Field-driven femtosecond magnetization dynamics induced by ultrastrong coupling to THz transients

TL;DR

This paper demonstrates ultrastrong, coherent coupling between a single-cycle THz pulse and ferromagnetic thin film magnetization, enabling visualization of ultrafast dynamics far beyond natural electron precession speeds, with implications for high-speed data storage.

Contribution

It provides experimental evidence of ultrastrong, coherent light-magnetization coupling and models this interaction with a renormalized Landau Lifshitz equation, revealing new ultrafast magnetic phenomena.

Findings

Magnetization dynamics occur on a THz timescale, two orders faster than Larmor precession.

Coherent optical coupling can be described by a renormalized Landau Lifshitz equation.

Potential for higher data storage speeds through ultrafast magnetic control.

Abstract

Controlling ultrafast magnetization dynamics by a femtosecond laser is attracting interest both in fundamental science and industry because of the potential to achieve magnetic domain switching at ever advanced speed. Here we report experiments illustrating the ultrastrong and fully coherent light-matter coupling of a high-field single-cycle THz transient to the magnetization vector in a ferromagnetic thin film. We could visualize magnetization dynamics which occur on a timescale of the THz laser cycle and two orders of magnitude faster than the natural precession response of electrons to an external magnetic field, given by the Larmor frequency. We show that for one particular scattering geometry the strong coherent optical coupling can be described within the framework of a renormalized Landau Lifshitz equation. In addition to fundamentally new insights to ultrafast magnetization dynamics the coherent interaction allows for retrieving the complex time-frequency magnetic properties and points out new opportunities in data storage technology towards significantly higher storage speed.