Magnetic susceptibility and zero-field muon spectroscopy $(\mathrm{ZF}-{\mu }^{+}\mathrm{SR})$ measurements of monoclinic ${\mathrm{Nb}}_{12}{\mathrm{O}}_{29}$ 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 $S=\frac{1}{2}$ linear Heisenberg chain, gives an exchange energy of $J/k=12.71\mathrm{}\mathrm{K}$ and substantially reduced $g=\mathrm{0.556.}\mathrm{}$ In this magnetically dilute system, the observation of an oscillating signal in the $\mathrm{ZF}-{\mu }^{+}\mathrm{SR}$ experiments is a clear and unambiguous signature of static magnetic correlations. We argue that this is due to an incommensurate arrangement of the ${\mathrm{Nb}}^{4+}$ electronic moments, similar to a spin-density wave. This phase sets in below $T_f\approx 10\mathrm{K}$ 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.

• Received 23 August 2001

DOI:https://doi.org/10.1103/PhysRevB.65.134405