Size dependence in the magnetization reversal of Fe∕Gd multilayers on self-assembled arrays of nanospheres

The size dependent magnetization reversal of Fe∕Gd multilayers on self-assembled silica nanospheres has been studied by scanning transmission x-ray microscopy. Caps on 800nm spherules show S-shaped hysteresis loops while the caps on 330 and 160nm spherules exhibit rectangular loops. This drastic change is attributed to a transition from a radial to a uniaxial magnetization distribution with decreasing diameter and results from the increasing exchange interaction between adjacent moments. Furthermore, element selective studies reveal a transition from an aligned to a twisted magnetic state with reducing size of the nanocaps.