Magnetization switching in bistable nanomagnets by picosecond pulses of surface acoustic waves

We perform a theoretical investigation of the magnetization switching in polycrystalline Ni nanoparticles induced by ultrashort pulses of surface acoustic waves via magnetoelastic interactions. In our numerical simulations, a Ni nanoparticle is modeled as an ellipsoidal disk deposited on a dielectric substrate. The in-plane external magnetic field breaks the symmetry and allows us to adjust the height of the energy barrier between two metastable magnetization states of the free-energy density and dramatically lower the amplitude of elastic strain pulses required for magnetization switching. The switching threshold is shown to depend on the duration of an acoustic pulse, the magnetic shape anisotropy of an elliptical nanoparticle, the amplitude of the external magnetic field, and the magnetostriction coefficient. We introduce the magnetoelastic switching diagram, allowing for the simultaneous visualization of the switching threshold and its characteristic timescale as a function of various physical parameters.

Figure 1

Geometry of the problem: Absorption of an ultrashort laser pulse in a thin-film opto-acoustic transducer launches picosecond surface acoustic transients (SSLW and SAW pulses, see Sec. 3 for details), which propagate along the surface and interact with a single elliptical nanomagnet via magnetostriction mechanism. A constant in-plane magnetic field is used to modify the free energy of the nanomagnet.

Figure 2

(a) Dependence of the free energy on the azimuthal angle $\varphi$, calculated for different values of the external magnetic field. (b) Dependence of a small-angle FMR precession on the magnetic field. (c) Influence of the static strain $e_{xx}=\pm 5\times {10}^{-4}$ on the nanomagnet's free energy at $B=30\mathrm{mT}$.

Figure 3

Magnetization dynamics and switching induced by a single SAW pulse (dashed line) with different amplitudes: (a) no switching at $\eta =1.8\times {10}^{-4}$, (b) switching at $\eta =3.2\times {10}^{-4}$, (c) no switching at $\eta =6.5\times {10}^{-4}$. Duration of the SAW pulse is $\tau =280\mathrm{ps}$, the pulse is centered at $t_0=0.8\mathrm{ns}$. Magnetic field is $B=30\mathrm{mT}$.

Figure 4

Magnetization dynamics and switching induced by a single SAW pulse (dashed line) with constant amplitude $\eta =5\times {10}^{-4}$ for three different values of magnetic field: (a) no switching at $B=20\mathrm{mT}$, (b) switching at $B=27\mathrm{mT}$, (c) no switching at $B=35\mathrm{mT}$. Duration of the SAW pulse is $\tau =280\mathrm{ps}$, the pulse is centered at $t_0=0.8\mathrm{ns}$.

Figure 5

(a) Magnetoelastic $\eta -\tau$ switching-time diagram for $B=30\mathrm{mT}$. Dashed areas are not switched, the color within the switched areas maps the switching time in picoseconds (see the color bar and text for details). (b) Magnetization trajectories for different strain amplitudes $\eta =1.8,3.2,6.5,10\times {10}^{-4}$. The switching time for $\eta =3.2\times {10}^{-4}$ and $\eta =10\times {10}^{-4}$ is 260 ps and 170 ps, respectively. The duration of the SAW pulse $\tau =280\mathrm{ps}$. Dashed lines show five switching trajectories under the influence of random thermal noise (see text for details). Panel (c) demonstrates how thermal noise affects the magnet-elastic $\eta -\tau$ switching-time diagram: The boundaries between different regions look fuzzy, while the structure of the diagram persists.

Figure 6

(a) Magnetoelastic $\eta -B$ switching-time diagram for $\tau =280\mathrm{ps}$. Dashed areas are not switched, the color maps within the switched areas map the switching time in picoseconds (see text for details). (b) Magnetization trajectories for different values of the magnetic field $B=20,27,35\mathrm{mT}$ and the SAW amplitude $\eta =5\times {10}^{-4}$. Panel (c) demonstrates how thermal noise affects the magnetoelastic $\eta -B$ switching-time diagram: The boundaries between different regions are particularly affected for smaller ($B<5\mathrm{mT}$) and larger ($B>33\mathrm{mT}$) magnetic fields.