Terahertz macrospin dynamics in insulating ferrimagnets

We investigate numerically the excitation of nonlinear magnetic interactions in a ferrite material by an energetic pump pulse of terahertz (THz) radiation. The calculations are performed by solving the coupled Maxwell and Landau-Lifshitz-Gilbert differential equations. In a time-resolved THz pump/THz probe scheme, it is demonstrated that Faraday rotation of a delayed THz probe pulse can be used to map these interactions. Our study is motivated by the ability of soft x-ray free electron lasers to perform time-resolved imaging of the magnetization process at the submicrometer and subpicosecond length and time scales.

Figure 1

(a) The THz pump and probe pulses. (b) A schematic diagram showing the initial polarization convention. Both $E_{\mathrm{THz}}$ and $H_{\mathrm{THz}}$ lie in the plane of the sample surface. A unit sphere is shown to represent the magnetization state. A trajectory of the magnetization vector, after excitation by the THz pulse, is plotted on the sphere, with red arrows to indicate the time evolution.

Figure 2

(a), (b) Time evolution of the THz field and magnetization components at a depth of 50 µm inside the sample. Space-time map of the THz pump magnetic field (c) and of the out-of-plane magnetization (d).

Figure 3

The input THz fields (a), and the transmitted THz fields at a pump-probe delay of (b) 10 ps and (c) 20 ps. The transmitted probe pulses are shown on a magnified scale at the right of the figure. (d) Faraday rotation of the transmitted THz probe pulse in comparison with $m_{\perp }$ at a depth of 50 µm.