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

Novel storage concepts in spintronics based on antiferromagnets (AFMs) propose encoding information by the direction of the alignment of the staggered magnetization or Néel vector. This approach leverages the intrinsically fast THz dynamics of AFMs and their stability against external magnetic fields. Interest in this approach has surged with the prediction of a Néel spin-orbit torque (NSOT) in metallic collinear AFMs with specific symmetry, expected to rotate AFM sublattice spins perpendicularly to a driving bulk current. Early experiments on CuMnAs and Mn₂Au reported current pulse-induced reversible resistance changes, interpreted as evidence for NSOT-driven Néel vector rotation. However, alternative mechanisms like current-induced heating, electromigration, and rapid quenching-induced structural and magnetic modifications were later identified as capable of producing similar resistance changes. Furthermore, external strain-assisted Néel vector manipulation in Mn₂Au was also demonstrated. Therefore, direct microscopic evidence of current-driven staggered magnetization alignment is crucial. Previous studies on CuMnAs and Mn₂Au showed only minor current-induced modifications, with limited reversibility and stability, insufficient for identifying the switching mechanism or demonstrating applicability in memory devices. Microscopic imaging provides direct insights into current-driven Néel vector reorientation mechanisms, and has been shown to involve thermo-magnetoelastic coupling effects in NiO/Pt bilayers, acting similarly to spin-orbit torques on the Néel vector. This necessitates the experimental demonstration of current-induced NSOT for validation.

The research builds upon prior work investigating current-induced switching in antiferromagnets, particularly CuMnAs and Mn₂Au. Studies initially reported reversible resistance changes, attributed to NSOT, but later findings highlighted the role of competing mechanisms such as Joule heating, electromigration, and structural changes induced by rapid quenching. These alternative mechanisms can produce resistance changes mimicking those of NSOT-driven switching. The influence of external strain on Néel vector manipulation in Mn₂Au has also been demonstrated. The limitations of previous research include the observation of only minor current-induced changes and limited reversibility, hindering definitive conclusions about the switching mechanism and practical applications. This study directly addresses these limitations through microscopic imaging and detailed analysis of switching behavior under various conditions.

The researchers investigated epitaxial Mn₂Au(001) thin films (45 nm thick) grown on a Ta/Mo/MgO substrate and capped with SiN. Samples were patterned into cross and stripe structures using optical lithography and Argon ion beam etching. Current pulses were applied using Keithley sourcemeters, with pulse lengths ranging from 10 µs to 1 ms. X-ray magnetic linear dichroism photoelectron emission microscopy (XMLD-PEEM) was employed to directly image the Néel vector orientation before and after current pulse application. The influence of current density and pulse length on Néel vector switching was systematically studied. To investigate the role of thermal activation, experiments were conducted with varying pulse lengths and corresponding temperature increases, determined through ex-situ resistance measurements during pulse application. Current polarity dependence was investigated using a stripe structure aligned along the hard [100] direction to assess the presence of NSOT. An 8-terminal device was used to measure both longitudinal and transverse sample resistance changes associated with Néel vector reorientation. Long-term stability of the switched Néel vector was assessed over several months after current pulse application. The temperature increase during current pulses was calculated by comparing the sample's resistance at the end of the current pulses with the temperature dependent resistance obtained from low-current measurements in a cryostat. In situ resistance measurements were performed during pulse application. The raw transport measurement data is deposited in the Zenodo database.

The study demonstrated complete, remanent, and reversible Néel vector switching of Mn₂Au(001) thin films in cross structures using single current pulses, achieving complete switching even with a single 10 µs pulse. The required current densities were similar for different pulse lengths (1 ms and 10 µs), despite significant differences in the resulting temperature increase (70 K, 45 K, and 20 K). The transition from the beginning of Néel vector reorientation to complete switching occurred within a narrow range (≈20%) of the maximum required current density. Current polarity-dependent reversible domain wall motion indicated the action of NSOT on AFM domain walls. The switched domain configurations exhibited long-term stability (maintained for four months after switching). Measurements using an 8-terminal device revealed alternating longitudinal resistance changes consistent with the Néel vector reorientation observed by XMLD-PEEM, with a maximum ΔRlong/Rlong ≈ 1 × 10⁻³. This is consistent with the previously determined anisotropic magnetoresistance (AMR) of Mn₂Au. The negative AMR (ρ⊥ > ρ||) supported the NSOT-driven switching mechanism. The electrical signal showed no sign of decay, further supporting the stability of the switched state.

The findings directly demonstrate current-driven Néel vector switching in Mn₂Au, overcoming limitations of previous studies that showed only partial and unstable switching. The observed complete and reversible switching, even with single, short pulses, highlights the potential of Mn₂Au for high-speed, low-energy memory applications. The demonstration of NSOT-driven domain wall motion reinforces the understanding of the underlying switching mechanism. The long-term stability of the switched states addresses a key challenge for practical device implementation. The relatively small magnitude of the magnetoresistance signal points to the need for exploring alternative readout mechanisms for enhanced sensitivity in antiferromagnetic spintronics applications. The study provides compelling evidence supporting the viability of antiferromagnetic spintronics for memory device development.

This work demonstrates complete, reversible, and long-term stable Néel vector switching in Mn₂Au thin films using single current pulses. This finding, supported by microscopic imaging and electrical measurements, highlights the potential of Mn₂Au for high-performance antiferromagnetic memory applications. The low heating achieved during switching suggests potential for ultrafast operation and energy efficiency. Future research could focus on optimizing device structures for enhanced signal readout and exploring other metallic antiferromagnets with potentially larger magnetoresistance effects for improved device performance.

The study primarily focuses on epitaxially grown Mn₂Au thin films; results might not be directly transferable to other growth methods or film thicknesses. The relatively small magnitude of the observed magnetoresistance signal necessitates the exploration of alternative readout schemes for enhanced sensitivity in practical devices. The temperature increase during pulse application, while low, might still play a role, though not a dominant one, in the switching process and could be further minimized for even faster and more energy-efficient devices.