Disrupted antiferromagnetism in the brannerite MnV2O6

Magnetic order in brannerite type MnV 2 O 6 has been studied by magnetization measurements and low temperature powder neutron diffraction. A sharp transition to a three-dimensionally ordered antiferromagnetic state is observed at T N =20 K. Neutron diffraction at 5 K shows the spin structure to have a (cid:1) 0 0 1/2 (cid:2) propagation vector with Mn 2+ moments of 4.77 (cid:1) 7 (cid:2) (cid:1) B ordered parallel to b . Observed ferromagnetic order within chains of edge-sharing MnO 6 octahedra is consistent with a positive Weiss temperature of 5.8 K. The long range coherence of the magnetic order is limited, with a coherence length of 900 Å resulting from dilution of the Mn sites by 3% V due to intrinsic antisite disorder. The magnetic behavior of MnV 2 O 6 is markedly different to that of the analogue CuV 2 O 6 in which spin chain correlations were reported.