SPECIFIC-HEAT OF AMORPHOUS RARE-EARTH-TRANSITION-METAL FILMS

Microcalorimeters have been used to measure the temperature dependence of the specific heat C-p(T) of amorphous RxFe100-x (R=Gd, Tb) thin fi lms prepared by both sputtering and e-beam coevaporation. a-TbxFe100-x films possess large randomly oriented local magnetic anisotropy and l arge exchange coupling; they are considered random-anisotropy magnets. By varying growth temperature and by annealing, films of the same com position but with very different macroscopic anisotropy constant K-u w ere prepared and studied. K-u reflects the degree of nonrandomness in the local anisotropy axis directions. a-GdxFe100-x films possess negli gible local and macroscopic anisotropy. All samples show a relatively sharp peak in C-p(T) at the Curie temperature T-c determined by magnet ization measurements, indicative of a phase transition, independent of the magnitude of K-u. Effective critical exponents of alpha= alpha' = - 0.6 to -0.7 and a critical amplitude ratio of 1.5-2.5 are measured for reduced temperatures down to 0.02. Nearly all possible magnetic en tropy is developed below T-c, unlike what is seen in spin glasses. Inc reased growth or annealing temperature causes a small but systematic i ncrease in T-c, in the inverse high-field susceptibility chi and in th e homogeneity of the sample; K-u by contrast increases with growth tem perature, but decreases with annealing.