Uncovering the role of the density of states in controlling ultrafast spin dynamics

TL;DR

This study demonstrates that the efficiency of ultrafast demagnetization in ferromagnetic materials is governed by the density of unoccupied electronic states, confirming the role of optical intersite spin transfer predicted by theory.

Contribution

The paper provides experimental evidence linking the density of unoccupied states to demagnetization efficiency, validating the theoretical concept of optical intersite spin transfer.

Findings

Demagnetization efficiency increases from Fe to Ni in elemental magnets.

Multi-component systems show enhanced demagnetization proportional to unoccupied states.

Ab initio calculations accurately reproduce experimental trends.

Abstract

At the ultrafast limit of optical spin manipulation is the theoretically predicted phenomena of optical intersite spin transfer (OISTR), in which laser induced charge transfer between the sites of a multi-component material leads to control over magnetic order. A key prediction of this theory is that the demagnetization efficiency is determined by the availability of unoccupied states for intersite charge transfer. Employing state-of-the-art magneto-optical Kerr effect measurements with femtosecond time resolution, we probe this prediction via a systematic comparison of the ultrafast magnetic response between the 3d ferromagnets, Fe, Co, and Ni, and their respective Pt-based alloys and multilayers. We find that (i) the demagnetization efficiency in the elemental magnets increases monotonically from Fe, via Co to Ni and, (ii), that the gain in demagnetization efficiency of the multi-component system over the pure element counterpart scales with the number of empty 3d minority states, exactly as predicted by the OISTR effect. We support these experimental findings with ab initio time-dependent density functional theory calculations that we show to capture the experimental trends very well.