  Magnetic thin film heterostructures have been widely studied for fundamental
interests in the emergence of novel phenomena accompanied by the
heterointerface formation as well as their promising practical potential.
Combining x-ray magnetic circular dichroism with scanning tunneling microscopy,
we show for Mn/Fe thin film heterostructures that the interfacial factors
dominating electronic and magnetic properties of the entire system dynamically
change with the amount of the Mn overlayer. Element specific magnetization
curves of the Fe layer exhibit a two-step spin reorientation transition from
out-of-plane to in-plane direction with increasing the Mn coverage.
Atomic-scale characterizations of structural and electronic properties in
combination with the first-principles calculations successfully unravel the
roles of the entangled interfacial factors, and clarify the driving forces of
the transition. The first step of the transition at a low Mn coverage is
dominantly induced by the formation of FeMn disordered alloy at the
heterointerface, and the electronic hybridization with interfacial FeMn ordered
alloy is dominant as the origin of the second step of the transition at a high
Mn coverage.
