Quasi-one-dimensional magnetic properties of ${\mathrm{NiNb}}_{2-x}{\mathrm{V}}_x{\mathrm{O}}_6$ compounds synthesized at high pressure in a nonstandard columbite-type structure

We report on the low-dimensional magnetic behavior of the series of compounds ${\mathrm{NiNb}}_{2-x}{\mathrm{V}}_x{\mathrm{O}}_6$ ($0\le x\le 2$), with a columbite-type crystal structure stabilized at high pressure and temperature. Based on susceptibility, magnetization, and specific-heat measurements, the system is characterized as presenting quasi-one-dimensional magnetism, with ${\mathrm{Ni}}^{2+}$ magnetic moments that can be modeled as Ising spins, placed along zigzag chains in the crystal structure. The low-temperature phase is found to consist of an antiferromagnetic arrangement of ferromagnetic chains, and a metamagnetic transition to uniform ferromagnetic order is observed under magnetic fields slightly above ${\mu }_{0}H=1\mathrm{T}$. We discuss the effects of substituting vanadium for niobium, maintaining the same crystal structure along the whole series of samples. In particular, the long-range magnetic order, most clearly seen for $x=0$, tends to be suppressed as the vanadium content is increased. The exchange interactions are quantified, revealing that the ferromagnetic intrachain interactions vary from about $7\mathrm{K}$ for ${\mathrm{NiNb}}_2{\mathrm{O}}_6$ to $2\mathrm{K}$ for ${\mathrm{NiV}}_2{\mathrm{O}}_6$, remaining one order of magnitude larger than the mean antiferromagnetic interchain coupling.

Figure 1

Refined XRD pattern for the sample ${\mathrm{NiV}}_2{\mathrm{O}}_6$ recorded at room temperature, showing the experimental data (black points), the calculated curve from Rietveld refinement (red line), the difference between data and fitting (blue line at the bottom), and the expected positions of Bragg peaks for the Pbcn space group (green vertical marks). The inset depicts a unit cell of the structure, showing oxygen octahedra surrounding the cations.

Figure 2

Magnetic susceptibility as a function of temperature for ${\mathrm{NiNb}}_{0.5}{\mathrm{V}}_{1.5}{\mathrm{O}}_6$ with a measuring field ${\mu }_{0}H=10\mathrm{mT}$. The inset shows the temperature derivative of the product $T\chi$, whose peak is used to define $T_{\mathrm{N}}$.

Figure 3

Inverse of the magnetic susceptibility as a function of the temperature for all the samples of the series ${\mathrm{NiNb}}_{2-x}{\mathrm{V}}_x{\mathrm{O}}_6$. Symbols represent experimental data taken at ${\mu }_{0}H=2\mathrm{T}$, while lines are fittings to the Curie-Weiss law.

Figure 4

Left: Magnetic interchain exchange interactions ($J_1,J_2$) and unit-cell parameters on the $ab$-plane of the Pbcn columbite-type structure. Blue spheres represent ${\mathrm{Ni}}^{2+}$ cations. Right: Schematic representation of a chain of oxygen octahedra surrounding Ni ions along the $c$ axis, indicating the Ni-O-Ni angle $\theta$ quoted in Table 1.

Figure 5

Isothermal magnetization curves taken at $T=1.8\mathrm{K}$ (top) and their numerical derivatives (bottom) for the indicated samples of the ${\mathrm{NiNb}}_{2-x}{\mathrm{V}}_x{\mathrm{O}}_6$ compounds.

Figure 6

Isothermal magnetization curves for ${\mathrm{NiNb}}_2{\mathrm{O}}_6$ (top), ${\mathrm{NiNbVO}}_6$ (center), and ${\mathrm{NiV}}_2{\mathrm{O}}_6$ (bottom), at the quoted temperatures.

Figure 7

(a) Specific heat as a function of the temperature for samples $x=0$ and 2. (b) Example of extraction of the magnetic specific heat by subtracting a Debye-model contribution, as explained in the text.

Figure 8

Magnetic specific heat of the samples ${\mathrm{NiNb}}_2{\mathrm{O}}_6$ (top) and ${\mathrm{NiNbVO}}_6$ (bottom), plotted as $C_m/T$ vs $T$ (crossed lines), and the corresponding amount of entropy $\mathrm{\Delta }S$ obtained from its numerical integration (dashed lines). The theoretical specific heat (continuous line) and entropy (dotted line) for isolated Ising chains are included for comparison.