Anomalous hysteresis behavior in seed-free ultrathin films of amorphous-CoFeB alloy

Abstract Amorphous-Co20Fe60B20 (a-CFB) films are deposited by rf-magnetron sputtering on a self-oxidized Si (100) substrate with different thicknesses ranging from 0.7 nm to 20 nm. The 5-nm-thick a-CFB film is capped with a tungsten (W) layer for comparison. The small surface roughness observed in atomic force microscopy images indicates uniformity, moderate corrosion resistance, and good structural quality. The grazing-incidence X-ray diffraction spectra and bright-field transmission electron microscopy measurements confirm the amorphous nature of the CFB layer. The W capping is of mixed α+β phase in the experimental thickness regime. Energy-dispersive X-ray spectroscopy mapping indicates a homogeneous composition. In-plane and out-of-plane hysteresis loops obtained from the vibrating sample magnetometry technique show a transition from an ‘upright S’ to ‘nearly rectangular’ shape via a ‘completely inverted’ profile. A self-sustained tilted magnetic anisotropy is stabilized in a seed-free environment based on the direct substrate-to-magnet interaction at the ultra-thin regime. The interface anisotropy is estimated to be 0.06 erg/cm2. The complex anisotropic behavior of relatively thick films originates from the interplay between interface anisotropy, conventional shape anisotropy, growth-induced anisotropies, and inhomogeneity-induced anisotropies. This work may provide valuable insights into improving the functionality of amorphous soft magnetic alloys.