A low-temperature magnetic phase diagram under magnetic fields along the orthorhombic $a$ axis of an isosceles-triangular-lattice antiferromagnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ was investigated through single-crystal neutron diffraction measurements made at temperatures down to $T=0.5\mathrm{K}$. We produced a phase diagram that consists of three magnetically ordered phases, i.e., the antiferromagnetic (AF), the incommensurate sinusoidal magnetic (IC), and the induced ferromagnetic (IFM) phases, which were characterized by the propagation wave vectors $\mathbf{Q}=\left(0\frac{1}{2}0\right)$, (0 $q$ 0), and (0 0 0), respectively. We found that a field-induced ferrimagnetic phase with $\mathbf{Q}=\left(0\frac{1}{4}0\right)$ that had been observed by previous neutron diffraction studies down to $T=1.8\mathrm{K}$ [H. Weitzel et al., Phys. Rev. B 62, 12146 (2000)] does not exist as a single equilibrium phase, but rather it always coexists with the other ordered phases near the triple point where the AF, IC, and IFM phases meet. We also found that the relaxation time of the system becomes extremely long below $T=0.6\mathrm{K}$ in comparison with our observation time; this was considered to possibly be the reason for magnetization plateaus appearing at $T=0.5\mathrm{K}$. These plateaus have a half-saturation magnetization, from which another field-induced state was inferred in previous magnetization measurements [J. Phys. Soc. Jpn. 63, 2706 (1994)].

• Received 13 June 2016

DOI:https://doi.org/10.1103/PhysRevB.94.134427