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

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 [Tb0.27Dy0.73](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 lambda(parallel to)(0.8 T) = 265 to 520 ppm accompanied ky an increase of the magnetic phase transition temperature from T-C = 333 to 592 K, while the increase of the coercive fields from mu(0)H(c) < 5 to 75 mT lies distinctively below 100 mT. These properties are suitable for applications of giant magnetostrictive films in microsystems where values of lambda > 500 ppm, T-C > 500 K and mu(0)H(c) much less than 100 mT are required. Establishing a nanocrystalline microstructure with grain sizes d < d(c) similar to 15 nm (d(c) is the critical grain diameter) smaller than the exchange length is essential for the combination of intrinsic magnetic properties (increased lambda and T-C) with soft magnetic properties (mu(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. (C) 1999 Elsevier Science B.V. All rights reserved.