Decomposition of crystalline ferromagnetic particles precipitated in amorphous paramagnetic Ni-P

Abstract

According to small-angle X-ray scattering, amorphous Ni-P contains some volume per cent of metal-rich precipitations. During annealing, their diameters grow from about 2 nm in the as-prepared state to about 6 nm. It is shown by magnetic measurements that the inclusions are ferromagnetic single-domain particles embedded in a paramagnetic matrix. At sufficiently high temperatures such systems behave like superparamagnets. At 4.2 K, thermal agitation is too weak to flip the elementary magnetic moments. In this case, magnetisation curves can allow the authors to determine the coercivities and average saturation magnetisations. Annealing induces the two quantities measured at 4.2 K to change proportional to each other. This experimental fact justifies the application of a theory developed for fine non-interacting ferromagnetic particles having a magnetic shape anisotropy. The increase in the spontaneous magnetisation is attributed to the decomposition of the inclusions. With the aid of an approximation procedure the starting value of the phosphorus content and its decrease during annealing are estimated. These results and the values derived for the shape anisotropy agree well with those obtained through small-angle X-ray scattering. The starting phosphorus concentration of the precipitations proved to be independent of their volume fraction and the average composition of the samples. The observed kinetics fit in with the assumption of an activation energy spectrum. The lower limit starts at about 1.4 eV and is shifted on annealing to higher values.