Concentration-tuned tetragonal strain in alloys: Application to magnetic anisotropy of FeNi1-xCox

We explore an opportunity to induce and control tetragonal distortion in materials. The idea involves formation of a binary alloy from parent compounds having body-centered and face-centered symmetries. The concept is illustrated in the case of FeNi1-xCox magnetic alloy formed by substitutional doping of the L1(0) FeNi magnet with Co. Using electronic structure calculations we demonstrate that the tetragonal strain in this system can be controlled by concentration and it reaches maximum for x = 0.5. This finding is then applied to create a large magnetocrystalline anisotropy (MAE) in FeNi1-xCox system by considering an interplay of the tetragonal distortion with electronic concentration and chemical anisotropy. In particular, we identify an ordered FeNi0.5Co0.5 system with MAE larger by a factor 4.5 from the L1(0) FeNi magnet.