ISBN 9783843925914

978-3-8439-2591-4, Reihe Physik

Stefan Günther
Ultrafast Probing of the Ferromagnetic Order Parameter

128 Seiten, Dissertation Universität Regensburg (2015), Hardcover, A5

Zusammenfassung / Abstract

Angular momentum conservation is a fundamental law in physics and applies to magnetism, where the magnetization is directly connected to the conserved quantity of angular momentum. After excitation with an intense laser stimulus, the system is in an extreme nonequilibrium condition and the magnetization is found to decay within several hundred femtoseconds. Due to angular momentum conservation, the magnetic moment needs to be transported from the spin system to the lattice on the timescale of the relevant underlying mechanism. Although tremendous experimental and theoretical efforts have been devoted to investigate various magnetic materials, no single key channel for angular momentum transfer, such as Coulomb, exchange, spin-orbit, electron-phonon, electron-magnon interactions, could be identified.

In this work, various effects and their contribution to ultrafast demagnetization in 3d-ferromagnets are discussed in detail. Typical demagnetization traces show a demagnetization on two different timescales and it is shown, that the slow part on the timescales of picoseconds is caused by the rather slow thermalization of the phonon system. The fast part of the demagnetization where the whole system is far of thermal equilibrium a rigid band structure approximation is shown to fail and additionally electron-magnon scattering is experimentally proven to have a huge share in ultrafast demagnetization. Additionally, ultrafast spin transport after laser excitation is measured with time- and spin-resolved photoemission spectroscopy in terms of a spin accumulation at the surface, which is directly driven by the thermalization of the magnetic system. Moreover magnetic alloys are investigated during an ultrafast triggered antiferromagnetic-ferromagnetic phase transition and the interplay of different magnetic sublattices of multisublattices magnets during ultrafast demagnetization is decoupled.