First-Principles Prediction of Possible Rare-Earth Free Permanent Magnet of Tetragonal FeCo with Enhanced Magnetic Anisotropy and Energy Product through Interstitial Nitrogen

Pushing a uniaxial magnetic anisotropy (K-u) up to an order of meV per atom in bulk structures composed of only 3d magnetic elements with large saturation magnetization (mu M-0(s)) and Curie temperature (T-c) has been a long-term goal of hard permanent magnet and spintronics studies. Herein, using ab initio calculations, we predict an unexpectedly large K-u up to 1/2 meV/formula unit in B2-ordered tetragonal FeCo, which is even comparable to or more than 50% of those for the currently known permanent magnetic materials with 4d, 5d, and 4f orbitals. This large K-u is associated with an interplay mechanism of the JahnTeller-like tetragonal distortion induced by doping with an interstitial 2p-electron element, herein nitrogen as an example, and the strong orbital hybridization between 3d and 2p states. Our findings demonstrate the feasibility of significant enhancements on the magnetic anisotropy and energy product of 3d magnetic metals through the interstitial doping with 2p-electron nonmetal elements.