Experimental confirmation of the delayed Ni demagnetization in FeNi alloy

TL;DR

This study confirms that in FeNi alloys, Fe and Ni demagnetize with similar timing after ultrafast excitation, using a reliable element-specific technique, clarifying previous uncertainties caused by measurement cross talk.

Contribution

The paper provides the first direct, cross-talk-free experimental verification of delayed Ni demagnetization in FeNi alloys, challenging prior results obtained with potentially nonlinear T-MOKE measurements.

Findings

Fe and Ni demagnetize with similar timing.

XMCD results align with T-MOKE despite known nonlinearities.

Provides reliable data on ultrafast spin dynamics in FeNi.

Abstract

Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials like magnetic alloys. Recently, though, it turned out that the commonly used technique of transverse magneto-optical Kerr effect (T-MOKE) in the EUV range may have linearity issues including unwanted cross talk between different elemental signals. This problem can be sizeable, which puts recent observations of ultrafast spin transfer from Fe to Ni sites in FeNi alloys into question. In this study, we investigate the Fe-to-Ni spin transfer in a cross-talk-free time-resolved X-ray magnetic circular dichroism (XMCD) experiment with a reliable time reference. We find a very similar Fe and Ni dynamics with XMCD as with T-MOKE from identical samples. Considering the non-linearities of the T-MOKE response, the agreement with our findings appears fortuitous. We discuss possible reasons why T-MOKE seems to give accurate results in this case. Our data provide the ongoing discussion about ultrafast spin-transfer mechanisms in FeNi systems with a sound experimental basis.