Subnanosecond magnetization reversal of a magnetic nanoparticle driven by a chirp microwave field pulse

We investigate the magnetization reversal of a single-domain magnetic nanoparticle driven by a linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that a down-chirp microwave pulse is solely capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of a constant-frequency microwave field. The fast reversal is due to the fact that the down-chirp microwave field pulse triggers stimulated microwave absorptions (emissions) by (from) the spin before (after) it crosses over the energy barrier. Applying a spin-polarized current additively to the system further reduces the microwave field amplitude. Our findings provide a way to realize low-cost and fast magnetization reversal.

Figure 1

(a) Schematic diagram of the system. $\mathbf{m}$ and $\mathbf{p}$ represent unit vectors of magnetization of free and fixed layers, respectively. A microwave field is applied onto the free layer, and an electric current flows through the spin valve. (b) The frequency of a down-chirp microwave (sweeping from $+f_0$ to $-f_0$).

Figure 2

(a) The time evolution of $m_z$ driven by different sources: red dashed line for down-chirp microwave pulse (DCMWP) of $f_0=21$ GHz, $H_{\mathrm{mw}}=0.045$ T, and $\eta =67.2{\mathrm{ns}}^{-2}$; blue solid line for constant-frequency microwave (CFMW) of amplitude 0.98 T and frequency 21 GHz; black dash-dotted line for CFMW of amplitude 0.045 T and frequency 21 GHz. (b) The dependence of switching times $t_{\text{s}}$ on the chirp rate $\eta$ for different microwave field amplitudes $H_{\mathrm{mw}}$. The vertical dashed lines are lower and upper limits of $\eta$ for magnetization switching. (c) Comparison of magnetization reversal times for different strategies. The horizontal axis is the field amplitude. The black solid line is the theoretical limit. Red squares/blue triangles are for the DCMWP/CFMW. Inset: Optimal chirp rates $\eta$ for different field amplitudes $H_{\text{mw}}$.

Figure 3

(a) Magnetization reversal driven by a down-chirp microwave field pulse of $f_0=21$ GHz, $H_{\mathrm{mw}}=0.045$ T, and $\eta =67.2{\mathrm{ns}}^{-2}$. The red line is for a complete pulse. The black line shows magnetization reversal if the pulse is turned off at $m_z=-0.004$. (b) Magnetization trajectory if the field is turned off at the moment when $m_z=-0.004$. (c) Trajectory of magnetization for a complete pulse. (d) Plot of the relative angle $\mathrm{\Phi }$ against time (blue line) and the time dependence of $m_z$ (red line). (e) Plot of the energy changing rate $I$ of magnetization against time (blue line) and the time dependence of $m_z$ (red line).

Figure 4

(a) Dependence of reversal time $t_{\text{s}}$ on the chirp rate for LP DCMWP of $H_{\text{mw}}=0.06$ T, $f_0=20$ GHz. (b) Time evolution of $m_z$ driven by LP DCMWP of $\eta =20{\mathrm{ns}}^{-2}$, $H_{\text{mw}}=0.06$ T, $f_0=20$ GHz. (c), (d) Phase diagram of magnetization reversal in terms of (c) CP and (d) LP DCMWP amplitude $H_{\text{mw}}$ and current density $J$. The pink region means the magnetization does not reverse or the reversal time is longer than 10 ns. The white region means the magnetization reverses within 10 ns.