λ-MnO2, a new frustrated antiferromagnet with the defect spinel structure (abstract)

MnO2 exists in a number of crystalline modifications. λ-MnO2, prepared by electrochemical delithiation of the spine Li2Mn2O4, has Mn4+, S=3/2, occupying the octahedral (16c) sites in Fd3m. The magnetic sublattice consists of a pyrochlore-type array of corner-shared tetrahedra and is thus a candidate for frustrated antiferromagnetism. The dc susceptibility data indicate, for very low applied fields of 45 G, a sharp maximum at 32 K but this is smeared to a broad maximum at 25 K for fields as modest as 300 G. Zero-field neutron diffraction data show long range order with K=(1/2 1/2 1/2) and Tn=32 K. Surprisingly, the long range order is robust to applied fields of 1.0 T and higher. Prominent diffuse scattering persists up to greater than 60 K due to short range order which is indicative of frustration. The properties of λ-MnO2 are compared with those of the spin-glass-like pyrochlore, Y2Mn2O7, which has the same magnetic species on the same sublattice.