Relativistic theory for coupled orbital and spin angular momentum dynamics in magnetic systems

TL;DR

This paper develops a relativistic framework to describe coupled spin and orbital angular momentum dynamics in magnetic systems, accounting for electromagnetic influences and conservation laws.

Contribution

It introduces a comprehensive relativistic theory for orbital and spin angular momentum dynamics, including their coupling and conservation properties in magnetic materials.

Findings

Total angular momentum J is conserved without external fields.

Application of electromagnetic fields breaks the conservation of J.

Atomic spin and orbital moments are not individually conserved under electromagnetic influence.

Abstract

We develop a complete relativistic theory to describe the dynamics of electronic angular momentum including both spin (S) and orbital (L) contributions in magnetic systems. We start with the relativistic Dirac-Kohn-Sham Hamiltonian under the influence of an electromagnetic field and apply a unitary transformation to formulate the extended Pauli Hamiltonian. Using the transformed semirelativistic Hamiltonian, we derive the angular momentum dynamics for the orbital and spin angular momenta. Thereby, we formulate the coupled dynamics of orbital and spin moments consistent with the relativistic Dirac framework. Considering especially the conservation of the total angular momentum, J = S +L, we show first that J is conserved in the absence of a spin-polarized Kohn-Sham exchange field, but is no longer conserved under the application of an electromagnetic field, e.g., laser pulse, THz field, etc. Second, considering magnetic systems with atomic spin and orbital momenta, we derive the coupled equations of motion of angular momenta dynamics whilst making the atomistic Heisenberg approximation for the exchange interaction. Our results suggest that, under these assumptions, the total angular momentum remains conserved, even with electromagnetic field, but atomic spin and orbital angular momenta individually are not conserved.