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

Abstract

Triangular-lattice cobaltates with glaserite-type $X_{2}Y$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 $\mathrm{N}{\mathrm{a}}_2\mathrm{SrCo}(\mathrm{V}{\mathrm{O}}_4{)}_2$. Room-temperature x-ray and neutron powder diffraction confirm that $\mathrm{N}{\mathrm{a}}_2\mathrm{SrCo}(\mathrm{V}{\mathrm{O}}_4{)}_2$ crystallizes in the monoclinic $P{2}_1/c$ space group with slightly distorted triangular layers of $\mathrm{C}{\mathrm{o}}^{2+}$ ions. Magnetization measurements reveal a ferromagnetic transition at $T{}_{\mathrm{C}}\approx 3.4\mathrm{K}$, where a sharp $\hat{I}»$-type anomaly is observed in the specific heat. The magnetic entropy recovered up to 55 K approaches 90$\%$ of $R\ln 2$, supporting an effective spin-1/2 state of Co${}^{2+}$ ions at low temperature. Neutron diffraction at 2.3 K (below $T_{\mathrm{C}}$) 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 $\hat{I}¼{}_{\mathrm{B}}$. Comparing with its sister compounds with a trigonal symmetry, $\mathrm{N}{\mathrm{a}}_2\mathrm{BaCo}(\mathrm{V}{\mathrm{O}}_4{)}_2$ with a collinear ferromagnetic structure and the recently discovered spin supersolid candidate $\mathrm{N}{\mathrm{a}}_2\mathrm{BaCo}(\mathrm{P}{\mathrm{O}}_4{)}_2$ with a distinct Y-like antiferromagnetic ground state, this study indicates the decisive role of the $T{\mathrm{O}}_4$ tetrahedra in tuning exchange interactions and contrasting magnetic behaviors of these glaserite-structure compounds.

10 pages, 6 figures