Ultrafast demagnetization in metallic ferromagnetic/antiferromagnetic heterostructures

Abstract

This thesis summarizes experimental work performed on thin-film metallic and magnetic systems by time-resolved soft X-ray reflectivity and magneto-optical Kerr effect measurements. The delay-time-dependent magnetization dynamics in Co, Mn, and Ni after laser excitation are investigated in layered systems. The work aims to study the effects of antiferromagnetic coupling on de- and remagnetization dynamics in an adjacent ferromagnetic film on the ultrafast timescale. The presented work bridges the timescale from the first femtoseconds to macroscopic dynamics after several microseconds. The thesis is divided into a literature review covering the treatment and scientific context of the investigated phenomena and experimental reports where the findings of this work are presented and discussed. The presentation of the experiments is divided into four chapters. First, differences between the ultrafast demagnetization in a Cu/Co/NiMn/Cu(001) system recorded at high and low sample base temperatures are investigated. The report shows that the antiferromagnetic alignment in the NiMn system, present at low temperature, facilitates a faster demagnetization of the adjacent Co layer compared to a magnetically disordered NiMn layer at high temperatures. The second investigation covers the timescale of laser excitation and presents experimental evidence of the optically induced intersite spin transfer in a (Co/Mn)x6/Cu(001) multilayer system. An increase in the magnetic contrast in the antiferromagnetically ordered Mn layer is observed. The magnetic contrast lives for the time of excitation and indicates a realignment of the magnetic moments of the interface layers of Mn in the direction of Co. The third chapter treats simulations of the X-ray reflectivity after laser excitation. The multilayer system investigated in the aforementioned experiment is analyzed with respect to its magneto-optical properties, and the observations recorded with the X-rays are directly correlated with the transient optical response function of the elements. Furthermore, we compare the results of the simulations of the X-ray reflectivity considering structural, electronic, and magnetic effects. The last presented study is concerned with time-dependent magnetization dynamics recorded in Pt/Co multilayers. We investigate the delay-time-dependent Kerr effect and present experimental evidence for different dynamics in a Pt/Co bilayer system compared to a Pt/MnPt/Pt/Co system. Lastly, we investigate the effects of the spatial pump-probe overlap on the magnetization dynamics and link the observation to thermal and lateral transport phenomena.