Specific heat of amorphous rare-earth–transition-metal films

Microcalorimeters have been used to measure the temperature dependence of the specific heat Cₚ(T) of amorphous RₓFe_100-x (R=Gd,0ex0exTb) thin films prepared by both sputtering and e-beam coevaporation. a-TbₓFe_100-x films possess large randomly oriented local magnetic anisotropy and large exchange coupling; they are considered random-anisotropy magnets. By varying growth temperature and by annealing, films of the same composition but with very different macroscopic anisotropy constant Kᵤ were prepared and studied. Kᵤ reflects the degree of nonrandomness in the local anisotropy axis directions. a-GdₓFe_100-x films possess negligible local and macroscopic anisotropy. All samples show a relatively sharp peak in Cₚ(T) at the Curie temperature Tc determined by magnetization measurements, indicative of a phase transition, independent of the magnitude of Kᵤ. Effective critical exponents of α=α^'=-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 entropy is developed below Tc, unlike what is seen in spin glasses. Increased growth or annealing temperature causes a small but systematic increase in Tc, in the inverse high-field susceptibility {χ} and in the homogeneity of the sample; Kᵤ by contrast increases with growth temperature, but decreases with annealing.