Importance of ideal grain boundaries of high remanent composite permanent magnets

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Abstract
The excellent magnetic properties of nanoscaled composite magnets can only be obtained by imposing strong requirements on the grain structure. The soft magnetic phase with a maximum extension smaller than 20 nm should be embedded into a hard magnetic environment. Furthermore the grain boundaries should be as ideal as possible. These requirements for the grain structure guarantee an optimal exchange hardening of the soft magnetic phase and therefore a high remanence and a sufficiently high coercivity. In the framework of computational micromagnetism it is possible to investigate the effect of nonideal grain boundaries on the magnetization reversal process of composite magnets in detail. Nonideal grain boundaries can be simulated by intergranular layers with material parameters deviating from their bulk values. It turns out that the remanence and the coercivity decrease approximately linearly with the simultaneous reduction of the exchange and the anisotropy constants near the grain boundaries. For a reduction of these intergranular material parameters smaller than 10% of their initial values, the remanence and coercivity strongly decrease by a factor of about 2.

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