Effect of 3d-metal doping on magnetic properties of Fe3Se4

Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ exhibits large magnetocrystalline anisotropy (MAE) and coercivity up 40kOe. The large anisotropy of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ should be accompanied by large magnetization for permanent magnet applications. The magnetization of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ suffers from antiferromagnetic (AFM) superexchange coupling of Fe across the Se planes. We present density functional theory study of the magnetic properties of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ doped with TM (Co, Cr, Ni and Mn), TM ions doped in Fe sites, Fe$_{\mathrm{3-x}}$(TM)$_{\mathrm{x}}$Se$_{\mathrm{4}}$ (x $=$ 0.5), to examine a potential increase of the magnetization and Curie temperature of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$. We performed screening of the exchange interactions and magnetization modifications upon the substitution of Fe by 3d-transition metals at various Fe sites in the Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ We find that the doping of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ with 3d-elements does not remove AFM coupling across layers. The increase in the strength of exchange interactions on doping with Cr should increase the Curie temperature of the system. We compare the results of doped alloy with the ones for Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$. This compound is analogous to Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ because Te has same electron configuration as Se, but Cr has only four d-electrons. We find that Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$ has ferromagnetic coupling and magnetization larger than one possible in Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$. Magnetization per unit cell is 18.24$\mu _{\mathrm{B}}$. MAE of this material is large (MAE $=$1.67 MJ/m$^{\mathrm{3}})$