Abstract:
Rare earth metals exhibit large, strain-dependent anisotropy associated with their strong spin-orbit coupling. There are both single-ion and two-ion contributions to the resulting magnetostriction. The linear, volume-conserving (Joulian) magnetostriction λ is an important functional property of ferromagnets that saturates with the magnetization, and depends on the directions of applied field and measured strain. However, the magnetocrystalline anisotropy also tends to impede the saturation of the magnetization and limit the usefulness of effect. This problem was elegantly overcome in the cubic Laves-phase ferromagnets RFe2 by using a mixture of rare-earths (Tb and Dy) with opposite signs of cubic anisotropy, but the same sign of linear magnetostriction. The (Dy,Tb)Fe2 alloys (‘Terfenol-D’) hold the record for room-temperature saturation magnetization, λs ≈ 2000 ppm.
More recently, a different magnetostriction mechanism has been identified in Fe-Ga alloys (Gafenol), which exhibit a magnetostriction of up to 300 ppm due to tetragonal nanoinclusions that exist throughout the cubic A2 matrix. The magnetostriction is then related to the field-induced tetragonality of the matrix. Remarkably, this effect can be enhanced by up to a factor of five by incorporating tiny traces (~ 0.1%) of barely soluble rare-earth atoms into the Fe-Ga alloys. Values of magnetostriction approach those of Terfenol-D, with one hundred times less rare earth. Systematic investigations [1] reveal that the enhancement depends partly on the volume of the rare earth (regardless of whether it is magnetic or not), and partly on its quadrupole moment. Prospects for producing melt-textured or single crystal alloys to rival Terfenol-D will be discussed, as will recent claims [2] that Fe-Ga alloys exhibit ‘Non-Joulian’ magnetostriction.
[1] He Yangkun et al, Giant heterogeneous magnetostriction in Fe-Ga alloys: effect of trace element doping, Acta Materialia (in press) 2016
[2] H. D. Chopra and M. Wuttig, Non-Joulian magnetostriction, Nature 521 340 (2015)
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