Dissipation of Spin Angular Momentum in Magnetic Switching

Applying one ultrashort magnetic field pulse, we observe up to 10 precessional switches of the magnetization direction in single crystalline Fe films of 10 and 15 atomic layers. We find that the rate at which angular momentum is dissipated in uniform large angle spin precession increases with time and film thickness, surpassing the intrinsic ferromagnetic resonance spin lattice relaxation of Fe by nearly an order of magnitude.

Figure 1

Magnetic pattern generated with a single electron bunch in a 15 ML $\mathrm{Fe}/\mathrm{GaAs}(001)$ epitaxial bcc Fe film. The magnetic image is obtained by SEMPA after sputtering off the capping layer of 10 ML Au. Prior to the field pulse, $\overrightarrow{M}$ is aligned horizontally to the right, shown in light gray. The regions were $\overrightarrow{M}$ has switched to the left are shown as dark gray. On the left and lower left side, the pattern is disturbed by motion of domain walls after exposure. In the center, a large spot due to beam damage appears. The framed part is shown at greater magnification in the middle with the fitted circles and at the bottom at still larger magnification exposing zigzag domain boundaries.

Figure 2

Magnetic pattern generated with a single electron bunch in a 10 ML $\mathrm{Au}/10\mathrm{ML}$ $\mathrm{Fe}/\mathrm{GaAs}(001)$ epitaxial Fe film, otherwise as in Fig. 1. No after pulse motion of domain walls occurred in this sample, but the pattern is less regular than with 15 ML.

Figure 3

Energy deposited in the spin system in units of the uniaxial in-plane anisotropy constant $K_u$ vs the precession angle $\phi$. Data points are for 10 ML Fe (squares) and 15 ML Fe (circles). The simulations are with the Gilbert damping $\alpha =0.004$ and no magnon scattering (solid line), and for 10 ML Fe (dotted line) and 15 ML Fe (dashed line) including magnon scattering. The inset shows the relative saturation magnetization $M(t)/M_s(0)$, where $t$ is the time after an exciting field pulse of amplitude $0.24\times {10}^6\mathrm{A}/\mathrm{m}$ for 10 ML Fe (dotted line) and $0.175\times {10}^6\mathrm{A}/\mathrm{m}$ for 15 ML Fe (dashed line). Without magnon scattering (solid line) we have $M(t)/M_s=1$.