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Abstract
The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization ${M}_{S}$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and ${M}_{S}$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of ${M}_{S}$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.
Highlights
For the operation of spintronic and magnonic devices, various relaxation processes following perturbation of the magnetic state by an external stimulus play a role as important as the excitation processes themselves
We have studied the laser-induced dynamics of magnetization and magnetic anisotropy in thin films of the ferromagnetic metal galfenol based on the analysis of the evolution of the hysteresis loops following ultrafast laser-induced heating
We have demonstrated that the abrupt decrease of magnetization and magnetic anisotropy parameters is followed by an exponential recovery occurring within a nanosecond and being slower for the thicker film
Summary
For the operation of spintronic and magnonic devices, various relaxation processes following perturbation of the magnetic state by an external stimulus play a role as important as the excitation processes themselves. Magnetic anisotropy parameters at equilibrium can be obtained with high precision using ferromagnetic resonance [22] or torque measurements [23] Since these techniques are incompatible with time-resolved measurements, the evolution of magnetic anisotropy following ultrafast laser excitation is usually evaluated indirectly by monitoring laser-driven precession. By studying laser-induced changes of magneto-optical hysteresis loops, we deduce the temporal evolution of the saturation magnetization and of the cubic and uniaxial magnetic anisotropy parameters independently This enables us to confirm that during relaxation at the nanosecond timescale the ratio between the magnetic anisotropy and saturation magnetization satisfies the established power-law relation for thermal equilibrium. This is followed by conclusions, where we discuss relevance of the results to a number of prospective applications
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