Abstract
When a magnetic film is excited by a femtosecond laser pulse, either with THz or with optical frequencies, then there is at least a partial demagnetization within a few hundred femtoseconds, followed by a remagnetization to the original state on a bit longer time scale. This phenomenon is caused by a complex interaction of light with quantum matter. This paper gives a review of the present knowledge of the underlying physics. It discusses first the situation of a direct change of the magnetization by its interaction with the electromagnetic wave of the laser pulse, which appears during THz laser pulses with small field amplitudes. Then it considers the situation of an indirect change which appears after THz laser pulses with large field amplitudes and after optical laser pulses. In these cases the laser photons primarily excite electrons, with subsequent modifications of their spin-angular momenta by spin-flip scatterings of these electrons at quasiparticles, either at other electrons or at phonons or at magnons. The contributions of these various spin-flip scatterings to demagnetization are investigated. Then the transfer of angular momentum from the electronic spin system to the lattice during ultrafast demagnetization is discussed by describing the lattice vibrations in terms of magnetoelastic spin-phonon modes. Finally, the effect of electronic correlations in the sense of the density-matrix theory is investigated.
Highlights
The reason is that it is caused by Manfred Fähnle, Michael Haag et al.: Review of Ultrafast Demagnetization After Femtosecond Laser Pulses: A Complex Interaction of Light with Quantum Matter a complex interaction of light with quantum matter, and that the theoretical description requires very sophisticated methods
First the direct change of the magnetization by its interaction with the electromagnetic wave of the laser pulse which appears during THz laser pulses with small field amplitudes [5] is discussed
The calculation of the ultrafast demagnetization after fs optical laser pulses by spin-flip electron-phonon scatterings was performed on the basis of Fermi's golden rule for time-oscillating perturbations, here given by the time-oscillating perturbations produced by the phonons
Summary
At present this is the fastest possible macroscopic manipulation of the magnetization of a film, and it is of great interest from a fundamental point of view and from the view of a possible application in magnetic computers.
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