Thermal differential EXAFS

Abstract

Differential EXAFS (DiffEXAFS) is a new and novel technique for the study of small atomic strains. It relies on examining tiny differences in x-ray absorption spectra - taken under high-stability, low-noise conditions - generated by unit modulation of some sample bulk parameter. Initial experiments conducted by Pettifer et al. [64] to measure the magnetostriction of FeCo, revealed a sensitivity to atomic displacements of the order of one femtometre (10−15m). This was two orders of magnitude more sensitive than thought possible, based on conventional EXAFS techniques [16] [2]. The mandate for this thesis was to extend DiffEXAFS to the case of samples undergoing temperature modulation - to develop Thermal Differential EXAFS - and in doing so, demonstrate that DiffEXAFS is a generally applicable technique for studying small atomic strains. Topics covered here include the nature of Thermal DiffEXAFS signals, the design, manufacture, and characterisation of apparatus for Thermal DiffEXAFS experiments, and new analysis techniques developed to extract information from DiffEXAFS data. Thermal expansion coefficients have been determined for Fe and SrF2, for temperature modulation of the order of one Kelvin, proving the viability of the technique. Numerically, these were a Fe = (11.6±0.4)×10−6K−1 and a SrF2 = (19±2)×10−6K−1 respectively, which agreed with published values [52] [74]. In these measurements sensitivity to mean atomic displacements of about 0.3 femtometres was achieved. The more interesting case of thermally induced phase transitions has also been studied, with DiffEXAFS measurements taken through the Martensitic phase transition of the Heusler alloy Ni2MnGa. These revealed a hardening of the lattice as the transition was approached in the Martensite phase, agreeing with published trends [93][56], and an accompanying lattice contraction not seen previously