Magnetostrictive Rare Earth-Fe2 Compounds

By the early 1960’s, it was widely recognized that the rare earths possessed many extraordinary magnetic properties. Neutron diffraction measurements, for example, showed that the spin structures were much more complex than those of any of the classical ferromagnets or antiferromagnets. More importantly, in the heavy rare earth metals, the parallel coupling of large orbital and large spin angular momenta yielded huge magnetic moments of 9μ B and 10μ B, dwarfing the conventional values of 0.6 for Ni and 2.2 for Fe. Enormous magnetic anisotropies were also encountered in the heavy rare earth elements. In 1963, a breakthrough in magnetostrictive materials occurred with the measurement of the basal plane magnetostrictions of Tb and Dy at low temperatures (Legvold et al. 1963, Clark et al. 1963, 1965, Rhyne and Legvold 1965). These basal plane strains are 100 to 10000 times typical magnetostrictions and still remain today the largest known (~1%). Over wide temperature ranges, thermal expansions are dominated by the temperature dependences of the magnetostrains. Elastic moduli were found to be strongly influenced by the unprecedented magnetoelastic interactions. However, because of the low ordering temperatures of the rare earths the application of these magnetostrictive properties to devices operating at room temperature could not be achieved with the elements. Only Gd, which is essentially non-magnetostrictive, possesses a Curie point as high as room temperature.