Antiferromagnetic transition of spinel LiMn2O4 detected by a 7Li-NMR technique

A 7Li nuclear magnetic resonance (7Li-NMR) study was carried out to investigate magnetic properties of a polycrystalline sample of LiMn2O4 in the temperature range between 4.2 and 300 K. Both the shift (K) and the full width at half maximum (FWHM) of the 7Li-NMR line increased monotonically with lowering temperature from 300 to 40 K, then increased suddenly at around 40 K and looked to level off to constant values with further lowering temperature. The values of K and FWHM at 4.2 K were estimated to be about 0.7% and 3.2 kOe, respectively; additionally, FWHM at 4.2 K seemed to be independent of an external magnetic field. Furthermore, the temperature dependence of a spin-lattice relaxation rate (T1-1) exhibited a sharp maximum at around 40 K. Therefore, we concluded that the moments of Mn ions ordered antiferromagnetically below the Néel temperature TN=40 K. According to the analysis of the resonance peak, the spin structure of the ordered state was considered to be as follows; every Li+ has the same numbers of the nearest neighboring Mn3+ and Mn4+ ions to maintain local charge neutrality, and each of the Mn spins are randomly arranged parallel or antiparallel to the z-axis.