Electronic theory of ultrafast spin dynamics

TL;DR

This paper develops a quantum chemistry-based approach to study ultrafast spin dynamics in NiO using second harmonic generation, revealing subpicosecond demagnetization and the importance of magnetic fields and phonons in switching.

Contribution

It introduces a systematic quantum chemistry method to model and analyze ultrafast spin switching in NiO, including the effects of magnetic fields, phonons, and laser polarization.

Findings

Demagnetization and switching occur within subpicoseconds.

Linearly polarized light is more effective than circularly polarized light.

External magnetic fields and magnetic-dipole transitions are crucial for observing spin dynamics.

Abstract

NiO is a good candidate for ultrafast magnetic switching because of its large spin density, antiferromagnetic order, and clearly separated intragap states. In order to detect and monitor the switching dynamics, we develop a systematic approach to study optical second harmonic generation (SHG) in NiO, both at the (001) surface and in the bulk. In our calculations NiO is modeled as a doubly embedded cluster. All intragap \emph{d}-states of the bulk and the (001) surface are obtained with highly-correlational quantum chemistry and propagated in time under the influence of a static magnetic field and a laser pulse. We find that demagnetization and switching can be best achieved in a subpicosecond regime with linearly rather than circularly polarized light. We also show the importance of including an external magnetic field in order to distinguish spin-up and spin-down states and the necessity of including magnetic-dipole transitions in order to realize the $\Lambda$-process in the centrosymmetric bulk. Having already shown the effects of phonons in the SHG for the bulk NiO within the frozen-phonon approximation, and following the same trail of thoughts, we discuss the role of phonons in a fully quantized picture as a symmetry-lowering mechanism in the switching scenario and investigate the electronic and lattice temperature effects.