Spatial asymmetry of optically excited spin waves in anisotropic ferromagnetic film

TL;DR

This paper analytically and micromagnetically investigates how ultrafast laser excitation induces and controls spatial asymmetry in spin waves within anisotropic ferromagnetic films, revealing tunable phase, amplitude, and wavevector distributions.

Contribution

It introduces a novel analytical and micromagnetic approach to control the spatial asymmetry of magnetostatic waves via laser pulse parameters and magnetic field orientation.

Findings

Initial phase and amplitude are azimuthally asymmetric and tunable.

Wavevector distribution can be controlled, affecting wave propagation asymmetry.

Laser-induced torque terms have distinct azimuthal symmetries.

Abstract

We analytically discuss and micromagnetically prove the ways to tune the spatial asymmetry of the initial phase, amplitude, and wavevectors of magnetostatic waves driven by ultrafast laser excitation. We consider that the optical pulse heats a thin ferromagnetic metallic film and abruptly decreases the saturation magnetization and the parameter of uniaxial anisotropy. The two corresponding terms of laser-induced torque have different azimuthal symmetries, with the 4-fold symmetry of the demagnetization-related term, and the isotropic distribution of the anisotropy-related term. As a result, the initial phase and amplitude of excited magnetostatic waves have a non-trivial azimuthal distribution tunable with the angle between the external magnetic field and anisotropy axis, and the laser spot diameter. Moreover, the variation of these parameters tunes the distribution of wavevectors, resulting in additional asymmetry between the spectral components of the waves propagating in different directions.