EXCHANGE AND ANISOTROPY IN 3D-4F COMPOUNDS

High-performance permanent magnets are characterised by large values o f their remanence and coercive force, which lead to high values for th e energy product (BH)(max), and by a good thermal stability which allo ws the magnet to be used at elevated temperatures. These technical req uirements are directly related to the intrinsic properties of the mate rials of which the permanent magnets are composed. A high remanence ca n be obtained for materials possessing a high saturation magnetization . The thermal stability is largely related to the value of the Curie t emperature, while the coercive force depends on the magnetic anisotrop y. In the 3d-rich rare-earth-transition-metal compounds, high values f or the Curie temperature and for the saturation magnetization can be r eached for the transition-metal elements iron and cobalt, whereas a la rge and uniaxial anisotropy can be achieved for the rare-earth element s Nd, Pr or Sm in their proper crystallographic structures. Through an exchange coupling between the transition-metal and rare-earth magneti c moments, the 4f moment with its large 4f anisotropy is coupled to th e 3d moment, resulting in excellent magnetic properties for compounds like SmCo5, Sm2Co17 and Nd2Fe14B. To study the basic interactions in r are-earth magnets in full detail, applied magnetic fields are required of the same order of magnitude as the effective exchange and anisotro py fields which frequently reach values of 100 T or more in the 3d-4f intermetallics. The 3d-4f exchange interaction can be studied effectiv ely for the heavy rare-earth compounds on finely powdered material, a technique that fails for the light rare-earth compounds that are most attractive for applications. High-magnetic-field studies on single-cry stalline samples reveal both the exchange interactions and the magneti c anisotropy. Examples will be discussed for compounds belonging to se veral rare earth-cobalt/iron series with different stoichiometries.