Nonlinear quantum spin dynamics in magnetic systems

Project title: Nonlinear quantum spin dynamics in magnetic systems
Main applicant: Erik Sjöqvist, Materials Theory
Co-applicant: Olle Eriksson, Materials Theory
Grant amount: SEK 4 140 000 for the period 2026-2029

Atomistic spin dynamics studies the time evolution of magnetisation in magnetic systems from a microscopic perspective. A key tool in this field is the nonlinear Landau-Lifschitz-Gilbert (LLG) equation that describes damped dynamics of coupled spins. Although this equation treats the magnetisation as a collective effect arising from classical spins, by its very nature spin is a quantum-mechanical object. Therefore, atomistic spin dynamics will require a quantum description to make accurate predictions, especially since experiments on an increasingly shorter timescale will ultimately make the classical treatment obsolete. We have discovered a quantum analog of the LLG equation that shows non-trivial effects, such as entanglement-induced demagnetisation and unusual time-dependence of spin-spin correlation. The purpose of this four-year project is to extend, examine, and apply this quantum LLG (q-LLG) dynamics. We will justify the q-LLG dynamics and its extensions, by connecting them to time-local linear master equation dynamics and by comparing their classical limit with the classical LLG dynamics of multi-spin systems. We will develop a hybrid method that couples a quantum description with a classical one and apply this method to study the demagnetisation dynamics in magnetic materials. We further use the nonlinear master equation to analyze the formation and coherence of a spin qubit in a molecular nanomagnet and to analyze the role of quantum effects in magnetoreception.