Evidence of surface anisotropy in magnetic nanoparticles

Abstract Discontinuous magnetic multilayers provide a simple method of producing magnetic nanoparticles. Such systems can be viewed as two-dimensional nanosystems, depending on separation between the discontinuous magnetic layers via a non-magnetic spacer layer. We have studied the system whereby discontinuous layers of CoFe are separated by Al 2 O 3 interlayers of 40 A (nominal). A set of samples with nominal CoFe thicknesses between 7 and 13 A have been studied using ferromagnetic resonance (FMR). We have made measurements as a function of sample temperature in the range from 5 K to room temperature. The spectra display some strong enhancements of the resonance field at temperatures below 90 K, and are inversely proportional to the effective thickness of the CoFe. This indicates a strong surface contribution to the effective field and is interpreted as surface anisotropy. Variations in the linewidth indicate a change of behaviour which we equate to a transition of coupling regime from superferromagnetic in the thicker samples to superparamagnetic for the thinner samples.