Hard/soft permanent magnets have aroused many interests in the past two decades because of their potential in achieving giant energy products as well as their rich variety of magnetic behaviors. Nevertheless, the experimental energy products are much smaller than the theoretical ones due to the much smaller coercivity measured in the experiments. In this paper, the deterioration of the coercivity due to the interface atomic diffusion is demonstrated based on a three dimensional (3D) micromagnetic software (OOMMF) and a formula derived for the pinning field in a hard/soft multilayer, which can be applied to both permanent magnets and exchange-coupled-composite (ECC) media. It is found that the formation of the interface layer can decrease the coercivity by roughly 50%, which is responsible for the observed smaller coercivity in both composite and single-phased permanent magnets. A method to enhance the coercivity in these systems is proposed based on the discussions, consistent with recent experiments where excellent magnetic properties are achieved.
The hysteresis loops, angular distribution, nucleation field, coercivity and energy product are calculated by three-dimensional micromagnetic method for Nd2Fe14B/Fe65Co35 bilayers with a deviation of the easy axis, and the results are compared with the experimental results. The results show that obvious nucleation can be observed only when the β between the easy axis and the applied field is equal to 0°, and the nucleation field and the coercivity decrease as the thickness of the soft phase Ls increases. The remanence decreases and the squareness of the hysteresis loop weakens as β increases, leading to the decrease of the energy product while the largest maximum energy product (561.61 kJ/m3) occurs at Ls=1 nm and β=0°. The shapes of the hysteresis loops, the remanence and the coercivity obtained from calculations and experiments are close to each other.
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