Grain growth kinetics and coercivity of soft magnetic NiFe-alloys

Abstract
Using common models for the dependence of coercive force H_c on grain size and for grain growth kinetics a proportionality between H_c and (Dt)^-0.5 is deduced with diffusion coefficient D and annealing time t. This relation has been verified over a wide range of temperature and annealing time for a high purity commercial 80%-NiFe and a 50%-NiFe alloy. The model also allows to determine the activation energy of diffusion using the H_c-data and applying an Arrhenius-plot of In[(H_c-H_co)²t], H_co being the minimum attainable coercive force after annealing at very high temperatures. At higher temperatures an activation energy of 3.3 eV for 50%-NiFe and of 2.6 eV for the 80%-NiFe alloy could be deduced. corresponding to the activation energy of self diffusion of about 3 eV for this kind of alloys. At lower temperatures, that is below about 800°C, the activation energy becomes much smaller, and a value of 0,8 eV is obtained, corresponding to the activation energy of grain boundary diffusion.