  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.
