# Ultrafast magnetization dynamics in uniaxial ferrimagnets with compensation point. GdFeCo

## TL;DR

This paper develops a theoretical framework to describe ultrafast magnetization dynamics in uniaxial ferrimagnets near the compensation point, focusing on GdFeCo, and reveals critical behavior and control mechanisms for spin dynamics.

## Contribution

It introduces an effective Lagrangian approach for ferrimagnets near the compensation point, highlighting the role of external magnetic fields and ultrafast pulses in controlling magnetization dynamics.

## Key findings

- Non-zero torque only in non-collinear magnetic phase.
- Critical behavior of magnetization response near phase transition.
- Potential for controlling spin dynamics amplitude and timescale.

## Abstract

We derive an effective Lagrangian in the quasi-antiferromagnetic approximation that allows to describe the magnetization dynamics for uniaxial f-d (rare-earth - transition metal) ferrimagnet near the magnetization compensation point in the presence of external magnetic field. We perform calculations for the parameters of GdFeCo, a metallic ferrimagnet with compensation point that is one of the most promising materials in ultrafast magnetism. Using the developed approach, we find the torque that acts on the magnetization due to ultrafast demagnetization pulse that can be caused either by ultrashort laser or electrical current pulse. We show that the torque is non-zero only in the non-collinear magnetic phase that can be acquired by applying external magnetic field to the material. The coherent response of magnetization dynamics amplitude and its timescale exhibits critical behavior near certain values of the magnetic field corresponding to a spin-flop like phase transition. Understanding the underlying mechanisms for these effects opens the way to efficient control of the amplitude and the timescales of the spin dynamics, which is one of the central problems in the field of ultrafast magnetism.