2D Kagomé ordering in the 3D frustrated spinel Li2Mn2O4

Muon spin relaxation (μSR) experiments on the geometrically frustrated spinel oxide, Li2Mn2O4, show the development of spin correlations over a range of lengthscales with decreasing temperature. Increased relaxation below ~150 K is consistent with the onset of spin correlations. Below 50 K, spin order, on a lengthscale which is long range for the μSR probe, appears abruptly in temperature, consistent with prior neutron diffraction results. The oscillations in the zero field asymmetry are analysed using a three frequency model. By locating the muon site, this is shown to be consistent with the unexpected 2D structure on the Kagomé planes proposed originally from neutron data. Longitudinal field data demonstrate that some spin dynamics persist even at 2 K. Thus, a very complex magnetic ground state, featuring the coexistence of long lengthscale 2D ordering and significant spin dynamics, is proposed. This is unusual considering the 3D topology of the Mn3+ spins in this material.