First principles and atomistic calculation of the magnetic anisotropy of Y2Fe14B

We present a study of the effects of strain on the magnetocrystalline anisotropy energy and magnetic moments of Y2Fe14B bulk alloy. The study has been performed within the framework of density functional theory in its fully relativistic form under the generalized gradient approximation. We have studied seven different in-plane a lattice constant values ranging from 8.48 up to 9.08  Å with an increment of δa=0.1 Å. For each a value, we carried out an out-of-plane c parameter optimization, achieving the corresponding optimized lattice pair (a,c). We find a large variation in the site resolved magnetic moments for inequivalent Fe, Y, and B atoms for different lattice expansions and a negative contribution to the total moment from the Y sites. We find a strong variation in the magnetocrystalline anisotropy with the c/a ratio. However, the calculated variation when coupled with thermodynamic spin fluctuations is unable to explain the experimentally observed increase in the total magnetic anisotropy, suggesting that a different physical mechanism is likely to be responsible in contrast with previous interpretations. We show that opposing single- and two-ion anisotropy terms in the Hamiltonian gives good agreement with the experiment and is the probable origin of the non-monotonic temperature dependence of the net anisotropy of Y2Fe14B bulk alloy.