Temperature dependence of magnetic properties and phase transitions in a soft magnetic Co-based nanostructured alloy

We report the temperature dependence of magnetic properties and phase transitions in a soft-magnetic Co-based nanostructured alloy obtained from amorphous Vitrovac 6025. Homogeneous nanostructured samples are achieved by suitable thermal treatments below the conventional crystallization temperature and have ultrafine grain structures with average grain size of about 10 nm and volume fraction 0.6. The grains are embedded in a residual ferromagnetic amorphous matrix and this leads to superior soft-magnetic properties which, in our view, can be justified on the basis of the random anisotropy model. The temperature dependence of initial reversible permeability and coercive field emphasizes a different behaviour in the magnetic properties of nanostructured and amorphous samples. The sharpness in the Hopkinson peaks of the initial permeability is the best tool to show the first magnetic transition in both specimens: the results are compared with those detected by thermogravimetric curves and by magnetization intensity measurements. Differential thermal analysis is used to evaluate the transformation enthalpy of the amorphous-to-microcrystalline and the nano-to-microcrystalline processes, which both occur at about 820 K where another ferromagnetic phase appears.