Neutron-scattering studies of the geometrically frustrated spinelLiMn2O4

Abstract

The spinel structure oxide ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4},$ a geometrically frustrated antiferromagnet, was studied by neutron-scattering methods. Diffraction studies with unpolarized neutrons, $\ensuremath{\lambda}=2.52\AA{}$ and $E=12.9\mathrm{meV},$ are consistent with ${T}_{c}=65.5(5)\mathrm{K}.$ In agreement with previous results both Bragg peaks, indicative of long-range order, and a broad Lorentzian feature, centered at $Q=1.36{\AA{}}^{\ensuremath{-}1},$ indicative of short range order, coexist down to 2 K. The correlation length \ensuremath{\xi} associated with the short-range order is 3(1) \AA{}, 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 $\ensuremath{\lambda}=4.8\AA{}$ and 3.55 meV, quantitatively measure the total differential magnetic cross section, $d{\ensuremath{\sigma}}_{\mathrm{mag}}/d\ensuremath{\Omega}$ to be 1.05 b ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{f}.\mathrm{u}.}^{\mathrm{\ensuremath{-}}1}$ at 1.5 K, which is close to the value expected for a static spin model. $d{\ensuremath{\sigma}}_{\mathrm{mag}}/d\ensuremath{\Omega}$ increases by about 20% between 120 and 1.5 K below ${T}_{c},$ suggesting significant spin dynamics for $Tg{T}_{c},$ 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 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ at 80 K to 1.29 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ 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 \ensuremath{\sim}23% 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 ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ in the low-temperature structure of ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ and the inherent geometrical frustration of the Mn sublattice.