Crossover between intrinsic and temperature-assisted regimes in spin-orbit torque switching of antiferromagnetic order

Intensive studies have been made on antiferromagnets as candidate materials for next generation memory bits due to their ultrafast dynamics reaching picosecond time scales. Recent demonstrations of electrical bidirectional switching of antiferromagnetic states have attracted significant attention. However, under the presence of significant Joule heating that destabilizes the magnetic order, the timescales associated with the switching can be limited to nanoseconds or longer. Here, we present the observation of a crossover in the switching behavior of the chiral antiferromagnet Mn3Sn by tuning the magnetic layer thickness. While Joule heating interferes with switching in thicker devices, we find clear signatures of an intrinsic spin-orbit torque mechanism as the thickness is reduced, avoiding the heating effect. The suppression of heating enables switching without significant attenuation of the readout signal using pulses shorter than those required by temperature-assisted mechanisms. The crossover into the spin-orbit torque switching behavior clarifies the potential for achieving ultrafast switching as expected from the picosecond spin dynamics of antiferromagnets. Our results lay the groundwork for designing antiferromagnetic memory devices that can operate at ultrafast timescales.

💡 Research Summary

This paper investigates the thickness‑dependent spin‑orbit‑torque (SOT) switching mechanisms in the chiral antiferromagnet Mn₃Sn, a material that has attracted considerable interest for antiferromagnetic memory because of its large anomalous Hall effect (AHE) and picosecond spin dynamics. The authors fabricate Mn₃Sn(t)/Ta(5 nm)/AlOₓ(3 nm) heterostructures with Mn₃Sn thicknesses ranging from 15 nm to 200 nm on Si/SiO₂ substrates, using a single deposition run with a tilting shutter to ensure identical composition across all thicknesses (Mn₃.06Sn₀.94). Post‑deposition annealing at 500 °C after capping yields smooth interfaces (RMS ≈ 0.5–0.6 nm) and a preferential out‑of‑plane orientation of the kagome planes, as confirmed by AFM, TEM, XRD and EDX analyses. Hall bars (16 µm × 96 µm) are patterned for electrical switching experiments.

The switching protocol applies a write current pulse (initially 100 ms) together with a small in‑plane bias field (0.1 T) to break lateral symmetry. The Hall resistance R_H is monitored as a function of the write current magnitude. For a 40 nm Mn₃Sn layer (referred to as “Config. 1”), a clear sign reversal of R_H is observed once the current exceeds a threshold, reproducing earlier reports of bidirectional SOT switching in Mn₃Sn.

The central finding is a crossover between two distinct switching regimes as the Mn₃Sn thickness is varied. In thick films (≥ 40 nm), the required switching current density J_c^temp is essentially independent of thickness and relatively low. The authors attribute this to a “temperature‑assisted” mechanism: Joule heating raises the Mn₃Sn temperature close to its Néel temperature (T_N ≈ 420 K), destabilizing the antiferromagnetic order. During the subsequent cooling, the spin torque injected by the heavy‑metal (Ta) layer biases the re‑ordering, fixing the final magnetic state. Consequently, the switching speed is limited by the thermal cooling time, which for micro‑fabricated devices is on the order of sub‑microseconds—far slower than the intrinsic picosecond dynamics.

In contrast, when the Mn₃Sn layer is thinned below ≈ 30 nm, the switching current density J_c^int grows roughly linearly with thickness, as expected from a macro‑spin picture where the injected spin angular momentum must rotate the entire magnetic volume. Importantly, despite the higher current, the Hall signal retains its magnitude, indicating that the antiferromagnetic order remains intact throughout the pulse. This regime is identified as “intrinsic” SOT switching: the spin torque directly rotates the chiral order without the need for thermal destabilization. The authors demonstrate that even with much shorter pulses (down to 10 µs) the intrinsic switching persists, confirming that the limiting factor is not thermal cooling but the torque itself.

From the observed crossover, the authors estimate a critical thickness t_c ≈ 30 nm where the two mechanisms intersect, implying an exchange length ℓ_ex in Mn₃Sn of the order of tens of nanometers. This is significant because it shows that antiferromagnetic devices can sustain perpendicular order over much larger thicknesses than ferromagnets, yet still exhibit a thickness‑controlled transition between torque‑dominated and thermally‑assisted switching.

The work highlights several practical implications for antiferromagnetic memory design. To exploit the intrinsic, ultrafast switching potential (picosecond scale), device thickness must be reduced sufficiently to suppress Joule heating and keep the magnetic order stable during the write pulse. Conversely, thicker devices may be advantageous for thermal stability but will inevitably be limited to nanosecond‑scale operation due to the cooling bottleneck. The authors suggest that further miniaturization, improved heat sinking, and exploration of alternative heavy‑metal spin‑source layers could push the intrinsic regime to even lower current densities, making antiferromagnetic SOT memory competitive with existing ferromagnetic technologies.

In summary, the paper provides a comprehensive experimental demonstration of a thickness‑driven crossover from temperature‑assisted to intrinsic SOT switching in Mn₃Sn, quantifies the associated exchange length, and offers clear guidelines for engineering antiferromagnetic spin‑tronic devices capable of ultrafast, low‑energy operation.