Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO2

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO2 are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near T (N) indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T-T (N)) variation is satisfactorily described by a power-law dependence with a critical exponent beta = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO2 is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (similar to 0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn3+ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Neel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO2. As a result, a certain fraction of the manganese positions is occupied by lithium ions (Li-Mn) and vise versa (Mn-Li). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.