Control of crystallization and magnetic properties of CoFeB by boron concentration

Controlling the crystallinity of CoFeB is the most essential issue for designing various spintronics devices. Here we show the microstructure and magnetic properties of MgO/CoFeB/MgO structures for various boron concentration. We present the effect of boron on the crystallinity of CoFeB into two categories: the critical boron concentration (5 ~ 6%) at which CoFeB crystallizes and the effect of remaining boron (0 ~ 5%) in the crystallized CoFeB. And the trends of the saturation magnetization, exchange stiffness, exchange length, domain wall energy and Gilbert damping constant according to the boron concentration are provided. Abrupt variation of properties near the critical boron concentration (5 ~ 6%) and a noticeable change in the crystallized CoFeB (0 ~ 5%) are confirmed, revealing a clear causal relationship with the structural analysis. These results propose that the crystallization, microstructure, and major magnetic properties of CoFeB are governed by the amount of boron, and emphasize the need for delicate control of boron concentration.