Role of critical spin fluctuations in ultrafast demagnetization of transition-metal rare-earth alloys

TL;DR

This study investigates how critical spin fluctuations near the Curie temperature influence the ultrafast demagnetization process in transition-metal rare-earth alloys, revealing element-dependent dynamics and proposing a fluctuation-based mechanism.

Contribution

It introduces the role of critical spin fluctuations near Tc as a key factor affecting demagnetization rates in these alloys, supported by element-specific ultrafast measurements.

Findings

Demagnetization times are as fast as 280 fs for rare-earth elements below the compensation temperature.

Demagnetization times are limited to 500 fs when the laser-excited temperature is below Tc.

Critical spin fluctuations near Tc slow down the demagnetization process for 4f electrons.

Abstract

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co0.8Gd0.2, Co.74Tb.26 and Co.86Tb.14 alloys. Using element sensitivity of X-ray magnetic circular dichroism at the Co L3, Tb M5 and Gd M5 edges we evidence that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 fs is observed for the rare-earth in the alloy, when the laser excited state temperature is below the compensation temperature. It is limited to 500 fs when the laser excited state temperature is below the Curie temperature (Tc). We propose critical spin fluctuations in the vicinity of TC as the mechanism which reduces the demagnetization rates of the 4f electrons in transition-metal rare-earth alloys whereas at any different temperature the limited demagnetization rates could be avoided.