Coercivity and morphology in Fe/NiO films deposited on nanoporous Al 2O3 membranes

The occurrence of ferro-antiferromagnetic (F/AF) interfaces in Fe/NiO bilayers brings about the possibility of controlling the Fe layer hysteretic properties through the unidirectional anisotropy induced by the presence of unbalanced, antiferromagnetic phase moments at that interface. In this work, we report on the Fe thickness and temperature dependencies of the hysteretic parameters measured in Fe/NiO bilayers deposited on Al2O3 nanoporous membranes. The Fe/NiO bilayers grew in a six-fold columnar way around each one of the membrane nanopores. The in-plane hysteresis loops measured by field cooling the samples down to different temperatures below 300K, evidenced the occurrence of coercive forces larger than that associated to the Fe anisotropy field and also of significant loop shifts, both of them clearly linked to the occurrence of exchange bias at the Fe/NiO interfaces. The magnetization reversal mechanism active in the Fe/NiO bilayers essentially differs from that responsible for the reversal occurring in continuous films incorporating lithographed antidots, due to the fact that in our samples it does not involve either domain wall propagation or pinning, and the reversal events are spatially restricted to the Fe-covered, hexagonally closely packed dot-like tops of the NiO columns.