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

A survey of some important applications of Fe-57 Mossbauer spectroscopy to the study of local magnetic and structural characteristics of the main phas es in the rare-earth-transition metal (RE-TM) class of materials used in th e development of the new generations of high performance permanent magnets is presented. A brief history of the hard magnets evolution in the last dec ades is followed by the description and interpretation of the main effects on the hyperfine interactions revealed by the Mossbauer technique. The cont ribution of the distinct crystallographic iron sites to the saturation magn etization of the rare-earth iron compounds is discussed in terms of the cor responding types of magnetic coupling, based mainly on the iron-iron distan ces, the number of iron neighbors and the respective hyperfine fields. Thes e features are inferred from the energy of the resonant absorption by the i ron sites 4e/4c/8j(1)/8j(2)/16k(1)/16k(2) in the Nd2Fe14B, sites 6c/9d/18f/ 18h in the light RE2Fe17N3 and 8f/8i/8j in the RE(Fe12-xTMx) phases. The di rection of the easy axis of magnetization is discussed taking into account the effects due to the interaction between the local magnetization vector a nd the electric field gradients. Some other characteristics such as the occ upancy of the TM atoms in the 1:12 compounds as well as in some hybrid (FeTM) 2:17 structures are also discussed in terms of the changes in the relat ive subspectral areas. The effects regarding charge transfers and lattice e xpansions due to the incorporation of nitrogen and also of other different interstitial elements to the 2:17 rhombohedral structure are discussed cons idering the relative changes of the isomer shifts in comparison with the al terations in the parameter due exclusively to the respective volume expansi ons.