Anomalous Hysteresis Behavior in Sputter-deposited Ultrathin Films of Amorphous- CoFeB Alloy

TL;DR

This study investigates the magnetic hysteresis behavior of ultrathin amorphous CoFeB films deposited by sputtering, revealing anomalous hysteresis due to complex anisotropic interactions, with implications for magnetic alloy functionalities.

Contribution

It uncovers the anomalous hysteresis behavior in ultrathin amorphous CoFeB films and links it to complex anisotropic interactions, providing insights for magnetic material development.

Findings

Hysteresis loops transition from upright to rectangular shapes.

Self-sustained tilted magnetic anisotropy observed.

Interface anisotropy estimated at 0.06 erg/cm2.

Abstract

Thin amorphous-CoFeB (a-CFB) is deposited by rf-magnetron sputtering on a self-oxidized Si (100) substrate with different film thicknesses ranging from 0.7 nm to 20 nm. The 5-nm-thick a-CFB film is capped with a W layer for comparison. The surface morphology is investigated by using the atomic force microscopy technique. The low roughness of all the surface of the film indicates uniformity, moderate corrosion resistance, and good structural quality. The X-ray diffraction spectra reveal the amorphous nature of the CFB layer, while the W capping is of mixed phase in the experimental thickness regime. 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. The interface anisotropy is estimated to be 0.06 erg/cm2. The complex anisotropic behavior originates from the interplay between interface anisotropy, conventional shape anisotropy, growth-induced anisotropies, and inhomogeneity-induced anisotropies. In essence, effective anisotropy is responsible for the anomalous hysteresis behavior observed in these films, and this work might provide valuable insights to improve the functionalities of amorphous soft magnetic alloys.