Peculiarity of a magnetic structure in a quasi-one-dimensional columbite Co_0.4Ni_0.6Nb₂O₆

Magnetism remains a theme that has absorbed humanity for numerous generations. However, the long-range interaction combined with the complex systems magnetism emerges in to make it an extremely challenging subject. The importance of this field is primarily due to the future appeal of the electronic industry down to the atomic level. Quantum effects get more imperative at short scales, notably in lower-dimensional materials such as sheets and wires.¹ Nowadays, numerous composites, as CoNb₂O₆, have been synthesized serving as a model for the quantum spin system’s investigation with a one-dimensional arrangement of spins. This subject has proved therefore interesting that they been calling attention from many different scientific communities; like physicists,^2–6 engineers,^7–10 chemists^11–13 and biologists.¹⁴ Indeed, ANb₂O₆ compounds are considered prototype materials, seeing as they present low dimensional magnetic characteristics. These orthorhombic materials exhibit weak interactions in one-dimensional magnetic chains.^15–18 They present magnetic ordering, typically, below 10 K, in ordered antiferromagnetic (AF) phase, but they diverge from a simple nearest-neighbor Néel structure.

Some of the traditional experimental techniques for detecting and identify magnetic order and dynamics in solid-state materials is neutron measurements as scattering and diffraction. The Néel temperature obtained for CoNb₂O₆ was 2.95 K with a transition to an ordered spin structure from the paramagnetic phase (PM).¹⁷ Furthermore, its magnetic phase diagram was again investigated, linking powder and single-crystal neutron diffraction, by Schärf,¹⁹ Heid,^17,18 Kobayashi and coworkers^15,16 and Sarvezuk et al.^6,20 Below 2.96 K Schärf¹⁹ described as a screw magnetic structure, with the y-axis as a screw axis, and a screw angle of 133°. Such behavior changes because of the temperature decrease. In later work, Heid et al.¹⁷ presented an incommensurate propagation vector, (0,k_y,0), for this system, which took place in a very specific temperature range between 2.95 and 1.95 K. The value of ky changes over the temperature and decreases from 0.37 to 0.5, respectively, this means: below 1.95 K a (0,1/2,0) commensurate propagation vector is used. We have in the past proposed a (0,1/2,0) and (1/2,±1/2,0) propagation vector for cobalt columbite at 2.5 K and an incommensurate magnetic structure (0,0.4,0) at a lower temperature, down to 1.4 K, and suggests a continuous evolution of this incommensurate propagation vector.⁶ 

On the other hand, investigations of NiNb₂O₆ compound present ordered magnetic structure, due to triangular coordination on the ab planes. This also leads to an unusual magnetic behavior influenced by a few different exchange couplings. Nickel columbite orders from the paramagnetic phase to an ordered spin structure at 5.7 K, the magnetic atomic structure for NiNb₂O₆ had been reinvestigated by Heid et al.,^17,21 who showed that NiNb₂O₆ exhibits antiferromagnetic order with a canted magnetic structure. They determined this nickel columbite compound had a magnetic structure with two propagation vectors (0,1/2,0) and (1/2,1/2,0), through the combination of magnetic measurements and neutron diffraction.

Not long ago Munsie et al.^22,23 described NiNb₂O₆ with a new P4₂/n magnetic structure with propagation vector (1/2,1/2,1/2), showing that there is still much to investigate in this system. Compilation results obtained for magnetic structures of CoNb₂O₆ and NiNb₂O₆ are presented in Table I.

Here, we present a compilation of magnetic properties for Co_0.4Ni_0.6Nb₂O₆ by combining magnetic susceptibility, specific heat, and neutron diffraction studies, in this spin-chain compound, in order to reveal the magnetic structure present at low temperature.

Co_0.4Ni_0.6Nb₂O₆ powder sample was made with stoichiometric amounts of CoNb₂O₆ and NiNb₂O₆. The mixture of primary powders was pressed into pellets and heat-treated in the air at 1570 K for 30 h with a slow temperature decreasing to room temperature. Then, powdered to 44 microns. A large amount of sample is required for neutron diffraction (ND), about 2.5 g. Using X-ray diffraction (XRD), at room temperature, the sample pureness state was performed, before neutron diffraction measurements. The XRD pattern confirms the Pbcn space group symmetry and pure blended Co_0.4Ni_0.6Nb₂O₆ product. The ND was carried out at 2.5 Kelvin, with angular 2θ range from 10° to 80° and scan step of 0.05°, the wavelength was selected by a pyrolytic graphite monochromator using 2.52 Å, with the double-axis multi counter high-flux diffractometer D1B line, in collaboration with CNRS at the Institut Laue Langevin (ILL), Grenoble-FR.

Magnetic susceptibility (χ(T)) has been undertaken on powder samples using SQUID Quantum Design. The χ(T) was measured in a magnetic field of 50 Oe in the temperatures range 1.7–300 K. A PPMS calorimeter²⁴ was used to add information on magnetic transition temperature, specific-heat measurements were made from 1.9 to 300 K.

To extract the crystallographic and magnetic parameters data analysis of XRD and ND was done with FULLPROF²⁵ refinement software using the Rietveld refinement method. This article uses agreement factors defined in the Rietveld’s guidelines and can be found elsewhere.²⁶ 

The thermal evolution of magnetic susceptibility (χ) is displayed on the left side in Figure 1(a). A maximum on this curve is observed at low-temperature region (close to 4.5 K), which represents not a peak, but a broad maximum that characterizes low-dimensional magnetic materials. The antiferromagnetic transition does not occur at that point but the curve inflection region. These are better examined by plotting the derivative of the χ.T product with respect to T, which shows a well-defined peak at the temperature transition. This is shown in the inset, Figure 1(b), the transition is seen near 3.8 K which is an intermediate value between Co and Ni niobate.^6,17,21,27 This peak may be identified as the Néel temperature and coincides with the ordering temperature found from the specific-heat data showed at the right side of figure 1(c).

The Curie-Weiss law, χ(T) = C/(T – $θw$⁠)+$χo$ was fitted at sufficiently superior ordering temperature, higher than 40 K. The paramagnetic temperature $θw$⁠, Curie constant C and effective magnetic moments $μeff$ have been obtained by fitting the experimental data and are cataloged in Table II. The inclusion of $χo$ corresponds to a tiny correction for the temperature-independent diamagnetic susceptibility signal whose value was close to 10⁻⁷ emu/Oe. The paramagnetic temperature $θw$ is lower for the Co-pure sample and progressively increases for the Ni-pure sample. Another marked detail is the low and positive value of $θw$⁠, reflecting the dominance of ferromagnetic exchange interactions along the chains and due balance between Ferro and antiferromagnetic exchange interactions.

Regarding the X-ray diffraction, the diffractogram pattern refinement indicates the Co and Ni cations are fully ordered and forms a pure Co_0.4Ni_0.6Nb₂O₆ sample, but we should note that attempts to refine oxygen vacancies were unsuccessful within the experimental sensitivity. The columbite structure results from the stacking of slightly tilted oxygen octahedra surrounding the cations, forming zig-zag chains along the c direction.

Furthermore, low-temperature neutron diffraction measurement analysis can be seen in figure 2. This figure shows powder neutron diffraction data, recorded at 2.5 K, in Co_0.4Ni_0.6Nb₂O₆ (figure 2(a)), CoNb₂O₆ (figure 2(b)) and NiNb₂O₆ (figure 2(c)). For Co_0.4Ni_0.6Nb₂O₆ refinement, the identification of the magnetic Bragg reflections can be made by subtracting neutron diffraction pattern recorded at 20 K out of that at 2.5 K data,²³ and with the resulting pattern, only vector k = (1/2,±1/2,0) was required for indexing the magnetic phase.

Previous studies of magnetic structure in CoNb₂O₆ reported unique (0,1/2,0) or coupled (0,1/2,0) and (1/2,±1/2,0) propagation vector, for different temperatures. NiNb₂O₆, on the other hand, orders with (0,1/2,0) and (1/2,±1/2,0) and more recently a novel magnetic structure have been claimed (1/2,1/2,1/2), showing the contemporaneity of the research theme. Thus, comparing CoNb₂O₆ and NiNb₂O₆ magnetic structure, the pattern recorded at low temperature, reveals that Co_0.4Ni_0.6Nb₂O₆ has a peculiar behavior. Both Co and Ni pure columbite, present structures with two propagation vectors. For Co_0.4Ni_0.6Nb₂O₆ now we see one of them, that is (0,1/2,0), was annihilated, and only (1/2,±1/2,0) remain. This happens typically because of the cobalt and nickel spin correlation that makes the pattern peak reflection, relative to this propagation vector, fade. Accepting the importance of the ANb₂O₆ system into account, mainly due to its complex magnetic behavior, this result can further feed the experimental research of quantum criticality in Ising chains. The main results of the low-temperature neutron diffraction refinement are reported in Table III.

This magnetic structure changes most probably originate from Co-Ni combination, which modifies the magnetics interactions, since no crystal structure change has been observed, at this temperature. No significant lattice parameter deviation has been found from X-ray diffraction at room temperature data. This demands more exploration to better determine the exchange interactions involving Co and Ni cation substitution, to achieve a deeper understanding of the magnetic properties in this compound.

We have investigated the magnetic properties of Co_0.4Ni_0.6Nb₂O₆ with the focus on the low-temperature ordered phases. The compounds show that substitution of Co for Ni in NiNb₂O₆ preserves the orthorhombic crystal structure. Our susceptibility and specific heat results reveal the presence of magnetic transitions at 3.8 K. Examination of the magnetic properties shows that all the compounds display small Néel and Weiss temperatures, persistent with the quasi-one-dimensional nature of their magnetic subsystem. The neutron diffraction at 1.8 K has a remarkable result. Despite the fact the two extreme compounds CoNb₂O₆ as well NiNb₂O₆ show, at some point, the same magnetic structure containing two propagation vectors (0,1/2,0) and (1/2,±1/2,0), chemical disorder promotes a change in the long-range magnetic order in Co_0.4Ni_0.6Nb₂O₆, resulting in a unique (1/2,±1/2,0) propagation vector. The disappearance of this propagation vector for this transitional compound may occur from cation disorder, and consequently, it amends the competing interactions that tend to influence the ordered state. The magnetic rich variety behavior of Co_xNi_1-xNb₂O₆ compounds justifies further inspection, preferably with priority on the relative aspect of inter-and-intrachain couplings.

This work was supported in part by the Brazilian-France agreement CAPES-COFECUB cooperation program (N^o 600/08), by the Brazilian agency CNPq and UTFPR scientific dissemination program.