The role of dipolar interactions in magnetic nanoparticles: Ferromagnetic resonance in discontinuous magnetic multilayers

We have performed detailed ferromagnetic resonance (FMR) studies of [Co80Fe20(t)/Al2O3(40 angstrom)](10) (t=7-13 angstrom) discontinuous multilayers, with measurements taken as a function of the angle of the applied static magnetic field with respect to the sample at room temperature. We outline an approach to FMR in granular systems by using the magnetic dipolar interaction between magnetic nanoparticles. This can be easily incorporated into the free energy of the system and provides a method for evaluating the average interaction strength in particulate media. Angular dependent measurements show how FMR can be used to evaluate interparticle interactions. Our measurements indicate that there is a change of coupling regime, going from superferromagnetic, for thicker samples (with larger average particle sizes and smaller separations), to superparamagnetic in thinner samples. (c) 2007 American Institute of Physics.