Quasiballistic Magnetization Reversal

We demonstrate a quasiballistic switching of the magnetization in a microscopic magnetoresistive memory cell. By means of time resolved magnetotransport, we follow the large angle precession of the free layer magnetization of a spin valve cell upon application of transverse magnetic field pulses. Stopping the field pulse after a $180°$ precession rotation leads to magnetization reversal with reversal times as short as 165 ps. This switching mode represents the fundamental ultrafast limit of field induced magnetization reversal.

Figure 1

Magnetic memory cell used in the experiments. (a) Optical micrograph. Spin valve cell (SV) with electrical contacts (C1, C2, surrounded by the dotted lines) and buried pulse line (PL, marked by the white dashed line). (b) Sketch of the magnetic field configuration $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}$ (along $y$) is applied perpendicular to the initial and final magnetization ${\mathit{M}}_i$, ${\mathit{M}}_f$.

Figure 2

(a) Easy axis MR loop of the spin valve. (b) Time resolved MR response of the SV to magnetic hard axis field steps from 52 Oe (bottom) to 272 Oe (top) in 1 dB increments. The MR data is offset for clarity. Precessional oscillations of the free layer about $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}$ are clearly observed. (c) Precession frequency $f_{\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{c}}$ vs applied field amplitude from Fourier transforms of the data in (b), and calculated values (gray line). (d) Calculated trajectory of the magnetization $\mathit{M}$ in response o a field step of $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=81\mathrm{O}\mathrm{e}$. $m_X$, $m_Z$ are the normalized components of $\mathit{M}$ along the easy axis, and out of plane, respectively [cf. Fig. 1b].

Figure 3

Precessional switching of a SV cell. Calculated trajectories of precessional switching (a) and nonswitching (d) in the $m_X\mathrm{\text{−}}{m}_Z$ plane. For switching, the field pulse is stopped after a $180°$ precessional turn (a) ($H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=81\mathrm{O}\mathrm{e}$, $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=175\mathrm{p}\mathrm{s}$). Higher fields and longer pulse durations ($H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=205\mathrm{O}\mathrm{e}$, $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=240\mathrm{p}\mathrm{s}$) induce a full $360°$ rotation (d) (no switch). Field (b) and magnetization component $m_X$ (c) vs time for $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=81\mathrm{O}\mathrm{e}$, $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=175\mathrm{p}\mathrm{s}$. Gray dots: experiment; black lines: simulation. $m_X$ switches within $T_{\mathrm{s}\mathrm{w}\mathrm{i}\mathrm{t}\mathrm{c}\mathrm{h}}=165\mathrm{p}\mathrm{s}$. No residual precession is found indicating quasiballistic reversal. Field (e) and $m_X$ (f) vs time for $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=205\mathrm{O}\mathrm{e}$, $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=240\mathrm{p}\mathrm{s}$.

Figure 4

Higher order precessional switching: (a) First order switch. Measured $m_X$ vs time for $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=272\mathrm{O}\mathrm{e}$, $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=305\mathrm{p}\mathrm{s}$. After pulse decay, residual precession occurs [arrows (1)]. (b) Gray scale encoded map of $m_X$ as a function of time and pulse duration $T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}$. White: $m_X=1$; dark gray: $m_X=-1$. $H_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=272\mathrm{O}\mathrm{e}$ [21]. Pulse field decays to zero along the inclined dotted line. Higher order switching (white horizontal regions, switching order $n$ is indicated) occurs in phase with the precession at pulse cutoff. Zero order switching ($n=0$) is not accessible for the given pulse amplitude.