Nanoscale physics
Thermally-induced phase transitions in Pt/Tb/Fe trilayers
I.O. Kruhlov1, N.Y. Schmidt2, G.L. Katona3, M. Albrecht2, and I.A. Vladymyrskyi1
1 Metal Physics Department, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Prospect Peremogy 37, 03056 Kyiv, Ukraine
E-mail: vladymyrskyi@kpm.kpi.ua
2 Institute of Physics, University of Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany
3 University of Debrecen, Faculty of Science and Technology, Department of Solid State Physics, P.O. Box 400, H-4002 Debrecen, Hungary

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 upon post-annealing in vacuum up to 620 °C were investigated by x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry, Auger electron spectroscopy depth profiling, and magnetic properties measurements. Pronounced Pt and Tb diffusion intermixing is observed at 215 °C, leading to the formation of the binary Pt2Tb phase. Further annealing up to 280 °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 L10-FePt phase starts to form in parallel to the disappearance of the Pt2Tb phase at 450 °C. The final phase product at 620 °C is characterized by the coexistence of two remaining phases, L10-FePt and TbO2. Structural phase transitions have a significant impact on the films magnetic properties, in particular heat treatment leads to an enhancement of the coercivity. This fact is associated with the formation of the hard magnetic L10-FePt phase, which might be accompanied by grain isolation induced by partially Tb/TbO grain boundary filling.