New type of ferromagnetic insulator: Double perovskite La2NiMO6 (M=Mn, Tc, Re, Ti, Zr, and Hf)

Electronic structures of transition metal pnictides double perovskite La2NiMO6 (M=Mn, Tc, Re, Ti, Zr, and Hf) were shown as ferromagnetic (FM) insulators based on density functional calculation results. The FM state observed in La2NiMO6 (M=Mn, Tc, and Re) was most likely a mixture of high spin (HS) and low spin (LS) states; the electrons were transferred from the filled LS Ni eg states to the half-filled HS Mn (Tc). On the other hand, the FM state in La2NiMO6 (M=Ti, Zr, and Hf) was caused by the electron transfer from the half-filled LS Ti (Zr and Hf) eg orbital of HS Ni to the empty eg orbital of LS Ti (Zr and Hf). The FM insulating state of La2NiMO6 (M=Mn. Tc, Ti, Zr, and Hf) remained the same, whereas it changed from metal to insulator for La2NiReO6 based on the generalized gradient approximation+U calculation.