Abstract
In this work, the microscopic evolution of the optically induced magneto-structural phase transition in FeRh was investigated. The electronic structure evolution of the unoccupied states was measured with time-resolved X-ray absorption spectroscopy (tr-XAS) at Fe L3 edge. It was related to the modification of the exchange interaction and generation of ferromagnetic order with the help of X-ray absorption spectral simulations based on density functional theory. X-ray absorption spectra were calculated under dipole approximation and one-electron theory for different temperatures, lattice constants and magnetic states (FM and AFM). The theoretical calculations suggested that most significant contribution to electronic band structure change comes from the modification of magnetic ordering and thus exchange interactions. The experiments were performed on epitaxial FeRh thin films grown on single crystalline SiC membranes. A direct measurement of XAS spectral changes without the need for magnetic contrast and magnetic field provide the inherent timescales of the competing interactions. It was demonstrated that the electronic structure and exchange interactions in FeRh can be modified at ultrafast time scales by laser excitation. At later time scales, long range correlations also clearly modify the local electronic structure as observed at the post edge to the Fe L3 edge. Competition between the ferromagnetic exchange (Fe-Rh) and the antiferromagnetic exchange (Fe-Fe) leads to the short range FM state at picosecond time scale after the laser excitation, but long-range ferromagnetic state can only be stabilized at the time scale of 10 ps, which is similar to time scale of lattice expansion (14 ps) observed in other experiments. Connection was made between the microscopic ferromagnetic state and macroscopic magnetization by studying the spatial evolution of ferromagnetic domains using time-resolved small angle X-ray scattering (tr-SAXS). It was shown that difference signal (pumped-unpumped) gives primarily the ferromagnetic contribution of the phase transition. Nucleation and growth dynamics of FM domains, with range of 10 nm to 60 nm domains, was observed up to 100 ps timescales. FM domains which nucleate from the condensation/cooling of non-equilibrium FM fluctuations around 3-4 ps grow at different rates at later times. The correlation functions and correlation length evolution, calculated by the Fourier transform of fitted SAXS intensity, give three regions of dynamical slowing down of phase transition. The physics behind the dynamics can be sought out by comparing the evolution of correlation length with Ising model. In the first region from 3 to around 8 ps, where there is a exponential down in the evolution of correlation length, the dynamical exponent decays from 14 to 1. In this region, non-equilibrium FM fluctuations condense into FM nucleation centers. In intermediary region from 8 to around 23 ps, where growth rate is slower, the dynamical exponent decays from 1 to 0.3. In this region, nucleated domains grow in size and the FM order parameter does not remain conserved. In the last region from 23 ps onwards, the dynamical exponent decays from 0.3 to 0.2. In this region, FM domains grow at the expense of smaller FM domains by coalescence. The FM order parameter, in this region, remains nearly conserved as formation of new FM phase saturates. Thus, tr-XAS and tr-SAXS measurements give unified microscopic picture of spatial and temporal evolution of the photo-induced phase transition in FeRh. Nevertheless, more experimental and theoretical investigations, at the Rh L or M edge, are needed to clarify the dynamics of FMordering and role of Rh in stabilizing the FM state of FeRh at higher temperature.
In another part of thesis, X-ray spectral simulations were performed in order to simulate the thermal changes in X-ray absorption spectra of 3d transition metals (Ti, Co, Cr, Cu). It was observed that thermal changes in X-ray spectra, for most part, can be explained by electron reshuffling according to the Fermi-Dirac distribution. Some small features in spectral changes, specifically in case of Ti and Cr, might involve actual band structure changes. Further theoretical investigations are needed in this regard ( such as using time dependent density functional theory (TDDFT) or dynamical mean field theory (DMFT)) to accurately calculate the electronic correlations and many-body effects.
In relation to ongoing efforts to achieve sub-femtosecond time resolution in FEL experiments, novel arrival time measurement scheme based on spectral encoding of THz pulse emission excited by FEL pulses was presented. This scheme was shown to be capable of in-situ pulse-resolved timing jitter measurements with just μJ/cm2 fluence and time resolution of few femtoseconds. It makes it more suitable for future MHz rep rate FEL facilities comparing to other transient reflectivity techniques which require fluence near damage threshold (∼20 mJ/cm2). Further, it was shown that this technique can be used parasitically for FEL pulse diagnostics. Further investigations on different materials are needed to improve the sensitivity of in-situ measurement scheme and to extend the measurements to broad FEL energy range and pulse duration.
