Coexistence of Hard and Soft Magnetic Phases in Single Layer Amorphous Tb-Co Thin Films.

Amorphous Tb-Co thin films prepared as layers with a thin Ta underlayer and either a Ta or Pt overlayer (Ta(2 nm)/a-Tb_xCo_1-x (t)/ Ta or Pt (7 nm), with t varying from 1-16 nm), show evidence of both a soft and a hard magnetic phase despite no sign of this at room temperature. Low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature which we attribute to increased anisotropy energy at lower temperatures. The different t allowed us to separate out the effects of Ta or Pt overlayers. By decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft phase was isolated and found to be present in all the films which was also seen with x-ray reflectivity measurements, suggesting that magnetic nanoparticles form for the first 2 nm which are then buried by a homogeneous film on top responsible for the hard phase. This model was validated using micromagnetic simulations. The different effects of Ta and Pt capping layers on the magnetic properties lead to significant differences in magnetization, dominated by the enhanced anisotropy.