Single-Shot Time-Resolved Measurements of Nanosecond-Scale Spin-Transfer Induced Switching: Stochastic Versus Deterministic Aspects

Using high bandwidth resistance measurements, we study the single-shot response of tunnel junctions subjected to spin torque pulses. After the pulse onset, the switching proceeds by a ns-scale incubation delay during which the resistance is quiet, followed by a 400 ps transition terminated by a large ringing that is damped progressively. While the incubation delay fluctuates significantly, the resistance traces are reproducible once this delay is passed. After switching, the time-resolved resistance traces indicate micromagnetic configurations that are rather spatially coherent.

Figure 1

Experimental setup. A switch allows to exchange the low and high frequency configurations. The optical micrographs are the high frequency coplanar circuit ($500\times 500\mu {\mathrm{m}}^2$) and a zoom ($40\times 40\mu {\mathrm{m}}^2$) of its central part around the MTJ. The square shaped overlap of top and bottom triangular electrodes is $3\mu \mathrm{m}$ wide. The MTJ is buried below.

Figure 2

(a) Superimposition of 60 easy axis hysteresis loops. The cross symbols define the two remanent positions (b) Stoner astroid measurement. (c) Current versus voltage traces for applied field parameters taken at the center of the astroid. (d) time-resolved resistance of a device when subjected to a hard axis field of $-5\mathrm{mT}$, i.e., at the position of the $+$ symbol of panel (b).

Figure 3

(a) Power Spectra of the voltage noise emitted by a device when submitted to 140 mT and 0 V (red reference curve) or 0.1 V (black curve). Inset: zoom on a part of the field dependence of the square of the peak frequencies. (b) Power Spectra of the voltage noise in the two remanent states at the astroid center. The small ripple in the spectra is an artifact. Inset: sketch of the proposed micromagnetic configurations at remanence.

Figure 4

Inset: sketch of the incident and transmitted voltage pulses when a device switches from P to AP. Main panel: zoom on the plateau of the transmitted voltage pulse for a $-1\mathrm{V}$ incident voltage step: each of the 20 events is displayed in a different color.

Figure 5

Panel (a) top curve: Single-shot switching event measured with 3 GHz bandwidth. Second curve: same single-shot switching event measured with 13 GHz bandwidth. Third curve: another single-shot switching event, manually selected because it has the same switching instant, measured with 13 GHz bandwidth. Bottom curve; average of 20 single-shot switching events measured with 13 GHz bandwidth, those events having the same switching instant. Panel (B) average of single-shot switching events measured with 13 GHz bandwidth, those events having the same switching instant. From top to bottom curves, the switching instants are 0.7, 1.6, 2.8, 3.9, and 4.9 ns. The curves are vertically offset for clarity.