Elastic neutron diffraction study of long-range antiferro-magnetic order in the S=12 quantum chain system CuSb2O6

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Abstract

CuSb2O6 crystallizes with a monoclinically distorted variant of the trirutile structure type with Cu2+ ions arranged in square planar nets being structurally well separated from each other by Sb–O building blocks. Using elastic neutron diffraction we have established the low-temperature nuclear structure and antiferromagnetic (afm) arrangement of Cu moments at 2 K. The magnetic unit cell is doubled with respect to the nuclear cell along the a- and c-axes. The magnetic anisotropy in the ordered state suggests that Cu moments of magnitude 0.51(2)μB lie in the ab plane.

Introduction

Efforts to understand the underlying microscopic mechanism of high TC superconductivity have refocused attention on the magnetic behavior of quantum spin S=12 chain and ladder systems. The system CuSb2O6 crystallizes in a monoclinically distorted trirutile structure type. It has been reported to exhibit magnetic strong short-range order (SRO) signalled by a broad susceptibility maximum centered around 60 K [1]. The susceptibility was reported to fit reasonably well to an afm uniform S=12 Heisenberg chain (J=−49.6K) model with nearest neighbor coupling. A sharp decrease in the susceptibility below 8.6 K indicates the onset of long-range afm order due to interchain interactions. Initial powder neutron diffraction suggested a propagation vector k=(12,0,12) for the afm ordered phase. Various ordering models have been discussed in Ref. [2].

Section snippets

Experimental

Single crystals were grown by chemical vapor transport as described in detail previously [3]. Magnetic susceptibilities were measured in a MPMS7 magnetometer. Using the four circle diffractometer D10 (ILL, Grenoble) a single-crystal neutron diffraction measurement was carried out. The structure refinement was performed with standard routines of the Cambridge Crystallographic Subroutine Library [4].

Results and discussion

Fig. 1 displays the magnetic susceptibility and the heat capacity of a crystal of CuSb2O6 [4]. Long-rang order is evident from the heat capacity anomaly and the kinks in the susceptibility below TN=8.68(5)K. The fit of the susceptibility with the model of an afm S=12 Heisenberg chain with uniform nearest-neighbour coupling yields close to perfect agreement (|χmolfit−χmolexp|⩽0.4%;10K<T<350K) using an exchange parameter of J=−48.0(5)K. In addition to a phonon term (∝T3), the low-temperature heat

Summary

Magnetic susceptibility, heat capacity and neutron diffraction of a single crystal of CuSb2O6 indicate S=12 Heisenberg chain behavior above and long-range afm ordering below TN≈8.7K. The magnitude of the ordered Cu2+ moment amounts to 0.51 μB and they lie in the ab-plane with the major component along the b-axis.

Note added in proof

While proofreading we became aware of a neutron diffraction study on CuSb2O6 by Kato et al. [J. Phys. Soc. Jpn. 71 (2002) Suppl. 187 (2002)].

Acknowledgements

This work was supported by the DFG Schwerpunkt-programm, ‘Kollektive Quantenzustände in elektro-nisch eindimensionalen Übergangsmetallverbindun-gen’.

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