Hard magnetic property enhancement of Co₇Hf-based ribbons by boron doping

Hard magnetic property enhancement of melt spun Co₈₈Hf₁₂ ribbons by boron doping is demonstrated. B-doping could not only remarkably enhance the magnetic properties from energy product ((BH)_max) of 2.6 MGOe and intrinsic coercivity (_iH_c) of 1.5 kOe for B-free Co₈₈Hf₁₂ ribbons to (BH)_max = 7.7 MGOe and _iH_c = 3.1 kOe for Co₈₅Hf₁₂B₃ ribbons but also improve the Curie temperature (T_C) of 7:1 phase. The (BH)_max value achieved in Co₈₅Hf₁₂B₃ ribbons is the highest in Co-Hf alloy ribbons ever reported, which is about 15% higher than that of Co₁₁Hf₂B ribbons spun at 16 m/s [M. A. McGuire, O. Rios, N. J. Ghimire, and M. Koehler, Appl. Phys. Lett. 101, 202401 (2012)]. The structural analysis confirms that B enters the orthorhombic Co₇Hf (7:1) crystal structure as interstitial atoms, forming Co₇HfB_x, in the as-spun state. Yet B may diffuse out from the 7:1 phase after post-annealing, leading to the reduction of Curie temperature and the magnetic properties. The uniformly refined microstructure with B-doping results in high remanence (B_r) and improves the squareness of demagnetization curve. The formation of interstitial-atom-modified Co₇HfB_x phase and the microstructure refinement are the main reasons to give rise to the enhancement of hard magnetic properties in the B-containing Co₇Hf-based ribbons.