Exchange and anisotropy in 3d-4f compounds

Abstract

High-performance permanent magnets are characterised by large values of their remanence and coercive force, which lead to high values for the energy product ( BH) max, and by a good thermal stability which allows the magnet to be used at elevated temperatures. These technical requirements are directly related to the intrinsic properties of the materials of which the permanent magnets are composed. A high remanence can be obtained for materials possessing a high saturation magnetization. The thermal stability is largely related to the value of the Curie temperature, while the coercive force depends on the magnetic anisotropy. In the 3d-rich rare-earth-transition-metal compounds, high values for the Curie temperature and for the saturation magnetization can be reached for the transition-metal elements iron and cobalt, whereas a large and uniaxial anisotropy can be achieved for the rare-earth elements Nd, Pr or Sm in their proper crystallographic structures. Through an exchange coupling between the transition-metal and rare-earth magnetic moments, the 4f moment with its large 4f anisotropy is coupled to the 3d moment, resulting in excellent magnetic properties for compounds like SmCo 5, Sm 2Co 17 and Nd 2Fe 14B. To study the basic interactions in rare-earth magnets in full detail, applied magnetic fields are required of the same order of magnitude as the effective exchange and anisotropy fields which frequently reach values of 100 T or more in the 3d-4f intermetallics. The 3d-4f exchange interaction can be studied effectively 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-crystalline samples reveal both the exchange interactions and the magnetic anisotropy. Examples will be discussed for compounds belonging to several rare earth-cobalt/iron series with different stoichiometries.