[Paper Review] Laser induced ultrafast demagnetization: an \emph{ab-initio} perspective

Kevin Krieger, J. K. Dewhurst+3 more|arXiv (Cornell University)|Jun 25, 2014

Geophysics and Sensor Technology1 references3 citations

TL;DR

This study presents the first ab-initio time-dependent density functional theory (TDDFT) analysis of laser-induced ultrafast demagnetization in Fe, Co, and Ni, revealing a two-step mechanism: initial electron excitation followed by spin-flip transitions in localized electrons. The key finding is that demagnetization is dominated by spin-orbit-mediated transitions in remaining localized electrons, not delocalized excited states, and can be controlled via tunable laser parameters such as intensity, frequency, and duration, explaining experimental time-lags and polarization independence.

ABSTRACT

Time-dependent density functional theory is implemented in an all electron solid-state code for the case of fully non-collinear spins. We use this to study laser induced demagnetization in Fe, Co and Ni. It is shown that this demagnetization is a two-step process: excitation of a fraction of electrons followed by spin-flip transitions of the remaining localized electrons. These results successfully explain several experimental features such as the time-lag between the start of the pulse and demagnetization and spin-flip excitations dominating the physics. We further show that it is possible to control the moment loss by tunable laser parameters like frequency, duration and intensity.

Motivation & Objective

• To understand the microscopic mechanism of laser-induced ultrafast demagnetization in transition metals using a fully ab-initio approach.
• To resolve the long-standing ambiguity in experimental observations, such as the time-lag between laser pulse onset and demagnetization.
• To determine whether spin-flip transitions or delocalized electron dynamics dominate the demagnetization process.
• To investigate the role of non-collinear spin dynamics and the absence of coherent magnon contributions in ultrafast demagnetization.
• To explore the feasibility of controlling magnetic moment loss through tunable laser parameters like intensity, frequency, and duration.

Proposed method

• Implementation of time-dependent density functional theory (TDDFT) with fully non-collinear spin degrees of freedom in an all-electron solid-state code.
• Use of the time-dependent Kohn-Sham equation with spin-dependent effective potential including vector potential from laser field and spin-orbit coupling.
• Simulation of laser excitation using time-dependent external potentials with tunable parameters: intensity, frequency, duration, and polarization.
• Propagation of spin-resolved Kohn-Sham orbitals in real time to compute magnetic moment evolution and electron dynamics.
• Analysis of spin-flip transitions and electron excitation pathways via time-resolved spin and charge density tracking.
• Validation of results against experimental observations such as time-lag and polarization independence.

Experimental results

Research questions

• RQ1 What is the dominant microscopic mechanism behind ultrafast laser-induced demagnetization in Fe, Co, and Ni?
• RQ2 Does the demagnetization process arise primarily from delocalized excited electrons or from spin-flip transitions in localized electrons?
• RQ3 How does the time-lag between laser pulse onset and moment loss arise in the simulation?
• RQ4 To what extent can the magnetic moment loss be controlled by tuning laser parameters such as intensity, frequency, and duration?
• RQ5 Does the polarization of the laser field influence the demagnetization dynamics in bulk systems?

Key findings

• Laser-induced demagnetization in Fe, Co, and Ni proceeds via a two-step mechanism: initial excitation of a fraction of electrons followed by spin-flip transitions in the remaining localized electrons.
• The dominant contribution to demagnetization comes from spin-orbit-mediated transitions in localized electrons, not from delocalized excited electrons.
• A time-lag of up to ~15 fs between the start of the laser pulse and the onset of demagnetization is observed, increasing with longer pulse durations.
• In bulk Ni, up to 71% loss in magnetic moment is achieved with a 60 fs laser pulse at 10^14 W/cm² intensity.
• The amount of demagnetization can be tuned between 20% and 53% in Ni by varying the laser pulse frequency, with an optimal frequency dependent on the material's band structure.
• Joint tuning of laser intensity, frequency, and duration allows for identical final moment loss (e.g., ~20% reduction) via different dynamic paths, demonstrating control potential.