Neutron-scattering studies of the geometrically frustrated spinel LiMn2O4 -: art. no. 184424

The spinel structure oxide LiMn2O4, a geometrically frustrated antiferromagnet, was studied by neutron-scattering methods. Diffraction studies with unpolarized neutrons, lambda=2.52 Angstrom and E=12.9 meV, are consistent with T-c=65.5(5) K. In agreement with previous results both Bragg peaks, indicative of long-range order, and a broad Lorentzian feature, centered at Q=1.36 Angstrom(-1), indicative of short range order, coexist down to 2 K. The correlation length xi associated with the short-range order is 3(1) Angstrom, the order of the nearest-neighbor (nn) Mn-Mn distances. Quantitative estimates of the Bragg and Lorentzian contributions from 80 to 2 K indicate the growth of the Bragg at the expense of the Lorentzian component, that their integrated intensities become essentially equal below 20 K and that the total magnetic scattering is conserved within the E-transfer Q range studied. Further studies with neutron polarization analysis, using neutrons with lambda=4.8 Angstrom and 3.55 meV, quantitatively measure the total differential magnetic cross section, dsigma(mag)/dOmega to be 1.05 b sr-1 f.u.-1 at 1.5 K, which is close to the value expected for a static spin model. dsigma(mag)/dOmega increases by about 20% between 120 and 1.5 K below T-c, suggesting significant spin dynamics for T>T-c, which lie outside of the energy window of the neutron polarization analysis experiment. In addition, above T-c the Lorentzian peak broadens slightly and the center shifts from 1.36 Angstrom-1 at 80 K to 1.29 Angstrom-1 at 120 K. Inelastic magnetic scattering was measured using neutron polarization analysis over the energy range +2.0 to -4.9 meV at 1.5 and 80 K. At 1.5 K the scattering is essentially elastic with at most similar to23% of the spins fluctuating while at 80 K it is largely inelastic with at least 75% of the spins fluctuating. These results are consistent with the existence of a correlated paramagnetic state above T-c, which evolves with decreasing temperature into a ground state below 20 K in which about half the spins are long-range ordered and the other half are in a spin-glass-like configuration. This is in turn consistent with the presence of partial charge ordering between Mn3+ and Mn4+ in the low-temperature structure of LiMn2O4 and the inherent geometrical frustration of the Mn sublattice.