Non-conventional interaction contributions in permalloy/NiO composite thin films deduced from their static and dynamic magnetization behavior

A comparative study of thin permalloy films (10.4?nm) interfaced with NiO layers of various thicknesses (6 to 47?nm) is presented. The magnetic parameters deduced from different experimental techniques (Brillouin spectroscopy, ferromagnetic resonance and magnetometric (vibrating sample magnetometry and magneto-optical Kerr effect) investigations) show original characteristics which have not been previously evidenced: (i) for most of the samples studied, the exchange bias field is not parallel or perpendicular to the cooling field, in contrast with the in-plane anisotropy field; (ii) the magnetic properties under an out-of-plane applied magnetic field H do not match the calculated ones derived from the usual expression for the density of magnetic energy. We introduce a phenomenological additional term proportional to cos[?] where ? is the angle between H and the direction normal to the sample: this non-conventional interaction between NiO and permalloy layers allows us to fit all the experimental data.