# Temperature Dependence of Magnetic Excitations: Terahertz Magnons above the Curie Temperature

## TL;DR

This study investigates how terahertz magnons in an ultrathin ferromagnet behave across the Curie temperature, revealing they maintain their propagating nature even at high temperatures.

## Contribution

It provides the first detailed analysis of high-energy magnons' temperature dependence and their persistence above the Curie temperature using high-resolution spectroscopy.

## Key findings

- Magnon energy and lifetime decrease with temperature.
- Magnons retain their propagating character above TC.
- Temperature-induced damping varies with wave vector.

## Abstract

When an ordered spin system of a given dimensionality undergoes a second order phase transition the dependence of the order parameter i.e. magnetization on temperature can be well-described by thermal excitations of elementary collective spin excitations (magnons). However, the behavior of magnons themselves, as a function of temperature and across the transition temperature TC, is an unknown issue. Utilizing spin-polarized high resolution electron energy loss spectroscopy we monitor the high-energy (terahertz) magnons, excited in an ultrathin ferromagnet, as a function of temperature. We show that the magnons' energy and lifetime decrease with temperature. The temperature-induced renormalization of the magnons' energy and lifetime depends on the wave vector. We provide quantitative results on the temperature-induced damping and discuss the possible mechanism e.g., multi-magnon scattering. A careful investigation of physical quantities determining the magnons' propagation indicates that terahertz magnons sustain their propagating character even at temperatures far above TC.