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.

Direct measurements of heat capacity from 80 to 540 K of antiferromagnetic superlattices of NiO (high Néel temperature T N), CoO (low T N), and MgO (nonmagnetic) are used to study the effect of exchange coupling and layer thickness on magnetic ordering. NiO͞CoO superlattices with thin layers show a single heat capacity peak similar to a Ni 0.5 Co 0.5 O alloy; with increasing layer thickness, the peak splits into two maxima. Finite size effects are seen in uncoupled NiO and CoO. Observed shifts in T N show the importance of correlation lengths and spin fluctuations in the ordering.

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Small-angle neutron scattering ͑SANS͒ and magnetic-force microscopy ͑MFM͒ have been used to characterize the temperature dependence of the ferromagnetic correlation length and the domain structure in amorphous TbFe 2 below its magnetic ordering temperature. Amorphous TbFe 2 is classified as a random anisotropy magnet, in the exchange-dominated limit, and previous SANS observations had shown a correlation length limited to 50 Å at low temperatures. In the present study, samples were prepared by both sputtering and electron beam coevaporation and were either grown or preannealed at 200°C in order to permit measurements above T c without structural relaxation. Samples grown by vapor deposition processes possess a large macroscopic perpendicular anisotropy constant K u , which can be reduced or eliminated by annealing. A strong SANS signal is seen in all samples, with a magnitude strongly correlated with the temperature-dependent sample magnetization and with the inverse length scale of the domain structure seen in MFM. For all samples, the magnetic correlation length determined from SANS is 300-500 Å in the thermally demagnetized state, and increases beyond measurement range after magnetizing. This long correlation length is consistent with theoretical predictions of a ferromagnetic ground state in exchange-dominated random anisotropy magnets in the presence of coherent anisotropy. The SANS signal is dominated by a Lorentzian squared term, which is best understood as resulting from ferromagnetic domains with meandering domain walls, similar to the Debye-Bueche model developed for materials consisting of two strongly segregated, interpenetrating phases.