Optimization of magnetostriction, coercive field and magnetic transition temperature in nanocrystalline TbDyFe+Zr/Nb multilayers

Abstract The magnetostrictive properties of TbDyFe/Nb multilayers containing 2 at% Zr as an additive have been investigated after different annealing treatments for the (Terfenol-D near) composition of [Tb 0.27 Dy 0.73 ] 0.27 Fe 0.73 . The multilayer structure has been produced by ion-beam sputtering on a sapphire substrate. After 10 min annealing of the multilayers at temperatures from 873 to 973 K the parallel magnetostriction increased from λ ‖ (0.8 T)=265 to 520 ppm accompanied by an increase of the magnetic phase transition temperature from T C =333 to 592 K, while the increase of the coercive fields from μ 0 H c λ >500 ppm, T C >500 K and μ 0 H c ⪡100 mT are required. Establishing a nanocrystalline microstructure with grain sizes d d c ∼15 nm ( d c is the critical grain diameter) smaller than the exchange length is essential for the combination of intrinsic magnetic properties (increased λ and T C ) with soft magnetic properties ( μ 0 H c of a few mT) as typical for an amorphous microstructure. It is shown by microstructural XRD and TEM investigations that such a nanocrystalline microstructure can be realized by a suitable heat treatment of TbDyFe+Zr/Nb multilayers. Introducing Nb spacer layers effectively reduces grain growth for certain annealing temperatures while Zr is assumed to play a dominant role in forming nucleation centers of nanograins. In combination, both effects can be well used to optimize the magnetostrictive layer properties.