Canted ferromagnetic order in a distorted triangular-lattice magnet Na$_2$SrCo(VO$_4$)$_2$

TL;DR

This work investigates Na2SrCo(VO4)2, a glaserite-type triangular-lattice cobalt vanadate, to understand how lattice distortion tunes magnetic exchange. Through synthesis, X-ray and neutron diffraction, magnetization, and heat capacity measurements, the authors establish a monoclinic $P2_1/c$ structure with distorted Co triangles and identify a ferromagnetic transition at $T_C \approx 3.4~\mathrm{K}$. Neutron diffraction reveals a long-range canted ferromagnetic order with Co moments in the $ac$ plane and magnitudes near $2.6~\mu_B$ per Co. The results emphasize the decisive role of nonmagnetic tetrahedra, specifically the VO4 units, in mediating exchange and demonstrate how symmetry lowering controls magnetic ground states in glaserite cobaltates.

Abstract

Triangular-lattice cobaltates with glaserite-type $X_2Y$Co($T$O$_4)_2$ structure provide an ideal platform to investigate intriguing quantum magnetism. Here we report a comprehensive study of the structural and magnetic properties of a triangular-lattice cobalt vanadate $\rm Na_2SrCo(VO_4)_2$. Room-temperature x-ray and neutron powder diffraction confirm that $\rm Na_2SrCo(VO_4)_2$ crystallizes in the monoclinic $P2_1/c$ space group with slightly distorted triangular layers of $\rm Co^{2+}$ ions. Magnetization measurements reveal a ferromagnetic transition at $T\rm_C \approx 3.4~{\rm K}$, where a sharp $λ$-type anomaly is observed in the specific heat. The magnetic entropy recovered up to 55 K approaches 90$\%$ of $R{\rm ln}2$, supporting an effective spin-1/2 state of Co$^{2+}$ ions at low temperature. Neutron diffraction at 2.3 K (below $T_{\rm C}$) further confirms a long-range canted ferromagnetic order with the Co$^{2+}$ moments aligned in the $ac$ plane and the ordered moment size of $\sim$ 2.6 $μ\rm_{B}$. Comparing with its sister compounds with a trigonal symmetry, $\rm Na_2BaCo(VO_4)_2$ with a collinear ferromagnetic structure and the recently discovered spin supersolid candidate $\rm Na_2BaCo(PO_4)_2$ with a distinct Y-like antiferromagnetic ground state, this study indicates the decisive role of the $T{\rm O_4}$ tetrahedra in tuning exchange interactions and contrasting magnetic behaviors of these glaserite-structure compounds.

Figures (6)

• Figure 1: Crystal structure of NSCVO (a) with a monoclinic symmetry (space group $P \rm 2_1/c$), compared with that of NBCVO (c) with a trigonal symmetry (space group $P\bar{3}m1$) SANJEEWA201661. In NSCVO, $\rm Co^{2+}$ ions in the $bc$ plane form an isosceles triangular lattice with a top angle of 59.79° at room temperature. The $\rm O_3$ faces of the Co1O$_6$ octahedra defined by O1, O2 and O6 are tilted away from the $bc$ plane and also exhibit a finite in-plane rotation (b). In NBCVO, the $\rm Co^{2+}$ ions in the $ab$ plane form an equilateral triangular lattice, and $\rm O_3$ faces of the $\rm CoO_6$ octahedra, defined by three equivalent O1 atoms, are strictly parallel to the $ab$ plane (d).
• Figure 2: Room-temperature XRD (a) and NPD (b) patterns of polycrystalline NSCVO and the Rietveld refinements with the $R$-factors and $\chi^2$ provided. The red circles represent the observed intensities, and the black solid lines are the calculated patterns. The differences between the observed and calculated intensities are shown as blue solid lines at the bottom. The Bragg peaks of NSCVO are marked by green bars.
• Figure 3: (a) DC magnetic susceptibility ($\chi$) of polycrystalline NSCVO, measured in a magnetic field of 0.1 T. The inset shows the low-temperature part of $\chi$ below 10 K and its first derivative (d$\chi$/d$T$) with a dip around 3.5 K. (b) The inverse magnetic susceptibility (1/$\chi$) and Curie-Weiss fittings to the high-temperature (200-300 K) and low-temperature (5-10 K) ranges, as represented by the red and solid lines, respectively. The inset shows an enlarged view of the low-temperature region. (c) Isothermal magnetization of NSCVO measured at 1.8 K, with the low-field hysteresis loop highlighted in the inset. The solid line represents a linear extrapolation to subtract the Van Vleck paramagnetism.
• Figure 4: (a) Specific heat ($C_{\rm p}$, red circles) of polycrystalline NSCVO. The black dashed line is an approximation to the phonon specific heat ($C\rm_{ph}$) using a two-component Debye model, and the blue solid line corresponds to the magnetic specific heat ($C\rm_{mag}$) after substracting the phonon contribution. (b) $C_{\rm mag}/T$ (green circles) and the estimated magnetic entropy change ($\Delta S_{\rm mag}$, purple triangles) as functions of temperature, respectively. The horizontal dashed line marks $R{\rm ln}2$ as expected for a spin-1/2 system. The vertical dashed line marks 10 K, where most of the entropy is already recovered and $\Delta S_{\rm mag}$ reaches $0.84R{\rm ln2}$.
• Figure 5: Low-temperature NPD patterns collected at 10 K (a) and 2.3 K (b) as well as the Rietveld refinements. The nuclear and magnetic reflections are marked by green and brown vertical bars, respectively. After subtracting the nuclear scattering (10 K) from the 2.3 K data set, the net magnetic scattering (2.3 K$-$10 K) is obtained and fitted using different magnetic structures described by $\Gamma_1$, with the $a$-axis component only (top) and both $a$-axis and $c$-axis components (bottom), respectively. Clearly, the latter well accounts for the intensity of (1 0 0) magnetic reflection.