Thermodynamic Measurements of Applied Magnetic Materials.

摘要

The specific heat of a material offers a host of information about the energetics of the system, from the phonons and electrons to phase changes in the material and two-state systems. In order to measure the specific heat of small samples such as quenched high pressure materials or thin films, one must turn to microcalorimetry. This thesis discusses the application of microcalorimetry to small magnetic samples and the underlying physics illuminated by the technique.;The thesis first describes the measurement of the spinel and olivine phases of Fe2SiO4 and the technical development necessary to measure a metastable small (10-100mug) sample, obtaining the first direct measurement of the entropy difference between the two phases.;Focusing next on the canonical giant magneto-resistive system of Fe/Cr multilayers, first is discussed the contributions of disorder to the electrons and phonons in the system where it is determined that disorder and strain plays a dominant role in the electronic density of states for thin films of chromium and not the antiferromagnetic state of the film. Next it is determined that while sputtered Fe/Cr multilayers do exhibit an interfacial enhancement in the density of states due to interfacial alloying, the spin-dependent scattering is more dependent upon a well-defined quantum well structure.;Finally, described herein is the development of a new calorimeter based on the ion beam-assisted deposition (IBAD) of MgO in order to measure the specific heat of epitaxial thin films. After measuring the lattice parameters of the IBAD MgO through synchrotron X-ray diffraction (XRD) and proving through XRD that thin films could successfully be grown epitaxially on the device, it was used to measure the specific heat of Fe-Rh alloys with ferromagnetic and antiferromagnetic ground states. Fe-Rh alloys have been suggested for application to thermally assisted magnetic recording, but there is much debate in the literature about the theoretical origin of the AF>FM transition. By taking the difference in specific heat of the two alloys, a predicted Schottky anomaly is observed and discussed in the context of two thermal fluctuation models. Our photoemission and specific heat data show that the origin of the transition is not adequately explained by the difference in electronic densities of states.