Perpendicularly magnetized ferromagnetism in Mn/Al bilayer thin films on Si substrates induced by temperature dependent ion beam mixing

The perpendicularly magnetized τ-phase was achieved in Mn/Al bilayer thin films by ion beam mixing using 400 keV Xe ions. The effect of annealing and irradiation on the structural, microstructural, and magnetic properties of Mn/Al bilayer thin films was investigated in the present study. Mn/Al bilayer thin films (thickness ∼97 nm) were deposited on silicon substrates by the evaporation technique. These as-deposited films were irradiated to the fluences of 5 × 10¹⁶ and 1 × 10¹⁷ ions cm⁻² at three substrate temperatures (Room temperature ∼300 K, 450 K, and 623 K). As-deposited films were also annealed at 450 K and 623 K for 5 h. The ferromagnetic τ-phase was enhanced in annealed and irradiated thin films confirmed from synchrotron x-ray diffraction patterns, MFM images, and magnetic hysteresis curves. The average grain size estimated from the AFM study was found in the range of 78–192 nm. The domain patterns obtained in MFM images confirmed the ferromagnetic nature of the annealed and irradiated films. The hysteresis curves measured at 5 K show the maximum coercivity (7111 Oe) for the sample annealed at 450 K and the highest value of saturation magnetization (60 emu cc⁻¹) for the film irradiated to the fluence of 1 × 10¹⁷ ions cm⁻² at the substrate temperature of 450 K. Perpendicularly magnetized τ-phase was achieved in the films irradiated to the fluence of 1 × 10¹⁷ ions cm⁻² at different temperatures. The formation of the ferromagnetic phase by ion beam mixing can be attributed to the combined effect of ballistic mixing and thermal spike mechanisms. This study suggests that the ferromagnetic properties of MnAl thin films can be modified by selecting the proper annealing and irradiation conditions to make them useful for applications in spintronics and magnetic memory devices. (paper)