Searching for magnetically hard monoborides (and finding a few): A first-principles investigation

New hard magnetic materials with zero or low rare earth content are in demand due to the high and volatile prices of the rare earth metals. Among the candidates for such materials, we consider MnB, FeB and their alloys, since previous experiments suggest that FeB has a relatively high magnetic hardness of about 0.83 at room temperature. Using first-principles calculations, we examine the full range of alloys from CrB, through MnB, FeB, to CoB. Furthermore, we consider alloys of MnB and FeB with substitutions of 3$d$, 4$d$ and 5$d$ transition metals. For the above compositions, we determine magnetic moment, magnetocrystalline anisotropy energy and magnetic hardness. For the alloys with transition metals, we calculated also formation energies and Curie temperatures. The most accurate MAE calculations performed for (Fe-Co)B alloys using supercells indicate both semi-hard and hard magnetic compositions. The weak point of these alloys, however, is the relatively low values of Curie temperatures and magnetic moments. Furthermore, we classify considered MnB alloys substituted with transition metals as magnetically soft or semi-hard and FeB alloys with Sc, Ti, V, Zr, Nb, Mo, Hf, Ta, or W as magnetically hard (with magnetic hardness exceeding unity). However, the most promising elements for FeB alloying are Mn and Cr, because by increasing the magnetic hardness of the alloy (below unity), they increase or only a little decrease its Curie temperature, respectively.