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Magnetic phase transition and relaxation effects in lithium iron phosphate


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We report the observation of a para-antiferromagnetic transition at ∼50 K in lithium iron phosphate, LiFePO4 through DC magnetic susceptibility and Mössbauer spectroscopy. The ferrous ion Fe2+ (3d6, 5D) in LiFePO4 exhibits relaxation effects with a relaxation frequency ∼1.076 × 107 s−1 at 300 K. The temperature dependence of the frequency suggests the origin of the relaxation is of the spin–lattice type. The quadrupole splitting at low temperatures indicates that the excited orbital states mix strongly to the orbital doublet ground state via spin–orbit coupling. Modified molecular field model analysis yields a saturation value for hyperfine field Bhf ∼ 125 kOe. The anomaly in susceptibility and Mössbauer parameters below 27 K may be ascribed to a contribution of orbital angular momentum. The high value of the asymmetry parameter η (∼0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighborhood for the ferrous sites.