Temperature dependence of ferromagnetic resonance as induced by NiO pinning layers

Ferromagnetic resonance (FMR) experiments have been conducted near 9.5 GHz on permalloy (Py) thin films which are components of spin valves and related structures. These so-called giant magnetoresistance structures often use antiferromagnetic NiO to achieve pinning of one magnetic layer. Magnetic anisotropies acting on these pinned layers were deduced by observing their resonances for fields perpendicular to and in the sample plane. We used data taken from 4 to 600 K to identify potential mechanisms of pinning, anisotropy, and linewidth. The anisotropic exchange pinning and an isotropic downward FMR shift vanish at a blocking temperature well below the bulk Neél temperature of NiO. The strong temperature dependencies of the isotropic shift and linewidth may reflect the presence of different spin pinning subsystems and the different time scales of the FMR and low frequency or static measurements.