Uniaxial-pressure control of geometrical spin frustration in an Ising antiferromagnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ via anisotropic deformation of the isosceles lattice

We report neutron diffraction measurement results for an Ising antiferromagnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ under uniaxial pressure along the geometrically frustrated isosceles-triangular-lattice direction. We find that an onset incommensurate wave number at the Néel temperature increases with pressure from 0.378 to 0.411 at 400 MPa. The observations suggest that the anisotropic deformation of the lattice by the uniaxial pressure significantly modifies the spin frustration, leading to an increase in the nearest-neighbor to next-nearest-neighbor interaction ratio from 1.33 to 1.81.

Figure 1

(a) Crystal structure of ${\mathrm{CoNb}}_2{\mathrm{O}}_6$. Two different easy axes exist in the $a\text{−}c$ plane with a canting angle of $\pm 31{}^{\circ }$ from the $c$ axis, which originate from the two different octahedral ${\mathrm{CoO}}_6$ sites. (b) Isosceles-triangular lattice of magnetic ${\mathrm{Co}}^{2+}$ ions in the $a\text{−}b$ plane with interchain antiferromagnetic exchange interactions, $J_1$ and $J_2$. The hatched area corresponds to a chemical unit cell and the large arrows represent the uniaxial-pressure direction, as applied in the experiment.

Figure 2

Temperature dependence of magnetization at $H=50$ Oe on heating after zero-field cooling, taken at different uniaxial pressures along the $a$ axis. The data are shifted vertically down by 0.1 emu/g for clarity.

Figure 3

Temperature dependence of the neutron diffraction profile of the $\left(0k0\right)$ scans at (a) $P=0$ MPa and (b) $P=400$ MPa. In the IC phase, two magnetic reflections appear at $\left(0q0\right)$ and $(01-q0)$. The solid lines through the data represent least-square fittings, assuming Gaussian AF and IC peaks.

Figure 4

Temperature dependence of (a) the propagation wave number in the $b^{*}$ direction and (b) the integrated intensity of the $(01-q0)$ peak, before and after the application of uniaxial pressure. The inset in (b) shows the $P\text{−}T$ phase diagram obtained in this study. The circles and triangles in the inset denote magnetic phase transition temperatures, obtained by magnetization and neutron diffraction measurements, respectively. $T_2$ for the neutron diffraction data is defined as the crossing point of the two extrapolation lines of the integrated intensities of both the AF and IC phases.