# Ab initio dynamical exchange interactions in frustrated anti-ferromagnets

## TL;DR

This paper introduces an ab initio formulation for dynamical exchange interactions in frustrated antiferromagnets and explores how ultrafast laser pulses induce different types of spin excitations, including spin transfer and spin wave formation.

## Contribution

It develops a non-local, purely dynamical exchange interaction framework from first principles and analyzes laser-induced spin excitations in transition metal antiferromagnets.

## Key findings

- Identification of two distinct laser-induced excitations: Stoner-like spin transfer and ultrafast Heisenberg-like exchange modification.
- Demonstration that ultrafast laser pulses can trigger spin wave formation within hundreds of femtoseconds.
- Proposal of a new ab initio approach to study dynamical exchange interactions in frustrated antiferromagnets.

## Abstract

The ultrafast response to an optical pulse excitation of the spin-spin exchange interaction in transition metal anti-ferromagnets is studied within the framework of the time-dependent spin-density functional theory. We propose a formulation for the purely dynamical exchange interaction, which is non-local in space, and it is derived starting from {\it ab initio} arguments. Then, we investigate the effect of the laser pulse on the onset of the dynamical process. It is found that we can distinguish two types of excitations, both activated immediately after the action of the laser pulse. While the first one can be associated to a Stoner-like excitation and involves the transfer of spin from one site to another, the second one is related to the ultrafast modification of a Heisenberg-like exchange interaction and can trigger the formation of spin waves in the first few hundred femtoseconds of the time evolution.