Magnetic multilayers as a way to increase the magnetic field responsiveness of magnetocaloric materials.

The magnetocaloric response of Ni-Cu based multilayers has been studied with the aim of optimizing their magnetic field dependence. In contrast to the behavior of single phase materials, whose peak magnetic entropy change follows a power law with exponents close to 0.75, multilayering leads to exponents of -1 for an extended temperature span close to the transition temperature. This demonstrates that nanostructuring can be a good strategy to enhance the magnetic field responsiveness of magnetocaloric materials.

Structure and giant magnetoresistance behaviour of Co-Cu/Cu multilayers electrodeposited under various deposition conditions.

Electrodeposited Co-Cu/Cu multilayers were prepared under a variety of deposition conditions on either a polycrystalline Ti foil or on a silicon wafer covered by a Ta buffer and a Cu seed layer. X-ray diffraction (XRD) revealed a strong (111) texture for all multilayers with clear satellite peaks for the multilayers on Si/Ta/Cu substrates, in some cases for up to three reflections. Cross-sectional transmission electron microscopy investigations indicated a much more uniform multilayer structure on the Si/Ta/Cu substrates. The bilayer periods from XRD satellite reflections were in reasonable agreement with nominal values. An analysis of the overall chemical composition of the multilayers gave estimates of the sublayer thickness changes due to the Co-dissolution process during the Cu deposition pulse. The XRD lattice spacing data indicated a behaviour close to a simple "multilayer" Vegard's law which was, however, further refined by taking into account elastic strains as well. In agreement with the structural studies, magnetoresistance data also indicated the formation of more perfect multilayers on the smooth Si/Ta/Cu substrates. An analysis of the magnetoresistance behaviour revealed the presence of superparamagnetic (SPM) regions in the magnetic layers. The contribution of these SPM regions to the total observed giant magnetoresistance was found to be dominating under certain deposition conditions, e.g., for magnetic layer thicknesses less than 1 nm (about 5 monolayers).

Formation of nanocrystalline phases during thermal decomposition of amorphous Ni-P alloys by isothermal annealing.

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a-->ni by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) ni-->nii by isothermal annealing of the nanocrystalline ni state; (3) ni-->nii by linear heating of the ni state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.

Crystallite-size distribution and dislocation structure in nanocrystalline HfNi5 determined by x-ray diffraction profile analysis.

A rapidly quenched nanocrystalline Hf11Ni89 alloy was produced by melt-spinning. The x-ray phase analysis shows that the as-quenched ribbon consists mainly of nanocrystalline fcc HfNi5 although a small amount of Ni(Hf) solid solution is also detected. The crystallite size distribution and the dislocation structure of the dominant HfNi5 phase were determined by a recently developed method of diffraction profile analysis. In this procedure, by assuming spherical shape and log-normal size distribution of crystallites, the Fourier coefficients of the measured physical profiles are fitted by the Fourier coefficients of well established ab initio functions of size and strain peak profiles. The anisotropic broadening of peak profiles is accounted for by the dislocation model of the mean square strain in terms of average dislocation contrast factors. It was found that the median and the variance of the crystallite size distribution are 3.3 nm and 0.82, respectively. The dislocation density is 3.7 x 10(16) m-2 and the character of dislocations is nearly pure screw. The results obtained from x-rays were in good agreement with transmission electron microscopy observations.