Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation

TL;DR

This study demonstrates that ultrashort laser pulses can directly quench the exchange interaction in ferromagnetic metals, leading to a reduction in spin moments, with potential implications for controlling magnetism dynamically.

Contribution

It provides experimental and theoretical evidence that photoexcitation reduces exchange interaction in 3d ferromagnets beyond the rigid band approximation.

Findings

Photoexcitation quenches exchange interaction in ferromagnetic metals.

Spin moment reduction follows a Bloch-like law.

Experimental results agree with theoretical predictions.

Abstract

The exchange interaction among electrons is one of the most fundamental quantum mechanical interactions in nature and underlies any magnetic phenomena from ferromagnetic ordering to magnetic storage. The current technology is built upon a thermal or magnetic field, but a frontier is emerging to directly control magnetism using ultrashort laser pulses. However, little is known about the fate of the exchange interaction. Here we report unambiguously that photoexcitation is capable of quenching the exchange interaction in all three $3d$ ferromagnetic metals. The entire process starts with a small number of photoexcited electrons which build up a new and self-destructive potential that collapses the system into a new state with a reduced exchange splitting. The spin moment reduction follows a Bloch-like law as $M_z(\Delta E)=M_z(0)(1-{\Delta E}/{\Delta E_0})^{\frac{1}{\beta}}$, where $\Delta E$ is the absorbed photon energy and $\beta$ is a scaling exponent. A good agreement is found between the experimental and our theoretical results. Our findings may have a broader implication for dynamic electron correlation effects in laser-excited iron-based superconductors, iron borate, rare-earth orthoferrites, hematites and rare-earth transition metal alloys.