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. It is further found that the lithium intercalation substantially reduces the energy deviation of an antiferromagnetic state from a ferromagnetic phase, reflecting the experimental observation of lowering the Néel temperature from 125 K for FePO4 to 52 K for LiFePO4, which are fairly reproduced by the Monte Carlo simulation. In addition, density functional perturbation theory and ab initio molecular dynamic calculations indicate that LixFePO4 phases are thermodynamically stable. These findings open interesting prospects for exploiting electrochemical lithiation/delithiation process to manipulate magnetization direction in antiferromagnet spintronics.