Biased Quasiballistic Spin Torque Magnetization Reversal

We explore the ultrafast limit of spin torque magnetization reversal time. Spin torque precession during a spin torque current pulse and free magnetization ringing after the pulse is detected by time-resolved magnetotransport. Adapting the duration of the pulse to the precession period allows coherent control of the final orientation of the magnetization. In the presence of a hard axis bias field, we find optimum quasiballistic spin torque magnetization reversal by a single precessional turn directly from the initial to the reversed equilibrium state.

Figure 1

(a) Free layer easy axis TMR hysteresis loop and (b) switching asteroid. Configuration of $H_S$, $M$, and $M_P$ is sketched. (c) Experimental setup for electrical detection of ST precession.

Figure 2

Time-resolved TMR signal of ST and free precession. ${\mu }_0{H}_S=22.8\mathrm{mT}$. (a) ST precession during the rise of a 5 ns pulse of $j=3.2\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$ at $\varphi =-114°$. (c) Free precession after excitation by a 180 ps pulse of $j=9\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$ at $\varphi =84°$. (b),(d) Precession data after background subtraction [gray line in (a),(c)]. Parameters are derived from fitting a damped sinusoid (gray line).

Figure 3

(a),(b) Suppressed ringing of $M$ by variation of ST pulse duration. $\varphi =-104°$, ${\mu }_0{H}_S=24\mathrm{mT}$, $j=-7.8\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$. $T_{\mathrm{Pulse}}=450$, 600, 770, 930, 1100, and 1250 ps. Graphs are offset for clarity. For the black curves, ringing is coherently controlled after $n=2,3,4$ full precessional turns as sketched in (b). (c)–(f) Biased precessional ST reversal by $j=2.5\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$ with ${\mu }_0{H}_S=10\mathrm{mT}$ along the in-plane hard axis ($y$ axis). (c) ST reversal by 5 ns pulse. ST precession (i) and relaxation to reversed state (ii) after the pulse is observed. (d) Sketch of magnetic configurations and reversal trajectory. (e) ST reversal by 730 ps with strong ringing (iii) and low switching reliability. (f) Quasiballistic ST reversal by 1.1 ns pulse. The pulse terminates in the precession minimum of (c). Optimized reversal occurs by one full precessional turn.