Comparative Muon-Spin Rotation and Relaxation Study on the Zigzag Chain Compounds NaMn2O4 and Li0.92Mn2O4

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The magnetic nature of the CaFe2O4-type NaMn2O4 and Li0.92Mn2O4, in which Mn ions form a zigzag chain along the $b$-axis, have been investigated by muon-spin rotation and relaxation ($\mu^{+}$SR) and susceptibility ($\chi$) measurements in the temperature range between 1.9 and 250 K using polycrystalline samples. Weak transverse-field $\mu^{+}$SR measurements revealed the existence of a bulk antiferromagnetic (AF) transition for NaMn2O4 at $T_{\text{N}}=39$ K and for Li0.92Mn2O4 at $T_{\text{N}}=44$ K, although the $\chi(T)$ curve did not exhibit a clear anomaly around $T_{\text{N}}$ for either compound. Below $T_{\text{N}}$, however, the zero-field $\mu^{+}$SR spectra in both samples did not show the oscillatory signal characteristic of long range magnetic order. Instead, a slowly relaxing tail is observed. Comparisons with the calculated internal magnetic field for the AF ordered phase suggests that the AF spin structure is disordered and/or rapidly fluctuating. In the paramagnetic region above 100 K, a nuclear magnetic field in Li0.92Mn2O4 was found to be dynamic, whereas that in NaMn2O4 was almost static. This implies the diffusive motion of Li+ ions in Li0.92Mn2O4 even below 250 K.
2009-08-15