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.
Highlights
The field of spin-electronics comprises a variety of effects that often show up in inhomogeneous systems and as responses to a static cause
The attention has shifted to ultrafast processes which, for example, are pushed by femtosecond laser pulses or by electromagnetic terahertz radiation
The electron system is not coupled to thermodynamic baths (e. g., phonons or magnons)
Summary
The field of spin-electronics comprises a variety of effects that often show up in inhomogeneous systems and as responses to a static cause. On the femtosecond timescale time-dependent density-functional theory (TDDFT) offers the perhaps most fundamental and detailed description [1] Since such computations are very demanding, they are presently limited to a short duration (a few ten femtoseconds) and a small sample size (a few unit cells). Strong asymmetries of spin-dependent relaxation times and transition probabilities explain that after the laser excitation mostly hot majority-spin electrons traverse the interface and propagate rapidly from the ferromagnetic into the nonmagnetic region of the sample. This scenario is corroborated by experimental results on Co/Cu systems that show that demagnetisation is initially governed by spin-polarized transport starting at the interface [7]
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