Enhanced Exchange Coupling in TiO $_{2}$ -Doped FePt-C Nanocomposite Thin Films Induced by Surfactant Mediated Growth

We reported the TiO₂ doping effects on the microstructure and magnetic properties of FePt-C nanocomposite films (dual doping of C and TiO₂). X-ray diffraction measurements revealed that the (001) orientation was well kept and there was no observable deterioration in chemical ordering degree, indicating a negligible change in anisotropy energy with increased TiO₂ doping. The coercivity of FePt-C-TiO₂ films decreased, while the slope of the hysteresis loops increased, with TiO₂ volume fraction, implying an enhanced exchange coupling induced by TiO₂ doping. Angular-dependent switching fields of FePt-C-TiO₂ films depicted that the reversal mode was changed from coherent-rotation dominated to wall-motion dominated, possibly resulting from the enhanced exchange coupling. Cross-sectional transmission electron microscope images showed that the grain boundaries became less clear and grains either coalesced or contacted each other at increased TiO₂ doping. The compositional-depth profile revealed that the C was pushed onto the film surface due to TiO₂ doping. A surfactant mediated growth model was proposed to account for the observed enhancement of exchange coupling and the change of microstructure. This study showed that to reduce grain size and promote grain isolation, a proper selection of boundary materials with different surface free energy is needed.