FMR evidence of finite-size effects in CoCu granular alloys

Granular CoCu ribbons, as cast and heat treated, were studied by means of the X-band ferromagnetic resonance (FMR) as a function of temperature, in the range from 10 K to 250 K. The FMR spectra were fitted as a sum of absorption and dispersion functions. The effective anisotropy constant K-eff and average grain diameter of the magnetic grains were obtained from a model derived for identical independent particles with effective uniaxial anisotropy. K-eff is enhanced in comparison to values for the bulk materials, and also shows a decrease with increasing mean grain diameter. The asymmetry parameter for the FMR line shows both temperature and grain-size dependence. In addition, zero-field cooled and field cooled magnetization measurements at higher fields show irreversibility for smaller grains. These results indicate an important role of the finite grain size in the magnetic behavior of the granular alloys.