Magnetic ordering in the charge-ordered Nb12O29

Magnetic susceptibility and zero-field muon spectroscopy (ZF-μ+SR) measurements of monoclinic Nb12O29 are reported. The magnetic susceptibility shows a broad maximum at around 12 K with no divergence between the zero-field cooled and field-cooled susceptibility, implying low-dimensional antiferromagnetic ordering. Application of the Bonner-Fischer model, applicable for a uniform S = 1/2 linear Heisenberg chain, gives an exchange energy of J/k = 12.71 K and substantially reduced g=0.556. In this magnetically dilute system, the observation of an oscillating signal in the ZF-μ+SR experiments is a clear and unambiguous signature of static magnetic correlations. We argue that this is due to an incommensurate arrangement of the Nb4+ electronic moments, similar to a spin-density wave. This phase sets in below Tfâ??10 K and produces spontaneous muon-spin precession associated with two distinct internal local fields that saturate at 39.3(6) and 138(1) G at 60 mK.