Magnetic properties, nanostructure, and ordering kinetics of Nb additive FePtCu films

In this study,the (FePt)_94-xCu₆Nb_x (x=0, 2.87, 4.52, 5.67) alloy films were prepared by co-sputtering. The effects of Nb addition content and heat treatment on the microstructure and magnetic properties of the polycrystalline FePtCu films are reported. Our previous experiments showed that the ordering temperature of the (FePt) ₉₄Cu₆ films reduced to 320 °C, which is much lower than that of the FePt alloy. However, the grain growth after heat treatment limited the practical application in recording media. By adding the Nb content in the (FePt)₉₄Cu₆ film, the grain sizes of the films can be adjusted from 50 to 18nm, even for the films annealed at temperature as high as 600°C. DSC traces of as-deposited disorder films at different heating rates, to evaluate the crystallization of the order phase, revealed that the addition of Nb enhanced the activation energy of ordering from 87 kJ/mol to 288 kJ/mol for the (FePt)_94-xCu₆Nb_x (x=0 and 2.87, respectively) films. The reduction of the grain size and the corresponding increase in the activation energy of the Fe-Pt-Cu-Nb films might result from the precipitation of the Nb atoms around the ordering FePt phase. The (FePt) _94-xCu₆Nb_x (x=2.87) film showed a coercive force of 13.4 kOe and the magnetization of 687 emu/cc.