High-Field Magnetization Process and Crystalline Electric Field Interaction in Rare-Earth Permanent-Magnet Materials

Abstract

During the last two decades the performance of permanent magnets has been greatly improved by introducing rare-earth (R) elements to their constituents [1]. It is doubtless that the high coercivity of these magnets comes from the large magnetic anisotropy originated by 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 into the magnetic anisotropy. Since 1985, we have been investigating systematically the high-field magnetization in a series of Nd₂Fe₁₄B-type compounds using mainly single crystal samples [2]. On the other hand, we have developed a method of analyzing these magnetization curves which consists of a simplified Hamiltonian taking the exchange and crystal field at the R ions into account, with the Fe sublattice being treated phenomenologically [3]. The essential feature in this model is the coupling of the two different types of sublattices. One is the R sublattice, which gives a large magnetic anisotropy owing to the CEF interaction. Another is the Fe sublattice, which determines the large magnetization and high Curie temperature as a result of strong Fe-Fe exchange interactions. This method has proven to be applicable not only to the R ₂ M ₁₄B system, with M = Fe or Co, but also to the pseudo-ternary system (R _1−x R′ _x )₂Fe₁₄B [4] or other R-Fe-X systems such as R ₂Fe₁₇N _x [5]. In these systems an interplay among the R-Fe exchange interaction, CEF potential acting on R ions and a large magnetic moment of the Fe sublattice leads to a variety of magnetic properties such as a first-order magnetization process (FOMP) and spin reorientation (SR) transitions. In general, such SR transitions will be accompanied by a considerable lattice deformation, since there is a large orbital contribution to the R magnetic moments, resulting in a strong coupling between the spin and lattice systems. It is therefore of interest to investigate the magnetoelastic properties of these materials.