Seminar - Ultrafast x-ray diffraction of magnetic heterostructures: Strain as trigger and sensor of spin dynamics


Matias Bargheer (Institute of Physics, University of Potsdam and Helmholtz-Zentrum Berlin, Germany)  

CEITEC BUT, Purkyňova 123, large seminar room (S building)  

Ultrafast X-ray diffraction (UXRD) experiments provide a unique access to coherent longitudinal acoustic phonons (coherent strain wave packets) and heat transport at the nanoscale (flow of incoherent excitations). Bragg-peak shifts are especially useful experimental observables in nano-layered systems, and contemporary laser-based sources of hard x-rays with femtosecond pulse duration have sufficient x-ray flux and stability to analyze the dynamics of films with single-digit nanometer thickness. This presentation will show that UXRD can be considered as an emerging tool of materials science by highlighting various experiments on heterostructures composed of simple metals, semiconductors and insulators as well as nanoscale materials with long range order such as ferroelectric, ferroelastic and (anti-)ferromagnetic materials. Basic parameters such as sound velocity and heat conductivity can be accessed at the nanoscale, but we can also quantify the absolute amplitude of hypersound waves and discriminate different quasiparticles contributing to the thermal expansion.

In order to establish a basis of our analysis of ultrafast (non-)thermal expansion, I will discuss the concept of macroscopic Grüneisen-parameters for the electronic, magnetic, and phononic system as well as a thermodynamic framework of thermal expansion. Based on this description we can appreciate that ultrafast lattice contraction can result from ultrafast entropy changes in the spin system, which are termed “demagnetization” in the literature of ultrafast magnetism. In turn we understand how coherent and incoherent phonons drive higher-order standing spin waves. In UXRD experiments with double-pump excitation we can distinguish different contributions to the thermal expansion by saturating the excitation of one of the contributing quasiparticles.