Electrochemical manipulation of spin reorientation in FePO4 by Li-ion intercalation

Density functional theory plus U and Monte Carlo simulations reveal that the electrochemical lithiation/delithiation process has a dramatic effect on magnetic structure and magnetocrystalline anisotropy (MCA) of an olivine LixFePO4 (0 <= x <= 1) structure. We demonstrate that MCA undergoes a transition from the magnetic easy axis along the [100] direction for FePO4 (fully delithiated phase) to the [010] direction for the partially and fully lithiated phases of LixFePO4 with x >= 0.25. This magnetization reorientation is the result of the interplay between the lithiation/delithiation induced changes in the spin-orbit coupled 3d-electron orbital states near the Fermi level and the high-spin crystal field split of the Fe3+ and Fe2+ ionic states. Furthermore, the lithium intercalation substantially reduces the energy deviation of an antiferromagnetic state from a ferromagnetic phase, reflecting the experimental observation of lowering the Neel temperature from 125 K for FePO4 to 52 K for LiFePO4, which are fairly reproduced by the Monte Carlo simulation. These findings open interesting prospects for exploiting electrochemical lithiation/delithiation process to manipulate magnetization direction in antiferromagnet spintronics.