Two-Dimensional Short-Range Magnetic Order in the Tetragonal Spinel Li2Mn2O4

Metastable, tetragonal Li2Mn2O4, prepared by chemical insertion of Li into the cubic spinel, LiMn2O4, was studied by dc magnetic susceptibility and neutron diffraction. The susceptibility data show a very broad maximum at about 120 K, a sharp maximum at about 50 K, and no evidence for Curie−Weiss behavior even up to 600 K. This is consistent with the dominance of short-range, antiferromagnetic correlations attributed to the geometrical frustration inherent in the Mn3+ sublattice which is a slightly distorted pyrochlore lattice, a three-dimensional array of corner-sharing tetrahedra. The temperature dependence of two diffuse peaks in the neutron diffraction data correlate well with the 50 K anomaly but show, remarkably, an asymmetric Warren line-shape characteristic of two-dimensional correlations with no sign of long-range order down to 1.6 K. The magnetic reflections can be indexed as (2 0) and (1 3) of a √3 × √3 magnetic structure arising from a Kagomé layer. The structure and properties of tetragonal Li2Mn2O4 are compared with the thermodynamically stable orthorhombic form.