57Fe Moessbauer spectroscopy applied to the study of rare-earth iron permanent magnets

A survey of some important applications of 57Fe Moessbauer spectroscopy to the study of local magnetic and structural characteristics of the main phases in the rare-earth-transition metal (RE-TM) class of materials used in the development of the new generations of high performance permanent magnets is presented. A brief history of the hard magnets evolution in the last decades is followed by the description and interpretation of the main effects on the hyperfine interactions revealed by the Moessbauer technique. The contribution of the distinct crystallographic iron sites to the saturation magnetization of the rare-earth iron compounds is discussed in terms of the corresponding types of magnetic coupling, based mainly on the iron-iron distances, the number of iron neighbors and the respective hyperfine fields. These features are inferred from the energy of the resonant absorption by the iron sites 4e/4c/8j1/8j2/16k1/16k2 in the Nd2Fe14B, sites 6c/9d/18f/18h in the light RE2Fe17N3 and 8f/8i/8j in the RE(Fe12-xTMx) phases. The direction of the easy axis of magnetization is discussed taking into account the effects due to the interaction between the local magnetization vector and the electric field gradients. Some other characteristics such as the occupancy of the TM atoms in the 1 : 12 compounds as well as in some hybrid (Fe +TM) 2 : 17 structures are also discussed in terms of the changes in the relative subspectral areas. The effects regarding charge transfers and lattice expansions due to the incorporation of nitrogen and also of other different interstitial elements to the 2 : 17 rhombohedral structure are discussed considering the relative changes of the isomer shifts in comparison with the alterations in the parameter due exclusively to the respective volume expansions