Terahertz charge and spin transport in metallic ferromagnets: the role of crystalline and magnetic order

TL;DR

This study investigates how crystalline and magnetic order in CoFeB thin films affects charge and spin transport, revealing that structural order influences conductivity and ultrafast demagnetization, with complex dependencies on annealing temperature.

Contribution

It demonstrates the relationship between crystalline order, charge transport, and ultrafast demagnetization in ferromagnetic thin films, highlighting non-monotonic behavior with annealing.

Findings

Higher crystalline order increases conductivity.

Ultrafast demagnetization decreases with increased order, except at intermediate annealing.

Demagnetization correlates with in-plane magnetic anisotropy.

Abstract

We study the charge and spin dependent scattering in a set of CoFeB thin films whose crystalline order is systematically enhanced and controlled by annealing at increasingly higher temperatures. Terahertz conductivity measurements reveal that charge transport closely follows the development of the crystalline phase, with increasing structural order leading to higher conductivity. The terahertz-induced ultrafast demagnetization, driven by spin-flip scattering mediated by the spin-orbit interaction, is measurable in the pristine amorphous sample and much reduced in the sample with highest crystalline order. Surprisingly, the largest demagnetization is observed at intermediate annealing temperatures, where the enhancement in spin-flip probability is not associated with an increased charge scattering. We are able to correlate the demagnetization amplitude with the magnitude of the in-plane magnetic anisotropy, which we characterize independently, suggesting a magnetoresistance-like description of the phenomenon.