Spin waves excited by hard x-ray transient gratings

TL;DR

This paper demonstrates the use of ultrashort hard x-ray transient gratings to excite and probe magnetic and lattice dynamics, including spin waves, in a ferrimagnetic film, revealing new capabilities for wave vector access.

Contribution

It introduces a novel application of x-ray transient gratings for driving coherent phonons and magnons, expanding the toolkit for ultrafast magnetic dynamics studies.

Findings

Magnetization precession observed at spin wave frequencies.

Strain from thermal expansion drives the magnetization precession.

X-ray transient gratings can access wave vectors across the Brillouin zone.

Abstract

Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and non-magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.