Structure and Magnetic Properties of Vacuum-Annealed CoFeB Thin Films: From Amorphous Alloy to Metastable (Co,Fe)23B6 tau-Boride

Abstract

Controlled crystallization of amorphous alloys offers a powerful route to tailor magnetic and structural properties at the nanoscale. Thin films of CoFeB alloy are essential for the development of various spintronic devices. The crystallization mechanisms of CoFeB during the annealing process have been thoroughly investigated in earlier studies, demonstrating that boron diffuses from the amorphous film, allowing the remaining CoFe to form a body-centered cubic lattice. Here, a distinct transformation pathway in pulsed-laser-deposited amorphous Co40Fe40B20 films is revealed, where vacuum annealing drives the formation of a metastable tau-boride phase, (Co,Fe)23B6. Comprehensive structural characterization - combining X-ray diffraction, transmission electron microscopy, and compositional analysis - proves that tau-boride forms with high crystalline quality and minimal boron loss. Following the transition from amorphous CoFeB films to crystalline (Co,Fe)23B6, an improvement of magnetic properties is observed, with corresponding increases in such values as saturation magnetization, coercivity, loop squareness, and average magnetic moment. The reproducible stabilization of a boron-rich metastable phase in CoFeB thin films expands the known crystallization landscape of this technologically important alloy system. These findings provide new insight into phase engineering in transition-metal borides and open opportunities for designing nanostructured magnetic materials with tunable functionality for next-generation spintronic and nanoelectronic devices.