Anomalies in hysteresis properties of Fe20Ni80/Tb-Co films with unidirectional anisotropy

• Hysteresis properties of FeNi/TbCo films depend nonmonotonically on temperature.
• Ti spacer changes significantly the hysteresis properties of exchange coupled FeNi layer.
• Annealing changes hysteresis properties of exchange coupled FeNi layer irreversibly.
• Ti spacer improves thermal stability of unidirectional anisotropy of FeNi layer.

Specific features of magnetization process in permalloy soft magnetic layer exchange coupled to an amorphous ferrimagnetic Tb-Co layer with strong in-plane magnetic anisotropy were investigated. Anomalous large variations in coercivity Hc and unidirectional anisotropy field He versus temperature T and annealing temperature Ta dependencies were observed. The changes in Hc and He were shown to be reversible for temperature range 5–300 K and irreversible for annealing temperatures 400–650 K. A critical role of a nonmagnetic spacer in the Нс(Т) and Нe(Т) formation were demonstrated. In conclusion simple explanation based on an assumption of thermally-induced variation of the interlayer magnetic boundary position and interdiffusion process was proposed.