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Ph.D. thesis by Pedro Guzman, presenting work on a microminiature Mossbauer spectrometer for possible planetary missions, and work with a large spectrometer at a beamline at the Advanced Photon Source. The work at the advanced photon source showed the thermodynamic origin of the pressure-induced Invar effect in Fe-45%Ni as a balance between phonons and spins. 

The anomalously low thermal expansion of Fe-Ni Invar has long been associated with magnetism, but to date, the microscopic underpinnings of Invar behavior have eluded both theory and experiment. Here, we present nuclear resonant X-ray scattering measurements of the phonon and magnetic entropy under pressure. By applying a thermodynamic Maxwell relation to these data, we obtain the separate phonon and magnetic contributions to thermal expansion. We find that the Invar behavior stems from a competition between phonons and spins. In particular, the phonon contribution to thermal expansion cancels the magnetic contribution over the 0 - 3 GPa pressure range of Invar behavior. At pressures above 3 GPa the cancellation is lost, but our analysis reproduces the positive thermal expansion measured separately by synchrotron X-ray diffractometry. Ab initio calculations informed by experimental data show that spin-phonon interactions improve the accuracy of this cancellation over the range of Invar behavior. Spin-phonon interactions also explain how different phonon modes have different energy shifts with pressure. 

The heat capacities of nanocrystalline Ni3Fe and control materials with larger crystallites were measured from 0.4300 K. The heat capacities were integrated to obtain the enthalpy, entropy, and Gibbs free energy and to quantify how these thermodynamic functions are altered by nanocrystallinity. From the phonon density of states (DOS) measured by inelastic neutron scattering, we find that the Gibbs free energy is dominated by phonons and that the larger heat capacity of the nanomaterial below 100 K is attributable to its enhanced phonon DOS at low energies. Besides electronic and magnetic contributions, the nanocrystalline material has an additional contribution at higher temperatures, consistent with phonon anharmonicity. The nanocrystalline material shows a stronger increase with temperature of both the enthalpy and entropy compared to the bulk sample. Its entropy exceeds that of the bulk material by 0.4 kB/atom at 300 K. This is insufficient to overcome the enthalpy of grain boundaries and defects in the nanocrystalline material, making it thermodynamically unstable with respect to the bulk control material.