| Amorphous Gd-Si is a spin glass which has many characteristics of classic spin glasses but displays as yet unexplained phenomena, including large magnetic entropy and enhanced effective magnetic moment near the metal-insulator transition. Specific heat and thermal conductivity measurements are important contributions to the understanding of amorphous solids, spin-glasses and the physics of the metal-insulator transition.; This thesis presents specific heat and thermal conductivity measurements of a-Gd-Si and its non-magnetic analog, a-Y-Si, focusing on temperatures 3–100 K and in magnetic field up to 8 Tesla. It also describes extentions of Si-N membrane-based thin-film microcalorimetry necessary for these measurements.; The specific heat of a-Gd-Si shows the classic feature of a spin-glass, a broad peak at approximately 1.4 × T f. Subtraction of the specific heat of a-Y-Si, allows the extraction of the magnetic specific heat for Gd-Si, which is large at low temperatures, field dependent, and gives a large magnetic entropy that suggests that near the metal-insulator transition, localized electrons cooperate in the spin-freezing.; Thermal Conductivity in both Gd-Si and Y-Si is extremely low below 20 K, even for a disordered solid. This is due to the heavy rare-earth metal ions rattling in “cages” in the silicon matrix, which limits the phonon mean free path. |
