Hysteresis loops and coercivity mechanisms in sintered and nanocrystalline permanent magnets

摘要

The hysteresis loops of nanocrystalline (nc) permanent magnets (pms) produced by the melt-spin technique have been investigated for compositions based on the intermetallic compounds R_2Fe_(14)B (R = Nd, Pr) and the carbides Sm_2Fe_(17-x)Ga_xC_y. The following three types of pms have been studied: 1) High-coercivity pms with exchange decoupled grains. 2) High-remanence exchange-spring pms. 3) High-coercive-high-remanence composite pms with exchange coupled soft and hard magnetic grains. The temperature dependence of the coercive field mu _0H_C for all three types of pms obeys a relation for a modified nucleation field, H_C = (2 K_1 / J_s) alpha - N_(eff) M_S (K_1 = first anisotropy constant, M_S = spontaneous magnetization). For an analysis of the char-acteristic differences between the microstructural parameters alpha and N_(eff) as obtained for the three types of pms, computational micromagnetism on the basis of the Finite Element Technique is applied. This powerful method allows a quantitative analysis of the role of grain size, grain boundaries (gbs), texture of easy directions and of soft magnetic phases in composite materials. In order to obtain satisfactory results, a self-adapting algorithm has been developed where the mesh size is adapted to the gradients of the direction cosines of the spontaneous magnetization. It turns out that excellent magnetic properties of composite pms can only be obtained if the gbs are as ideal as possible. Remanence and coercive field are found to decrease linearly with a corre-sponding reduction of both, the crystal anisotropy and the exchange constant within the gbs. In composite pms the diameters of the soft magnetic grains should be smaller than twice the domain wall width, delta _B~(hard), of the hard magnetic phase in order to obtain a remarkable remanence enhancement. From these model calculations general rules for the development of optimized nc pms with large remanences and large coercivities are derived.