Marti et al. (Nature Materials 13, 367 (2014)) demonstrated the first room-temperature bistable memory based on a compensated antiferromagnet — a FeRh resistor written by field cooling through its antiferromagnet-to-ferromagnet transition and read via an antiferromagnetic analogue of anisotropic magnetoresistance — and posed the agenda of turning this proof of concept into a practical memory: writing by efficient heat- and spin–orbit-torque-based electrical schemes instead of field cooling, materials with stronger spin–orbit coupling and larger readout signals, and determination of switching speed, energy, endurance, scaling, and the microscopic switching dynamics. A decade of follow-up work resolved several components: current-induced Néel spin–orbit-torque writing is established in CuMnAs, Mn2Au and related films, terahertz-speed writing has been demonstrated, heat-assisted low-current writing was realized in FeRh itself, and large room-temperature readout has been achieved through all-antiferromagnetic tunnel junctions and spin-phase-change mechanisms. What remains open is the integrated objective: a room-temperature antiferromagnetic memory that simultaneously combines deterministic, artifact-free, full-area electrical writing at practical current densities, a robust and large electrical readout margin, and device-level endurance, retention, and scalability, together with a settled microscopic picture of the switching mechanism.

Background

An antiferromagnet (AFM) is a magnet in which equivalent magnetic moments on alternating sublattices point in opposite directions, so the net magnetization vanishes. The orientation of the staggered spin arrangement is described by the Néel vector. Because there is no net moment, AFMs produce negligible stray magnetic fields (allowing dense packing without bit-to-bit crosstalk) and are largely insensitive to external magnetic fields (helping data retention) — properties that ferromagnetic memories lack. However, the same compensation makes AFMs hard to write and to read by the electrical means used in ferromagnetic random-access memories.

FeRh is a metallic AFM with a first-order phase transition: near room temperature it is antiferromagnetic, and heating by roughly 100 K converts it to a ferromagnet (an AFM-to-FM transition). Marti et al. (Nature Materials 13, 367 (2014)) exploited this: they heated a FeRh resistor into its ferromagnetic phase, set the direction of the iron moments with an applied magnetic field, and cooled back to room temperature, where antiferromagnetic order re-forms with the spin axis predetermined by the field-cooled ferromagnetic state (the 'writing' step). The two stored states were read electrically through an AFM analogue of the anisotropic magnetoresistance (AMR) — the century-and-a-half-old effect in which a ferromagnet's resistance depends on the angle between its magnetization and the current direction, here acting on the Néel vector instead — supported by ab initio transport calculations. This established that an ohmic, room-temperature, bistable AFM memory element is possible, but its writing protocol (heating above a phase transition plus an external field) is slow and energy-hungry compared with the purely electrical writing of ferromagnetic spintronic memories.

The routes the paper and the surrounding literature flagged for making AFM memories practical are: (i) writing protocols that avoid field cooling, such as heat-assisted schemes adapted from magnetic recording, or materials that pass through a paramagnetic rather than ferromagnetic phase when heated; (ii) current-induced spin–orbit torques — relativistic torques in which an applied charge current, through spin–orbit coupling, generates effective magnetic fields acting on the AFM sublattices — enabling fully electrical, deterministic reorientation of the Néel vector, including the 'Néel spin–orbit torque' predicted for metallic AFMs such as CuMnAs and Mn2Au whose two spin sublattices are related by inversion symmetry; (iii) other metallic AFMs with higher ordering temperatures or stronger spin–orbit coupling that give a larger AFM-AMR (better room-temperature signal margins), or alternative readout mechanisms such as tunneling magnetoresistance (TMR, the resistance change of a magnetic tunnel junction between differently oriented magnetic layers); and (iv) the dynamical and device questions: switching speed limits, energy efficiency, miniaturization, endurance, and the microscopic dynamics of AFM switching. Reviews of the ensuing subfield of antiferromagnetic spintronics (Baltz et al., Rev. Mod. Phys. 90, 015005 (2018); Dal Din et al., npj Spintronics 2 (2024)) document both the rapid progress on these routes and the unresolved obstacles.

Problem Statement

Starting from the demonstrated room-temperature bistable antiferromagnetic memory in FeRh (written by field cooling through the antiferromagnet-to-ferromagnet transition and read via AFM anisotropic magnetoresistance), establish what is required to make a room-temperature antiferromagnetic memory practical, in the four directions posed by the source paper and the subsequent literature: (1) writing protocols that are more efficient, faster, and more scalable than field cooling — heat-assisted electrical writing schemes, optimized combinations of peak temperature and field, or materials that traverse an AFM-to-paramagnetic rather than AFM-to-ferromagnetic transition; (2) purely electrical writing by current-induced spin–orbit torques producing deterministic reorientation of the antiferromagnetic spin axes, including the theory of the staggered (Néel) torques in metallic antiferromagnets; (3) identification and experimental and theoretical characterization of other metallic antiferromagnets with higher ordering temperatures or stronger spin–orbit coupling that yield larger room-temperature electrical readout signals; and (4) the switching-speed limits, energy efficiency, device miniaturization, endurance, and microscopic dynamics of antiferromagnetic switching (including optical reorientation routes). In light of the demonstrated components described above — current-induced Néel spin–orbit-torque switching in CuMnAs, Mn2Au and related films, terahertz-speed writing, heat-assisted phase-transition-assisted writing in FeRh, and large readout via all-antiferromagnetic tunnel junctions and spin phase change — the question that remains open is what is required to complete the overall objective: an antiferromagnetic memory device operating at room temperature in which deterministic, reproducible, artifact-free electrical writing of the entire probed magnetic volume (with the switching mechanism identified as genuinely magnetic rather than thermal or resistive in origin), a robust and sufficiently large electrical readout margin, and device-level endurance, retention, and scalability to integrated bit arrays are simultaneously achieved and quantitatively characterized. What remains to be established, across those four directions, is which unresolved scientific questions must be answered for a practical device to satisfy all of these conditions simultaneously; the qualitative conditions involved — efficient and 'practical' writing, a 'sufficiently large' readout margin, and 'artifact-free' writing — are not numerically fixed by the sources and are part of the resolution boundary.

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