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

As importance of permanent magnet in daily-life is increased for energy conversion ranging from electric cars to wind power generators, research on the development of new high-efficiency and low-cost permanent magnet is actively being carried out [1]. The exchange-coupled hard and soft magnet has been considered as one of the prominent candidates for high-efficiency permanent magnet since it can compensate for relatively low magnetization of hard magnet and very low crystalline anisotropy of soft magnet and thus, it improves the energy product [2, 3]. In order to apply this prospect for high performance permanent magnet, various types of exchange-coupled magnet are proposed including multi-layer [4], mixture [5], and core/shell structure [6, 7]. The cylindrical core/shell structure are particularly advantageous owing to large interface for exchange coupling and versatility at controlling composition and demagnetization factor by the dimensions [8]. 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. To obtain hysteresis loop and the energy product, we computed equilibrium magnetization configuration of local energy minimum under external magnetic field by using a finite differential micromagnetic solver, Mumax [9]. The hysteresis loops were obtained from applying external magnetic field ranging from -10 T to 10 T along easy axis. Since the energy product corresponds to the energy stored in the stray field produced by the magnet itself, it should be calculated from Hd and B at the remanent state (zero external magnetic field). Thus, we obtained the maximum energy product for each cylindrical core/shell structure directly from the calculations of the remanent states for various aspect ratio of the cylinder. For finding optimized core/shell structure, the energy product is expressed as phase diagrams. We 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