to be distinguished^{32}.
Allelectrical antiferromagnetic memory devices based on these principles can show highly reproducible switching over many cycles (Fig. 2a), for current densities of 4 MA/cm^{2}, comparable to those required for switching of ferromagnetic devices^{7}, and for current pulse lengths down to picoseconds^{10}. Other notable features include a deterministic multilevel response, with potential applications for neuromorphic computing^{9}, and enhanced readout ascribed to fast quenching of the AF state^{13}. However, electrical readout signals in such devices may be prone to artefacts, for example due to nonmagnetic inhomogeneities generated by the electrical stress from largeamplitude current pulses. Indeed, superficially similar “electrical switching" behavior is reported in nonmagnetic devices subjected to current pulses on the order of 50 MA/cm^{2} ^{33,34}. Moreover, transport measurements probe only average properties over the electrical contact region, and thus provide limited information on the underlying physics of antiferromagnetic domain switching phenomena.
Synchrotronbased xray photoemission electron microscopy (XPEEM) provides a powerful means of directly visualizing the magnetic modifications induced by current pulses in AF devices. By tuning the incident xray beam to a core level absorption edge and varying its linear polarization, contrast between magnetic domains which are oriented perpendicular and parallel to the xray polarization direction can be resolved^{35}. Spatial resolutions below 50 nm can be achieved^{36}, and AF domain walls and structural defects can be separately resolved and correlated^{37}.
Switching of individual submicron antiferromagnetic domains in CuMnAs has been directly demonstrated using XPEEM^{8}. The AF magnetic moments rotate on average into a direction perpendicular to the current pulse, consistent with the expected direction of the currentgenerated spinorbit torque, but with considerable inhomogeneity due to local pinning (Fig. 2b). Currentinduced motions of 90^{∘} and 180^{∘} domain walls in CuMnAs under modest current densities (≈4 MA/cm^{2}) have also been directly observed using XPEEM^{11,15} (Fig. 2c). In Mn_{2}Au, homogeneous and reversible currentinduced switching has recently been demonstrated, without significant thermal activation^{19}. For both CuMnAs and Mn_{2}Au, the symmetry of the domain switching observed for moderate current densities is consistent with the spinorbit torque mechanism predicted for compounds with broken PT symmetry. With higher amplitude current pulses >10 MA/cm^{2}, the CuMnAs layer may be heated to the vicinity of its Néel temperature, resulting in a fragmentation of AF domains on lengthscales comparable to or even below the spatial resolution of the technique^{13}.
Interest in this field has stimulated the development of benchtop techniques for imaging antiferromagnetic domains. For example, spin Seebeck microscopy and magnetoSeebeck microscopy have been used to investigate currentinduced switching in NiO/Pt and CuMnAs, respectively^{12,23}. Both techniques rely on detection of thermoelectric voltages due to an opticallyinduced local heat gradient. Elsewhere, the large optical birefringence of NiO has been used to image AF domains in NiO/Pt structures with birefringence microscopy^{26,28}. Remarkably, reproducible switching of AF domains was observed even in electrically isolated regions of the NiO film, indicating that switching mechanisms beyond currentinduced torques need to be considered. It was shown that the observed switching in NiO/Pt can be ascribed to a thermally induced and spatially varying strain^{28}.
The thermomagnetoelastic effect observed in NiO/Pt devices^{28} provides an alternative mechanism to the spinorbit torque, and the two mechanisms can act cooperatively for orthogonally applied current pulses. For CoO/Pt devices, both thermomagnetoelastic switching and spinorbit torque driven switching were reported, depending on the current density^{38}. In CuMnAs and Mn_{2}Au, reversible Néel vector reorientation was observed for reversal of the current polarity, which cannot arise from thermal effects, confirming the key role of spinorbit torque switching in these materials^{11,15,19,39}.
Topological antiferromagnetic spin textures
As the size of conventional magnetic logic devices, based on the electrical switching of single domains, reaches a lower limit, attention has shifted towards finding novel device architectures to continue their downsize scaling. The most promising device designs within the last two decades have focused on topologically stable, nanoscale magnetic structures as the carriers of information. These include 2dimensional textures that resemble microscopic whirls in the magnetic order^{40,41,42}. The variation of spin through these textures ascribes to them a topological charge (or winding number), defined as^{43},
where m^{(k)} is the magnetization field and k = 1, 2 labels the two sublattices for the AF case. An AF skyrmion is composed of two topological objects with opposite winding numbers (Q^{(k)} = ± 1) which are strongly coupled through the AF exchange interaction^{44}. Q^{(k)} can be compactly expressed as the product between the polarity, \(p={\left.\frac{1}{2}\cos \theta (r)\right\vert }_{r = 0}^{\infty }\), and vorticity, \(w={\left.\frac{1}{2\pi }\Phi (\phi )\right\vert }_{\phi = 0}^{2\pi }\), where θ and Φ are the azimuthal and polar angle components of the magnetization density, and r and ϕ are the polar coordinates^{43,45}. Names are given to textures according to their topological charge; notably, skyrmions and antiskyrmions, with Q^{(k)} = ± 1, respectively; and merons (or halfskyrmions), which have halfinteger topological charge. The concept of defining magnetic textures by their topology is readily extended to 3dimensions, with examples including more exotic Bloch points^{46} and Hopfions^{47}. All textures with a finite topological charge are afforded topological protection. In AF materials, where dipoledipole interactions are not a limiting factor, this enables their stability, even down to ultrasmall sizes < 10 nm^{48}.
Despite intensive study of topological textures nucleated and controlled in ferromagnetic materials, and a recent demonstration of their use as active components in magnetic tunnel junctions^{49}, their implementation into practical devices has been hindered. A considerable, unwanted effect that is exhibited in their currentdriven motion is a transverse deflection, caused by a gyrotropic force originating from their topology, which reduces their suitability for racetrack device architectures. Furthermore, longrange dipole interactions inhibit skyrmion downscaling, making them susceptible to collapse at ultrasmall scales. A solution to both issues is provided by their AF counterparts, which also have benefits inherent to their compensated magnetic order: terahertz dynamics, negligible stray field, and robustness to external magnetic fields. The main alleviating factor in these AF topological textures is their compensated magnetic sublattices, which individually have a topological charge, but of opposite sign on each sublattice, so that associated gyrotropic effects cancel out^{44,50}.
In recent years, there has been an increase in the number of studies focused on topological textures in intrinsic AFs, but primarily in synthetic AF systems, where nucleation and stabilization mechanisms can be readily achieved using external magnetic fields and thermal effects^{51,52}. Despite the difficulty to nucleate and measure these textures in intrinsic systems, there have been a few breakthrough studies. In naturally occurring AF material αFe_{2}O_{3}, nucleation of topological textures was first demonstrated by thermal cycling using synchrotronbased imaging techniques^{53,54}, and nitrogenvacancy center magnetometry^{55}.
In the metallic AF CuMnAs, it was shown that meron and antimeron pairs can be nucleated on a 180^{∘} domain wall by an electrical pulse, as shown in Fig. 3a, b^{14}. Subsequent pulses result in the motion of the meronantimeron pairs along the domain wall, in the direction of the current pulse, as shown in Fig. 3diviii. The nucleation occurs stochastically due to the combined action of joule heating and spin torque effects, while their motion can be explained by a model based on currentinduced spin torques as the driving mechanism. The generated meronantimeron pairs are homochiral as a consequence of the anisotropy and domain wall geometry, which has been shown to be crucial for their coherent currentdriven motion^{56}. Reversible motion of these structures is achievable with current densities of (≈12 MA/cm^{2})^{14}.
In more general AF systems, the deterministic generation of homochiral topological textures remains a focus of interest. A proposed method to achieve this is to utilize systems with a symmetry breaking interfacial Dzyaloshinskii–Moriya interaction (iDMI)^{57}. This has been detailed in an analytic model of αFe_{2}O_{3}, and has been shown to be an effective stabilization mechanism of homochiral vortices and skyrmions in synthetic AF Pt/FeCoB/Irbased heterostructures^{51,58,59}.
Noncollinear antiferromagnets
A rapidly developing area in the field of antiferromagnetic spintronics is that of timereversal symmetry breaking in compensated (or mostly compensated) spin arrangements. For example, in noncollinear antiferromagnets with a threefold rotational sublattice transposing symmetry, the trigonal spin structure allows for breaking of the conventional t_{1/2}T and PT symmetries^{60,61}. The presence of these two symmetries imposes Treversal symmetry in the system. When excluded however, Tsymmetry may be broken. A signature of Tsymmetry breaking is an anomalous Hall effect (AHE)^{62}. The Hall vector, given by the offdiagonal components of the conductivity tensor h = (−σ_{yz}, −σ_{zx}, −σ_{xy}), transforms as a Todd axial vector, and so is most commonly associated with the uncompensated magnetization of a ferromagnet. For a collinear antiferromagnet with Tsymmetry breaking, the Hall vector depends on the orientation of the Néel vector and changes sign under its reversal (see Section “Altermagnetism”). In noncollinear systems however, the Hall vector follows the octupolar vector of the magnetic unit cell^{63}. In these systems an AHE can be measured even with complete compensation of all spins^{64}.
A sizeable AHE has been observed in hexagonal Mn_{3}Sn^{65} and Mn_{3}Ge^{66}, and cubic Mn_{3}Pt^{67}, where the spins form triangular arrangements (Fig. 4). Its thermal counterpart the anomalous Nernst effect can also be readily observed in these systems^{68,69}, as well as other firstorder magnetic effects including magnetooptical Kerr rotation^{70,71} and xray magnetic circular dichroism^{72}. The Mn_{3}X systems possess a small canted moment, around 23 orders of magnitude smaller than in typical ferromagnets, which allows a reversal of the spin configuration and resulting magnetic signals under modest magnetic fields^{65}. The characteristic properties of noncollinear AFs are compared to those of elemental ferromagnetic materials in Table 1.
Furthermore, the key ingredients for a practical roomtemperature antiferromagnetic memory device have recently been demonstrated, namely spin polarized currents^{61}, tunneling magnetoresistance (TMR)^{73,74} and currentinduced switching^{75,76,77}. In Mn_{3}Pt/MgO/Mn_{3}Pt, room temperature magnetoresistances of up to 100% were observed as well as exchange bias by a neighboring MnPt collinear antiferromagnetic layer^{73}. Beyond these “conventional” spintronic applications, the noncollinear antiferromagnets may offer new functionalities associated with their Weyl semimetallic band structure^{78} and novel micromagnetic properties^{79}, providing a rich playground for physics and nextgeneration technology.
Altermagnetism
An altermagnet is a collinear, compensated magnetic phase in which the magnetic sublattices are connected by a rotation (proper or improper, symmorphic or nonsymmorphic), but not translation or inversion symmetries.^{80}. The real space crystallographic rotational symmetry connecting the two opposite spin sublattices gives rise to an alternating spinsplitting of the Fermi surface in reciprocal space. Two prominent and already well studied examples are RuO_{2} and αMnTe^{81,82,83,84,85}. RuO_{2} crystallises in the rutile structural, space group P42/mnm. The unconventional antiferromagnetic order (Fig. 5) corresponds to the nonrelativistic point group ^{2}4/^{1}m^{2}m^{1}m^{80}. This spin group forbids inversion and translation sublattice transposing symmetries but exhibits a transposing symmetry containing a realspace fourfold rotation \({C}_{4}{t}_{\frac{1}{2}}\). Hexagonal αMnTe crystallises in the NiAstype crystal structure, space group P63/mmc, where the key altermagnetic symmetry is that of a sixfold rotation, \({C}_{6}{t}_{\frac{1}{2}}\)^{86}. It has been predicted and shown that an anomalous Hall effect can be measured in both of these systems^{82,83,87}. The Tsymmetry breaking arises due to the anisotropy of the local magnetization densities between the two magnetic sublattices, induced by nonmagnetic atoms at locally noncentrosymmetric positions^{81}.
Time reversal symmetry breaking phenomena such as the AHE, anomalous Nernst effect, Kerr rotation, Xray magnetic circular dichroism (XMCD), spin currents, giant and tunneling magnetoresistance, and spintorque most associated with ferromagnets have been predicted, and in some cases observed, in these compensated systems^{82,83,84,88,89,90,91}. This coexistence of ferromagnetic and antiferromagnetic like properties in a single collinear system was recently and elegantly elucidated by Šmejkal et al.^{80,81}. One of the recent but key experimental observations is that of their spinsplit band structure via angle resolved photoemission spectroscopy (ARPES)^{92,93,94,95}. Krempasky et al. have confirmed a sizeable spin splitting of ~0.5 eV predicted by theory in bulk altermagnetic αMnTe^{93}. The observed weak altermagnetic splitting predicted at k_{z} = 0 requires spinorbit coupling (SOC), unlike the larger altermagnetic splitting at finite k_{z} that persists without. The former has been shown, with Berry phase physics, to facilitate the measured dissipationless anomalous Hall currents. While the latter, larger spin split bands, highlight the possibility of robust quantum devices in these new materials^{81,82,83,91}. In epitaxial thin film MnTe it was found that the band splitting persists up to 267 K, significantly lower than the bulk magnetic transition temperature^{92}. There is also now evidence of spin splitting in another, high Néel temperature altermagnetic candidate CrSb^{94}.
A distinct feature of the AHE in these systems is its dependence on the Néel vector orientation with respect to the crystal axes. For RuO_{2}, the AHE is excluded by symmetry when the Néel vector is oriented along the [001] magnetic easy axis. As a consequence, the AHE shows negligible hysteresis and requires large fields for saturation (on the order of 50 T at 1.5 K)^{96}. In αMnTe, the AHE is allowed when the Néel vector is oriented along a [1\(\bar{1}\)00] axis, which can coincide with a magnetic easy axis for certain conditions^{86,97}. Hence, the AHE is hysteretic and has a \(\sin 3\phi\) dependence on the orientation of the Néel vector within the cplane. As with the Mn_{3}X noncollinear systems, the Néel vector can be reversed using a magnetic field due to the presence of a spinorbitinduced canted moment, which may be as small as 10^{−5} μ_{B} per Mn in the case of MnTe^{98}.
In RuO_{2} and other materials with sufficient symmetry lowering, the anisotropy of the bands between sublattices allows for a measureable bias in the oppositely spin split isosurfaces in reciprocal space under the application of an electric field^{81}. This allows for spin current generation as well as spin splitter like properties. An electrical spin splitter effect (SSE) with a 34^{∘} propagation angle between spinup and spindown currents was predicted with a corresponding chargespin conversion ratio of 28%^{90}. Shortly after, efficient dampinglike torques and spintocharge conversion due to the SSE were measured in RuO_{2}/ferromagnet bilayers^{99,100,101}. Unlike the conventional spin Hall effect, the SSE does not rely on the spinorbit interaction and is odd under time reversal. The origin of this effect is instead the anisotropic spinsplit Fermi surface as shown in Fig. 5.
Outlook
Electrical writing is a key component of all magnetic memory devices. This is now wellestablished in antiferromagnetic materials via the Néelorder spinorbit torque, although electrical artefacts which mimic AF domain switching have also been observed, as well as competing thermal and straininduced switching effects. The latter provide an alternative route to realize some of the potential advantages of AF materials for spintronics.
A principal stumbling block to the technological utilization of AF materials is the lack of a practical electrical readout mechanism. The anisotropic magnetoresistance and spinHall magnetoresistance are typically on the order of 0.1–1%, and effects associated with AF domain walls and spin textures may be even weaker. The need for sizeable electrical readout explains the current interest in noncollinear systems and collinear altermagnets, which combine spinpolarized transport with the compensated spin configurations. Tunneling magnetoresistance has been demonstrated using noncollinear antiferromagnets and has been predicted in allaltermagnet systems^{91}. The spinpolarized currents and giant magnetoresistance predicted and measured in certain altermagnets also shows promise for their role in future spintronics^{90,91,100}. Exploiting the full potential of these materials will require improved understanding and control of their magnetic domains, as well as exploration of their dynamical properties.
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Acknowledgements
This work was supported by the Leverhulme Trust Grant No: ECF2023755.
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Dal Din, A., Amin, O.J., Wadley, P. et al. Antiferromagnetic spintronics and beyond. npj Spintronics 2, 25 (2024). https://doi.org/10.1038/s44306024000290
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DOI: https://doi.org/10.1038/s44306024000290