Spin–orbit-torque (SOT) MRAM, prized for its high endurance and sub‑nanosecond switching, faces a reliability hurdle: high‑current write pulses can cause write probability to collapse or oscillate. Collaborating with Antaios, Spintec researchers have now revealed that this phenomenon stems from an intrinsic, largely deterministic back‑switching mechanism, and they demonstrate that lengthening the pulse fall time effectively suppresses it, substantially widening the viable write window.

The team mapped write‑error rate (WER) as a function of nanosecond SOT‑pulse amplitude on ~60‑nm CoFeB pillars on β‑W Hall crosses, applying magnetic fields along three axes. The maps consistently show three regimes: no switching, a low‑WER forward‑switching zone, and a high‑WER back‑switching region where the bit returns to its initial state. This pattern, observed on both 100‑nm pillars and full single‑cell MTJs (75‑nm dot on a 130‑nm‑wide β‑W track) with practical 10‑ns pulses, confirms that back‑switching is a systematic dynamical outcome, not random noise.

Macrospin simulations explain the behavior: strong spin‑orbit torques drive the magnetization near an unstable in‑plane equilibrium; after the pulse, small thermal fluctuations can trigger relaxation back to the original state. Rather than modifying materials, the researchers tackled the problem with pulse shaping. By increasing the fall time from 2 ns to 4 ns, the torques remain active during relaxation, steering the magnetization safely away from the instability. Experimentally, this widened the forward‑switching current window and, on a full SOT‑MRAM cell, yielded a WER below 10⁻⁵ across an extended current range.

The work, supported by EU MSCA ITN SPEAR (Grant 955671), Région Auvergne‑Rhône‑Alpes (19‑009938‑01‑MAPS), and RENATECH/PTA (ANR‑22‑PEEL‑0015), is detailed in *Phys. Rev. Appl.* 24, 064038 (2025) and open‑access via HAL (hal‑05411536).

Spin–orbit-torque (SOT) MRAM writing exhibits an unusual current dependence: at high current amplitudes, the write probability can drop to zero or even oscillate as the current increases. In collaboration with Antaios, we showed that this behavior originates from an intrinsic, largely deterministic back-switching mechanism, and we demonstrated that tailoring the pulse fall time significantly widens the reliable write window.

Experimental WER versus SOT write current for 10-ns pulses. (a) 100-nm CoFeB(0.9 nm) pillars on β-W Hall crosses, measured at HX=950Oe for pulse fall times of 2, 3, and 4 ns. (b) 75-nm MTJ dot on a 130-nm-wide β-W(4 nm) SOT track, measured at Hx=800Oe with a 2-ns fall time.

SOT-MRAM is particularly attractive for replacing the memories closest to processing units, as it combines high endurance with sub-nanosecond switching times. Reliable writing is therefore essential for this technology to scale and mature. However, perpendicularly magnetized SOT-MRAM devices may enter a back-switching regime in which increasing the current causes the bit to relax back to its initial state. We studied ~60-nm CoFeB pillars on β-W Hall crosses and mapped the write-error rate (WER) as a function of nanosecond SOT-pulse amplitude under magnetic fields applied along x, y, and z. These WER maps consistently reveal three regions: no switching, deterministic forward switching (low WER), and a high-WER zone where the magnetization returns to the starting state. This provides clear evidence that back-switching is a systematic dynamical outcome rather than random noise. Importantly, the effect is observed with practical nanosecond-scales write pulses and is reproduced across device configurations, from 100-nm pillars on β-W Hall crosses to a full SOT-MRAM single cell (MTJ dot on a W SOT track), highlighting its relevance for circuit-level operation.

Realistic macrospin simulations reproduce the maps and clarify the mechanism: at large currents, spin-orbit torques drive the magnetization close to an energetically unstable in-plane equilibrium; after the pulse, small thermal fluctuations during relaxation can send it back to the original state, depending on the torque balance. Guided by this picture, we mitigate back-switching without changing materials by shaping the write pulse. Lengthening the fall time keeps the torques active during relaxation and steers the magnetization away from the unstable point. Experimentally, increasing the fall time (2→4 ns) suppresses back-switching and widens the forward-switching window; on a complete SOT-MRAM single cell, this approach achieved WER

Team: Spinorbitronics

Collaborations: Antaios

Funding: EU Horizon 2020 MSCA ITN SPEAR (Grant 955671), Région Auvergne-Rhône-Alpes Pack Ambition Recherche (19-009938-01-MAPS), and RENATECH/PTA support (ANR-22-PEEL-0015)

Further reading: Intrinsic back-switching phenomenon in spin-orbit torque MRAM devices, K. Ray, J. Vigier, P. Usé , S. Martin, N. Lefoulon, C. Bouard, M. Drouard, G. Gaudin, Phys. Rev. Appl. 24, 064038 (2025).

Open access: hal-05411536

Contact: Gilles Gaudin

The post Pulse Shaping Suppresses Intrinsic Back-Switching in SOT-MRAM appeared first on Spintec.