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 $\Theta \simeq -22.5$ K, indicating dominant antiferromagnetic interactions. At $T_{\rm N} = 3.44$ K, CaCoV$_2$O$_7$ shows antiferromagnetic order, accompanied by a weak net ferromagnetic moment of $\sim 0.05$~$\mu_{\rm B}/{\rm Co}^{2+}$. Neutron powder diffraction confirms the formation of antiferromagnetic order below $T_{\rm N}$. 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.