Perpendicularly magnetized L 1₀-FePt nanodots exchange-coupled with soft magnetic Ni₈₁Fe₁₉

We studied exchange-coupled bilayers consisting of a perpendicularly magnetized L1₀-FePt layer and a soft magnetic Ni₈₁Fe₁₉ (Permalloy; Py) layer, where the Py layer was thick enough to form spatially twisted magnetic structures. The Py layer showed in-plane magnetization in the case of unpatterned thin film because of its demagnetizing field even though Py was exchange-coupled to the perpendicularly magnetized L1₀-FePt layer at the interface. After patterning the FePt/Py bilayer into nanosized dots with cylindrical shape, the effective perpendicular magnetic anisotropy was induced. Exchange-coupling behavior, i.e., spring back behavior, was observed when the minor magnetization curves were measured. In addition to this behavior, the magnetic domain observation suggested that a spatially twisted magnetic structure was formed with sweeping the perpendicular magnetic field.