Magnetic coupling at ferromagnetic rare earth / transition-metal interfaces: A comprehensive study

Thin film magnetic heterostructures with competing interfacial coupling and Zeeman energy pro- vide a fertile ground to study phase transition between different equilibrium states as a function of external magnetic field and temperature. A rare-earth (RE) / transition metal (TM) ferro- magnetic multilayer is a classic example where the magnetic state is determined by a competition between the Zeeman energy and antiferromagnetic interfacial exchange coupling energy. Techno- logically, such structures offer the possibility to engineer the macroscopic magnetic response by tuning the microscopic interactions between the layers. We have performed an exhaustive study of a nickel/gadolinium system by using the element-specific measurement technique x-ray magnetic circular dichroism, and determined the full magnetic state diagrams as a function of temperature and magnetic layer thickness. We explain our result based on a modified Stoner-Wohlfarth formal- ism and provide evidence of a thickness-dependent phase transition to a magnetic fan state which is critical in understanding magnetoresistance effects in RE/TM systems. The results provide im- portant insight for spintronics and superconducting spintronics where engineering tunable magnetic inhomogeneity is key for certain applications.