Ultrafast spin dynamics in inhomogeneous systems: a density-matrix approach applied to Co/Cu interfaces

TL;DR

This study uses a density-matrix approach to simulate ultrafast spin dynamics in Co/Cu interfaces, revealing the critical role of interfaces and orbital reflow in demagnetization and spin transport on femtosecond timescales.

Contribution

It introduces a density-operator simulation method for ultrafast spin dynamics in inhomogeneous systems, highlighting interface effects and orbital reflow phenomena.

Findings

Interfaces significantly influence ultrafast transport and demagnetization.

Orbital reflow from Cu to Co affects spin dynamics.

Minority-spin currents propagate into Cu layers.

Abstract

Ultrafast spin dynamics on femto- to picosecond timescales is simulated within a density-operator approach for a Co/Cu bilayer. The electronic structure is represented in a tight-binding form; during the evolution of the density operator, optical excitation by a femtosecond laser pulse, coupling to a bosonic bath as well as dephasing are taken into account. Our simulations corroborate the importance of interfaces for ultrafast transport phenomena and demagnetisation processes. Moreover, we establish a reflow from Cu $d$ orbitals across the interface into Co $d$ orbitals, which shows up prominently in the mean occupation numbers. On top of this, this refilling manifests itself as a minority-spin current proceeding several layers into the Cu region. The present study suggests that the approach captures essential ultrafast phenomena and provides insight into microscopic processes.