Thermally-induced phase transitions in Pt/Tb/Fe trilayers

Structural phase formation induced by thermally-activated diffusion processes and intermixing of layered structures is a promising reaction pathway for the synthesis of unexpected (metastable) thin film materials. Furthermore, the sequence of structural phase transitions during annealing can substantially differ from those predicted by corresponding bulk phase diagrams due to the underlying grain boundary diffusion mechanisms as well as surface and interface effects. In this study, phase transitions in Pt/Tb/Fe trilayers occurring during post-annealing up to 620 degrees C were investigated by various techniques including x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry, Auger electron spectroscopy depth profiling, and magnetic properties measurements. At an annealing temperature of 215 degrees C pronounced Pt and Tb diffusion intermixing is observed leading to the formation of the binary Pt2Tb phase. Further annealing up to 280 degrees C is accompanied by the appearance of the chemically disordered A1-FePt phase and significant Tb segregation to the outer surface forming an oxide layer. The chemically ordered L(1)0-FePt phase starts to form in parallel to the disappearance of the Pt2Tb phase at 450 degrees C. The final phase product at 620 degrees C is characterized by the coexistence of two remaining phases, L(1)0-FePt and TbO2. Structural phase transitions have a significant impact on the films magnetic properties - heat treatment leads to an enhancement of the coercive field. This fact is associated with the formation of the hard magnetic L(1)0-FePt phase, which might be accompanied by grain isolation induced by partially Tb/Tb-O grain boundary filling.