Three-dimensional magnetic ordering in the quasi-one-dimensional Ising magnet CoNb2O6 with partially released geometrical frustration

The formation of three-dimensional magnetic ordering has been studied on a quasi-one-dimensional magnet CoNb2O6 by mean-field calculations as well as neutron scattering measurements down to T = 1.5 K under magnetic fields up to H(parallel to c)similar to 600 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 to c-T magnetic phase diagram and give evidence for a high degeneracy of ground states by calculating the H-parallel to 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. [S0163-1829(99)11121-4].