Current-driven writing process in anti-ferromagnetic Mn₂Au for memory applications

The study addresses current-driven manipulation of the Néel vector in metallic antiferromagnets (AFMs), a key avenue for AFM spintronics where information is encoded in the direction of the staggered magnetization. AFMs offer intrinsically fast THz dynamics and robustness against external magnetic fields. A relativistic Néel spin-orbit torque (NSOT) has been predicted for certain metallic collinear AFMs, enabling current-induced rotation of sublattice spins perpendicular to the driving current. Early experiments in CuMnAs and Mn₂Au reported reversible resistance changes consistent with NSOT-driven switching; however, subsequent work showed that thermal effects, electromigration, and quench-induced structural changes can mimic such signals. Strain-assisted manipulation has also been demonstrated, and prior microscopic imaging typically found only partial and not fully reversible domain switching. Direct microscopic, device-relevant demonstrations of current-driven Néel vector alignment are therefore crucial to identify the switching mechanism and validate applicability for memory. Here, the authors demonstrate complete, remanent, and reversible Néel vector switching in epitaxial Mn₂Au(001) thin films patterned as device cross structures, compatible with MRAM. They observe current-polarity-dependent reversible domain wall motion indicative of NSOT and show that thermal activation is not necessary for switching in their films, supporting fast and energy-efficient operation.

The paper situates its work within the development of AFM spintronics, noting predictions of NSOT in specific AFMs and initial demonstrations of electrical switching in CuMnAs and Mn₂Au via resistance changes. It highlights confounding effects reported later, including pulse-induced heating, electromigration, and quench-induced structural/magnetic changes, which can also alter resistance. Strain-assisted Néel vector control in Mn₂Au has been shown. Prior microscopic studies in CuMnAs and Mn₂Au generally observed only minor, partially reversible domain modifications, insufficient to unambiguously attribute switching to NSOT or to demonstrate device readiness. Recent findings in NiO/Pt bilayers revealed thermo-magnetoelastic effects that can mimic spin-orbit torques, reinforcing the need for direct microscopic evidence of current-induced NSOT in metallic AFMs like Mn₂Au.

Epitaxial Mn₂Au(001) thin films (thickness 45 nm) were grown on MgO(001) substrates with a double buffer layer of 20 nm Mo(001) and 13 nm Ta(001). Films were capped with 2 nm SiN and patterned by optical lithography and Ar ion beam etching into device geometries: cross structures oriented along the easy (110) axes to allow pulsing parallel/perpendicular to the Néel vector, and single stripes aligned along easy [110] or hard [100] directions to study current-direction dependence and homogeneity. XMLD-PEEM (X-ray magnetic linear dichroism photoelectron emission microscopy) was used to image AFM domains and Néel vector orientation after pulsing. In-situ electrical manipulation during PEEM was performed using Keithley 2601B-PULSE (MAX IV) and Keithley 2461 (Diamond) source meters integrated with the X-PEEM setup. For transport characterization, an 8-terminal cross device enabled longitudinal (R_long) and transverse (R_trans) resistance measurements after each pulse. Ex-situ resistance was measured with a Keithley 6220 precision current source (probe current 50 µA) and a Keithley 2182A nanovoltmeter in Delta mode (averaging 200 readings per data point). Pulse–probe automation used an Agilent 34970A Switch Unit and a Keithley 2430 Pulse SourceMeter. Current pulse protocols included trains of 100 pulses at 1 ms length (10 ms off time) and single pulses of 100 µs or 10 µs; bipolar pulses were applied along orthogonal easy axes for switching in cross structures. Ohmic heating during pulses was quantified using an auxiliary lithographic pattern enabling 4-probe measurements of the central cross region; the maximum resistance at pulse end was compared with a room-temperature–extrapolated R(T) from low-current cryostat measurements to deduce ΔT. Time-resolved resistance during pulses was also recorded using the SourceMeter SENSE inputs and an oscilloscope to ensure consistency. XMLD-PEEM images were analyzed to identify domain reorientation and domain wall motion under varying current densities and polarities.

• Complete, reversible reorientation of the Néel vector across the central 10 µm × 10 µm area of cross-shaped Mn₂Au(001) devices was achieved using current pulses alternated along orthogonal easy (110) directions, as directly imaged by XMLD-PEEM (90° contrast inversion).
• Comparable switching thresholds were found for disparate pulse protocols: J_1ms ≈ 2.6 × 10^11 A/m² for 100 pulses of 1 ms (10 ms off), J_100µs ≈ 2.7 × 10^11 A/m², and J_10µs ≈ 3.0 × 10^11 A/m² for a single 10 µs bipolar pulse. Despite vastly different thermal loads, switching currents were similar, indicating thermal activation is not required in these films.
• Pulse-induced temperature rises estimated from resistance measurements were ΔT(1 ms) ≈ 70 K, ΔT(100 µs) ≈ 45 K, and ΔT(10 µs) ≈ 20 K.
• In single stripes along an easy [110] axis pulsed with 1 ms currents of increasing density, the switched area increased homogeneously and the transition from initial reorientation to complete switching occurred within ≈20% of the maximum required current (onset near J_rms ≈ 2.46 × 10^11 A/m²).
• For stripes along the hard [100] axis, no net Néel alignment developed in the central region, but partial reversible reorientations occurred that depended on current polarity, with reversible domain wall motion consistent with NSOT action. Local regions with current flowing along an easy direction showed expected local alignment.
• Electrical readout in 8-terminal devices showed alternating R_long and R_trans values consistent with Néel vector reorientation. A maximum ΔR_long/R_long ≈ 1 × 10^−3 was observed, consistent with previously measured low-temperature AMR in Mn₂Au (AMR_Mn2Au ≈ −1.5 × 10^−3). The negative AMR (ρ_⊥ > ρ_∥) matches the larger R_long when the Néel vector is perpendicular to the current direction as expected for NSOT-driven switching. No decay of the electrical signal was observed over time (per Supplementary Information).
• The current-driven Néel configurations were long-term stable; XMLD-PEEM images showed the aligned domain state persisted for at least four months after writing.

The results directly demonstrate device-relevant, current-driven, complete and reversible Néel vector switching in epitaxial Mn₂Au films with minimal heating and without requiring thermal activation. XMLD-PEEM imaging confirms that the entire active region can be deterministically reoriented with single pulses, and that the written states are robust for months, meeting a key requirement for AFM MRAM. The current polarity dependence of reversible domain wall motion, particularly under hard-axis pulsing, supports the presence of a Néel spin-orbit torque acting on AFM domain walls. Transport measurements corroborate the magnetic imaging, with AMR-consistent changes in both longitudinal and transverse resistances and no observable time-dependent decay. Together, these findings address ambiguities from prior studies by providing microscopic evidence of NSOT-driven switching and by ruling out the necessity of thermal activation in high-quality epitaxial Mn₂Au, advancing the feasibility of fast and efficient AFM spintronic memory.

This work establishes that epitaxial Mn₂Au(001) thin-film devices enable complete, remanent, and reversible Néel vector switching using single current pulses with low associated heating and without thermal activation. The switching is robust over months and exhibits current-polarity-dependent reversible domain wall motion indicative of NSOT. Electrical readout via AMR is consistent with the imaged magnetic states. These results demonstrate the viability of Mn₂Au for AFM MRAM and related spintronic applications. Future directions include optimizing device geometries and materials to further reduce switching currents, exploring ultrafast pulse regimes, and developing enhanced read-out schemes beyond AMR to increase signal levels for practical integration.

• The anisotropic magnetoresistance readout signal is relatively small (ΔR/R ≈ 10^−3), which may limit electrical readout sensitivity in practical devices and motivates alternative readout approaches.
• Slight inhomogeneities in Néel vector alignment across device areas were noted, reflected in minor differences between longitudinal and transverse resistance changes.
• Under hard-axis pulsing, no net alignment was achieved in the central stripe region; only partial, polarity-dependent reversible reorientations were observed, indicating geometry- and direction-dependent efficacy of switching.