# Magnon activation by hot electrons via non-quasiparticle states

## TL;DR

This paper investigates how hot electrons generated by femtosecond laser pulses transfer energy and magnetic moment to localized spins in half-metallic ferromagnets through non-quasiparticle states, using quantum kinetic theory.

## Contribution

It introduces a quantum kinetic model for magnon activation via non-quasiparticle states in ferromagnetic semiconductors after laser excitation.

## Key findings

- Magnons remain non-equilibrium for microseconds after laser pulse.
- Non-quasiparticle states facilitate energy transfer from hot electrons to spins.
- The derived kinetic equation describes magnon distribution dynamics.

## Abstract

We consider the situation when a femtosecond laser pulse creates a hot electron state in half-metallic ferromagnet (e. g. ferromagnetic semiconductor) on a picosecond timescale but do not act directly on localized spin system. We show that the energy and magnetic moment transfer from hot itinerant electrons to localized spins is facilitated by the so-called non-quasiparticle states, which are the scattering states of a magnon and spin-majority electron. The magnon distribution is described by a quantum kinetic equation that we derive using the Keldysh diagram technique. In a typical ferromagnetic semiconductor such as EuO magnons remain essentially in non-equilibrium on a scale of the order of microsecond after the laser pulse.