2D Kagomé ordering in the 3D frustrated spinel Li2Mn2O4

Journal of Physics Condensed Matter (Impact Factor: 2.22). 09/2005; 17(41):6469. DOI: 10.1088/0953-8984/17/41/017
Source: arXiv

ABSTRACT 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.

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    ABSTRACT: Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S=1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon-spin relaxation (μSR), and 89Y NMR-including magic-angle spinning (MAS) NMR data have been collected. La2LiMoO6 crystallizes in P21/n with a=5.59392(19) Å, b=5.69241(16) Å, c=7.88029(22) Å, and β=90.2601(30)° at 299.7 K, while Ba2YMoO6 is cubic, Fm3m, with a=8.39199(65) Å at 297.8 K. Ba2YMoO6 shows no distortion from cubic symmetry even at 2.8 K in apparent violation of the Jahn-Teller theorem for a t2g1 ion. 89Y NMR MAS data indicate about a 3% level of Y/Mo site mixing. La2LiMoO6 deviates strongly from simple Curie-Weiss (C-W) paramagnetic behavior below 150 K and zero-field-cooled/field-cooled (ZFC/FC) irreversibility occurs below 20 K with a weak, broad susceptibility maximum near 5 K in the ZFC data. A Curie-Weiss fit shows a reduced μeff=1.42μB, (spin only=1.73μB) and a Weiss temperature, θC, which depends strongly on the temperature range of the fit. Powder neutron diffraction and heat capacity show no evidence for long-range magnetic order to 2 K. On the other hand oscillations develop below 20 K in μSR indicating at least short-range magnetic correlations. Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below 150 K with a nearly spin only μeff=1.72μB and θC=−219(1) K. There is no discernible ZFC/FC irreversibility to 2 K. Heat capacity, neutron powder diffraction, and μSR data show no evidence for long-range order to 2 K but a very broad, weak maximum appears in the heat capacity. The 89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70 K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/T1. These results are discussed and compared with those from other isostructural S=1/2 materials and those based on S=3/2 and S=1.
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