Magnetization Reversal Properties of Fe/NiO/Fe(001) Trilayers

We have investigated the magnetic properties of epitaxially grown Fe/NiO/Fe(001) trilayers for different thickness combinations of the NiO spacer and of the Fe overlayer. The magneto optical Kerr effect has been exploited to study the in-plane magnetization reversal processes in the iron layers. Further information has been obtained by means of spin polarized inverse photoemission spectroscopy. We show the existence of a critical value t(c) of the NiO thickness t(AFM) for the magnetic coupling between the Fe layers at zero applied field: for t(AFM)<t(c) the magnetization directions align perpendicularly, while the alignment is collinear for thicker spacers. A phenomenological model has been developed to reproduce the reversal properties of the trilayer and determine the dependence of the coupling energy between the Fe ferromagnetic layers as a function of their relative orientation. We find that the observed behavior cannot be reproduced by a Slonczewski’s coupling term, but rather it is obtained when a biquadratic term is employed.