The Practical Energy Product of Cylindrical Core/shell Composed of Soft- and Hard-magnetic Materials

Abstract

Recently, as the demand for devices using magnetic energy of permanent magnets such as electric cars and wind power generators has increased, researches have actively carried out to develop new high-efficiency and low-cost permanent magnets. The exchange-coupled magnet is a good candidate for high-efficiency permanent magnet by exchange-coupling high anisotropic magnets having low magnetization (hard magnet) with high saturation magnet despite low anisotropy (soft magnet). Such structures are expected to improve the energy product as a combination of high coercive force of the hard phase and high saturation magnetization of the soft magnet. In order to apply this prospect for high performance permanent magnet, various types of exchange-coupled magnet are investigated including multi-layer, mixture, core/shell structure, and so on. The cylindrical core/shell structure are particularly advantageous owing to large interface and versatility at controlling composition and demagnetization factor by the dimensions. Because the energy product corresponds to the energy stored in the stray field produced by the magnet itself, it should be measured from Hd and B at the remanent state. Thus, we investigated the energy product of the cylindrical core/shell structure considering the demagnetization field, which affect not the maximum energy product, but the practical energy product at zero external field. In this work, we adopted cylindrical core/shell structure composed of the soft magnetic shell (FeCo) and the hard-magnetic core (Sm2Co17) as a model system. With the model system, we computed minimization of the Gibbs free energy of the model system made up with 2×2×2 nm3 sized cell, which is smaller than the exchange length of two materials, by using a finite differential micromagnetic solver. To estimate the energy product, the hysteresis loops were calculated by applying external magnetic field ranging from -10 T to 10 T along easy axis and B- and H- fields were obtained directly from them. The model consists of a hard-magnetic core having the dimension of maximizing the theoretical energy product is enveloped by diverse thickness of the soft magnetic shell, and the energy product and nucleation field calculated from the hysteresis loops are expressed as phase diagrams illustrated below figure 1. We finally expanded the model to iterative array structure of the cylindrical core/shell, and the results show great prospect for applying to bulk permanent magnet with high value of energy product