Magnetic Phase Transition and Relaxation Effects in LiFePO4

TL;DR

This study investigates the magnetic phase transition and relaxation phenomena in LiFePO4, revealing a para-antiferromagnetic transition at 50 K, spin-lattice relaxation effects, and orbital contributions affecting magnetic properties.

Contribution

It provides new insights into the magnetic behavior and relaxation mechanisms of LiFePO4 using magnetization and Mössbauer spectroscopy, including a modified molecular field model analysis.

Findings

Para-antiferromagnetic transition at ~50 K

Spin-lattice relaxation effects with frequency ~1.076×10^7 s^-1 at 300 K

Orbital angular momentum influences below 27 K

Abstract

We report the observation of para - antiferromagnetic transition at ~ 50 K in lithium iron phosphate, LiFePO4 through DC magnetization and M\"ossbauer spectroscopy. The Ferrous ion Fe2+ (3d6, 5D) in LiFePO4 exhibits relaxation effects with a relaxation frequency ~1.076 \times 10(rise to 7) s-1 at 300 K. The temperature dependence of the frequency suggests the origin of the relaxation is spin-lattice type. The quadrupole splitting at low temperatures indicates 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 ~125 kOe. The anomaly in magnetization and M\"ossbauer parameters below 27 K may be ascribed to contribution of orbital angular momentum. The high value of the asymmetry parameter ({\eta} ~ 0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighbourhood for the ferrous sites.