Single-Shot All-Optical Switching in CoFeB/MgO Magnetic Tunnel Junctions

TL;DR

This work demonstrates single-shot all-optical switching (AOS) in CoFeB/MgO magnetic tunnel junctions (MTJs) without rare-earth elements by engineering asymmetric laser heating through the capping-layer thickness, enabling deterministic P-to-AP reversal in full-film stacks and detection via TMR in microscale devices. The authors quantify threshold fluences for P-to-AP switching $F_P$ and multidomain formation $F_{MD}$, showing distinct dependencies on cap thickness and MgO thickness, and propose that precise energy absorption control between the free and reference layers drives switching, with possible contributions from ultrafast spin transport and HAMR-like thermal effects. While AP-to-P switching is not observed under these conditions, the results reveal a viable route to merge AOS with MTJ-based memory platforms, emphasizing thermal management and layer-absorption engineering for practical devices. This study advances the integration of ultrafast optical control with spintronic memory technologies, potentially enabling ultrafast, energy-efficient switching in STT-MRAM architectures like CoFeB/MgO MTJs.

Abstract

We demonstrate single shot al optical switching (AOS) in rare earth free CoFeB/MgO magnetic tunnel junctions (MTJs), a material system widely adopted in spin transfer torque magnetic random access memory (STT MRAM). By tuning the capping layer thickness, we show that precise heat control enables deterministic magnetization reversal from parallel (P) to antiparallel (AP) state. Furthermore, we detect magnetization reversal in a micro scale MTJ device via the tunnel magnetoresistance (TMR) effect. Our findings suggest that ultrafast spin transport or dipolar interactions or a combination of both may play essential roles in the switching process. This work represents a significant step toward integrating AOS with MTJ technology.

Figures (7)

• Figure 1: MTJ stack and its characterization. (a) Schematic of the MTJ stack structure used in this study. (b) $M$–$H$ curves measured for MTJ stacks with $t_\text{MgO} = 1.3$ and 2.0 nm. The inset shows a magnified view. Arrows indicate the corresponding magnetic configurations.
• Figure 2: Single-shot magnetization switching observed via MOKE imaging in MTJ stacks with a Ru capping layer thickness of $t_\mathrm{Ru} = 5.0$ nm. (a) $t_\mathrm{MgO} = 1.3$ nm, (b) $t_\mathrm{MgO} = 2.0$ nm. For both cases, MOKE images are shown after laser irradiation, starting from the P (blue) and AP (red) states.
• Figure 3: Summary of single-shot switching experiments in MTJ stacks. (a, b) Evolution of the threshold fluences for P-to-AP switching ($F_{\text{P}}$) and multidomain formation ($F_{\text{MD}}$) as a function of capping layer thickness $t_{\mathrm{cap}}$ for (a) $t_\mathrm{MgO} = 1.3$ nm and (b) $t_\mathrm{MgO} = 2.0$ nm. (c) Calculated laser energy absorption in the capping, free, and reference layers as a function of $t_{\mathrm{cap}}$. Closed (open) symbols represent MTJ stacks with Ru (Pt) capping layers.
• Figure 4: Single-shot switching detected via the TMR effect. (a) Optical microscopy image of the MTJ device. (b) Schematic of the experimental setup. (c, d) (c,d) Time dependence of MTJ resistance in response to a square-wave magnetic field (black lines) and to irradiation of the P (red line) and AP state (blue line) by single laser pulses with the timing indicated by the laser pulse arrows. The pulse durations and fluences were (c) 35 fs and 21.6 mJ/cm$^2$, and (d) 10 ps and 41.0 mJ/cm$^2$, respectively.
• Figure 5: (a, b) Evolution of the threshold fluences for P-to-AP switching ($F_{\text{P}}$) and multidomain formation ($F_{\text{MD}}$) as a function of pulse duration for (a) $t_\mathrm{MgO} = 1.3$ nm and (b) $t_\mathrm{MgO} = 2.0$ nm.