Antiferromagnetic order of ferromagnetically coupled dimers in the double pyrovanadate CaCoV${}_2$O${}_7$

Abstract

Magnetic properties of the pyrovanadate CaCoV${}_2$O${}_7$ have been studied by means of the bulk magnetization and neutron powder diffraction measurements. Magnetic susceptibility in the paramagnetic phase shows Curie-Weiss behavior with negative Weiss temperature $\hat{I}˜\simeq -22.5$ K, indicating dominant antiferromagnetic interactions. At $T_{\mathrm{N}}=3.44$ K, CaCoV${}_2$O${}_7$ shows antiferromagnetic order, accompanied by a weak net ferromagnetic moment of $\sim 0.05$~$\hat{I}{¼}_{\mathrm{B}}/{\mathrm{Co}}^{2+}$. Neutron powder diffraction confirms the formation of antiferromagnetic order below $T_{\mathrm{N}}$. It was further confirmed from the magnetic structure determination that the two Co${}^{2+}$ ions in the adjacent edge-sharing octahedra have almost parallel (ferromagnetic) spin arrangement, indicative of a formation of a ferromagnetic spin dimer. The antiferromagnetic order is, in turn, stabilized by sizable inter-dimer antiferromagnetic interactions.