Manipulation of the coercivity of FeCoCr films through artificial defects engineering based on Bi doping

Abstract

FeCoCr thin films have been considered as promising candidates for magnetic code disk materials used in high-precision magnetic rotary encoders. However, realizing high coercivity with a sufficient remanence is a critical issue for magnetic recording stability from the point of view of practical applications. We prepared Bi-free and Bi-doped FeCoCr(250 nm)/Ta(10 nm) films on Si substrates using magnetron sputtering followed by vacuum annealing at 650 °C for 30 min. The in-plane coercivity of Bi-free and Bi-doped FeCoCr samples are 414 Oe and 930 Oe respectively, indicating that the coercivity can be greatly enhanced by Bi doping. X-ray photoelectron spectroscopy (XPS) reveals that Bi atoms diffuse to the film surface and then sublimate during high-temperature annealing process. Positron annihilation spectroscopy (PAS) indicates that the diffusion of Bi atoms increases the density of defects in the film. Scanning transmission electron microscopy (STEM) analysis shows that the diffusion behavior of Bi atoms not only refines the grain size but also results in a uniform distribution of the Fe-Co-rich (α1 phase) phase with the Cr-rich (α2 phase) phase. Magnetic force microscopy (MFM) analysis observes that the introduction of Bi decreases the magnetic domain size and increases the magnetic domain density resulting in the enhancement of coercivity.