Three-dimensional magnetic ordering in the quasi-one-dimensional Ising magnetCoNb2O6with partially released geometrical frustration

The formation of three-dimensional magnetic ordering has been studied on a quasi-one-dimensional magnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ by mean-field calculations as well as neutron scattering measurements down to $T=1.5\mathrm{}\mathrm{K}$ under magnetic fields up to $H_{\parallel c}\sim 600\mathrm{Oe}.$ Measurements of a deviation of the magnetic Bragg scattering function from the delta function in the ordered state reveal a surprisingly rich variety of the magnetic formation arising from an isosceles triangular arrangement of the magnetic chain with competing interchain interactions in the $a-b$ plane. The competing interactions result in quasidegenerate ground states with different propagation wave numbers along the $b^{*}$ direction in the sinusoidally amplitude-modulated incommensurate magnetic (IC) phase. Our mean-field calculations qualitatively reproduce the complicated $H_{\parallel c}-T$ magnetic phase diagram and give evidence for a high degeneracy of ground states by calculating the $H_{\parallel c}-T$ dependence of the free energy curve in the propagation wave number space. In addition, a partial cancellation of the exchange field at the apex site from the base sites on the isosceles triangular lattice leads to a quasi-long-range ordering along the a axis in both IC and antiferromagnetic states where the correlation length along the a axis depends on the propagation wave number along the $b^{*}$ direction.