Stability phenomena in amorphous rare earth-transition metal films

TbFe films have been prepared with a range of compositions near to the room temperature compensation point and subjected to annealing at 100°C for up to 800 hours. Measurements of magnetic properties as a function of anneal time revealed a decrease in coercivity coupled with an increase in saturation magnetisation for all compositions. Changes in Msappeared to conform to a Johnson-Mehl-Avrami. exponent of approximately 0.5. Rutherford back-scattering spectrometry and electron microscopy revealed that the most likely cause of the observed changes in magnetic properties was oxygen diffusion resulting in the formation of TbOnoxides in turn leading to local diffusion, segregation and crystallisation of b.c.c, α-iron. The addition of an aluminium protective overlayer stablised the films against these changes, at least for the time and temperature ranges studied.