Indirect Evidence of Microcrystallite Formation in an Amorphous Ferrimagnet

Amorphous Tb-Fe has been prepared by magnetron co-sputtering onto substrates held nominally at room temperature. Samples with compositions ranging from 12 to 40 at.%Tb have been examined by transmission electron microscopy. They appear dense and featureless and show no sign of crystallinity. However, when samples are oxidized by annealing in room air at 200 degrees C, a relatively abrupt change at 22 at.%Tb is found in the microstructure. Specifically, samples containing more than 22 at.%Tb grow apparently Tb-oxide and Fe-oxide layers, while samples containing less than 22.%Tb grown an Fe-oxide consisting of huge crystals, along with an apparently amorphous Tb-oxide. Magnetic measurements of the details of the direction of the easy axis in these samples have also an extremely abrupt change at the same composition. These results, which have obvious technological implications for magneto-optic recording, will be interpreted in terms of microcrystalline formation in the Fe- rich amorphous Tb-Fe to oxidation. The composition at which they form, 22 at.%Tb, is believed to be related to a particular phase in the equilibrium phase diagram. These results are further investigated by annealing and by varying the substrate during deposition.