Robust magnetic order upon ultrafast excitation of an antiferromagnet

TL;DR

This study investigates the ultrafast magnetic order dynamics in an antiferromagnet using a three temperature model, revealing strong exchange coupling and the need for beyond mean-field theories for accurate descriptions.

Contribution

The paper applies the microscopic three temperature model to layered antiferromagnet GdRh$_2$Si$_2$ and compares it with experimental ultrafast magnetic dynamics data.

Findings

Both surface and bulk magnetic orders show two-step demagnetization with similar timescales.

The model overestimates demagnetization, suggesting additional effects beyond mean-field are important.

Strong exchange coupling between localized 4f and conduction electrons influences ultrafast magnetic behavior.

Abstract

The ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, we apply a M3TM to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh$_2$Si$_2$. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time- and angle-resolved photoemission spectroscopy and time-resolved resonant magnetic soft x-ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two-step demagnetization behaviors with two similar timescales (<1 ps, ~10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than our experimental observation, which can be phenomenologically described by a transient, fluence-dependent increased N\'eel temperature. Our results indicate that effects beyond a mean-field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.