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Abstract
Co-Ag heterogeneous alloys films having 5 at% Co are produced by sputtering and annealed after deposition to relieve stress and promote particle growth. X-ray diffraction suggests the as-deposited state consists of a single fcc alloy phase, with local density fluctuations resulting from the immiscible nature of Co and Ag leading to the formation of Co-rich and Ag-rich regions. Annealing is seen to drive progressive separation and growth of the Ag-rich and Co-rich areas.
Characterizations of magnetic properties indicate the Co precipitates are ferromagnetically ordered and have a uniaxial anisotropy. A Curie-Weiss analysis of the inverse initial dc susceptibility indicates the as-deposited film has net antiferromagnetic interparticle magnetic interactions, while the annealed sample has non-interacting particles. Fitting the magnetization curves to a Langevin function with a lognormal volume distribution indicates the films have a narrow particle size distribution.
The thermal activation behavior of the annealed sample is investigated through the use of dynamic susceptibility measurements made with a high sensitivity ac susceptometer and a SQUID magnetometer, which span 8 decades in frequency. The Neel model of thermal activation is first applied to the in-phase susceptibility data following a generally-accepted conventional analysis taken from the spin glass literature. Trends in the data are consistent with the Neel model, but values for the prefactor and the most probable energy barrier to reversal from this analysis are unphysical. A new method for applying the Neel model is presented, and allows, for the first time, correct application of this model to dynamic susceptibility data from a distributed system. This analysis of the dynamic susceptibility data yields physically meaningful results, provides a direct measure of the distribution of energy barriers, and derives a scaling relationship allowing data at different frequencies to be scaled onto a universal energy barrier curve. The universal energy barrier distribution determined with this analysis implies, through a frequency-dependent scaling relationship required for measurements at different frequencies to truly scale onto a universal curve, the existence of a distribution in the attempt frequency, $\Gamma\sb0$ and possible finite size effects.
