Giant heterogeneous magnetostriction in Fe–Ga alloys: Effect of trace element doping

Abstract

Enhanced magnetostriction in iron-rich Fe–Ga alloys has been attributed to a heterogeneous nanostructure with tetragonal inclusions, although direct experimental evidence for their structure was lacking. Here we use transmission electron microscopy to show that melt-spun, (001) textured Fe83Ga17 ribbons contain 3 nm inclusions with c-axis Ga–Ga pairs aligned in a tetragonal L60-type structure; the induced tetragonality of the entire A2 matrix is observed directly by synchrotron X-ray diffraction. Trace doping with 0.2 atomic % nonmagnetic elements such as La or Pb increases the magnetocrystalline anisotropy and greatly enhances the magnetostriction. Rare-earth dopants from La to Lu produce a quarter-shell variation of the magnetic anisotropy; the crystal field parameter A20 is determined to be −15 Ka0−2. The best trace dopants are the light rare earths Ce and Pr that give a transverse magnetostriction of up to −800 ppm, as these elements soften the tetragonal modulus via their crystal field interaction. A new model is proposed to explain nanoheterogeneous magnetostriction, where the Ga–Ga pairs remain fixed, but the tetragonal axis of the matrix can be realigned in a magnetic field by a series of small deformations of the A2 matrix. These results signal a new approach to creating highly-magnetostrictive materials.