Role of spatiotemporal nonuniformities in laser-induced magnetization precession damping

TL;DR

This study reveals that the observed damping anomaly in laser-induced magnetization precession is due to inhomogeneous excitation effects and interference, not an intrinsic material property.

Contribution

It demonstrates that inhomogeneous excitation and local magnetization interference explain damping anomalies, challenging the macrospin model.

Findings

Interference of local magnetizations causes damping time decrease.

Inhomogeneous relaxation distorts the precession envelope.

Dipole fields induce non-monotonic frequency behavior.

Abstract

Laser-induced magnetization precession measurements in ferromagnets often reveal an anomalous decrease in the damping time near a field-induced second-order spin-orientation transition, a behavior that cannot be described by the linearized Landau-Lifshitz-Gilbert equation. Here we demonstrate that this anomaly is not a material property but results from interference of precessing local magnetizations within the inhomogeneously excited region. By combining pump-probe experiments, analytical modeling that accounts for the finite sizes of the pump and probe spots, and micromagnetic simulations, we show that the standard macrospin approach fails to capture the observed dynamics. The inhomogeneous relaxation of magnetic parameters within the excitation area distorts the measured precession envelope, while dipole fields give rise to a temporally non-monotonic term in its frequency. Our results highlight the critical role of excitation locality in a vicinity of critical fields.