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

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.