A new microcalorimeter for measuring heat capacity of thin films in the range 1.5-800 K is described. Semiconductor processing techniques are used to create a device with an amorphous silicon nitride membrane as the sample substrate, a Pt thin film resistor for temperatures greater than 40 K, and either a thin film amorphous Nb-Si or a novel boron-doped polycrystalline silicon thermometer for lower temperatures. The addenda of the device, including substrate, is 4X 10m6 J/K at room temperature and 2~ 10e9 J/K at 4.3 K, approximately two orders of magnitude less than any existing calorimeter used for measuring thin films. The device is capable of measuring the heat capacity of thin film samples as small as a few micrograms.

We report results of specific heat measurements from 5 to 70 K on thin films of the giant negative magnetoresistive spin glass a-Gd x Si 12x and its nonmagnetic analog a-Y x Si 12x . The specific heat of a-Gd x Si 12x samples is significantly greater than that of a-Y x Si 12x samples below 50 K. The resulting magnetic entropy of a-Gd x Si 12x exceeds the total calculated entropy available from Gd moments alone. We suggest that the additional entropy is provided by the interaction of barely localized electron spins with the Gd moments.

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Reaction of solids of pure D 2d and D 2 C 84 , as well as mixtures of these isomers, with an excess of potassium produces the cubic phases K 8+x C 84 , with multiple occupancy of the octahedral site by potassium. Analysis of X-ray powder diffraction data reveals that the anions adopt well-defined orientations in the solid, driven by the anion-cation contacts. Electron paramagnetic resonance measurements on the solid fullerides indicate that the electronic properties depend on the particular isomer of C 84 used as host.