Nucleation fields in an exchange spring hard magnet

A detailed study of magnetization processes has been carried out on a new type of hard magnetic material. The material Nd_3.8Fe_73.3V_3.9B_18.0Si_1.0 is unique among hard magnets due to the exchange coupling of two ferromagnetic materials, one of which is soft and the other hard. The combination of the two materials gives a composite material, with a high magnetic saturation and coercivity. The irreversible components of magnetization for the material were examined using the demagnetizing remanence curve (DCD) and the isothermal remanence curve (IRM) for the magnetizing case. The nature of the interactions in the material were also studied using the delta M plot obtained via the DCD and IRM remanence curves. The reversible components were obtained from the M-H loop data, M_rev=M_tot-M_irr. Measurements of magnetic time dependence have also been carried out to provide fluctuation field data, H_f, from which activation volumes of reversal, v, for various fields have been obtained. The mean activation field H_a, obtained by extrapolating the linear portion of the fluctuation field data to zero, is the same as the mean nucleation field H_n obtained from the reversible components, M_rev of the demagnetizing remanence curve. Assuming spherical volumes of reversal, the dimensions of the soft and hard phases, obtained from the mean volume of activation, v(H_n) are close to the critical dimensions of the soft and hard phases.