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

Abstract

Microcalorimeters have been used to measure the temperature dependence of the specific heat ${C}_{p}(T)$ of amorphous ${R}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ $(R=\mathrm{Gd},\phantom{\rule{0ex}{0ex}}\mathrm{Tb})$ thin films prepared by both sputtering and e-beam coevaporation. $a\ensuremath{-}{\mathrm{Tb}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ 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 ${K}_{u}$ were prepared and studied. ${K}_{u}$ reflects the degree of nonrandomness in the local anisotropy axis directions. $a\ensuremath{-}{\mathrm{Gd}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ films possess negligible local and macroscopic anisotropy. All samples show a relatively sharp peak in ${C}_{p}(T)$ at the Curie temperature ${T}_{c}$ determined by magnetization measurements, indicative of a phase transition, independent of the magnitude of ${K}_{u}.$ Effective critical exponents of $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=\ensuremath{-}0.6$ 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 ${T}_{c},$ unlike what is seen in spin glasses. Increased growth or annealing temperature causes a small but systematic increase in ${T}_{c},$ in the inverse high-field susceptibility \ensuremath{\chi} and in the homogeneity of the sample; ${K}_{u}$ by contrast increases with growth temperature, but decreases with annealing.