High-Field Investigations on Exchange Coupling in R-Fe Intermetallics and Hard/Soft Nanocomposite Magnets

Abstract

The performance of permanent magnets has greatly been improved by introducing rare-earth elements (R) to their constituents (Herbst, 1991). It is doubtless that the high coercivity of these magnets comes from the large magnetic anisotropy originating from the crystalline electric field (CEF) acting on R ions with large orbital angular momentum. Magnetization measurements up to the high-field region where the hard-axis magnetization saturates are indispensable in order to obtain the basic insight of the magnetic anisotropy. Since the discovery of sintered Nd-Fe-B magnets (Sagawa et al., 1984), we have been investigating systematically high-field magnetization processes in a series of Nd₂Fe₁4B-type compounds using mainly single crystal samples (Nakagawa et al., 1990). On the other hand, we developed a method of analyzing these magnetization curves, which consists of a simplified Hamiltonian taking exchange and crystal field at R ions into account, with Fe sublattice being treated phenomenologically (Yamada et al., 1988). Similar analyses were made by Givord et al. (1988), Cadogan et al. (1988) and Radwanski et al. (1990b). A comparison of crystalline magnetization process (CEF) and molecular field parameters determined by these groups has been made by Nakagawa et al. (1990). The essential feature in these analysis models is the coupling of the two different types of sublattices. One is the R sublattice, which gives large magnetic anisotropy owing to the CEF interaction.