Magnetic structure and interactions in the quasi-one-dimensional antiferromagnet ${\text{CaV}}_2{\text{O}}_4$

${\text{CaV}}_2{\text{O}}_4$ is a spin-1 antiferromagnet, where the magnetic vanadium ions have an orbital degree of freedom and are arranged on quasi-one-dimensional zigzag chains. The first- and second-neighbor vanadium separations are approximately equal suggesting frustrated antiferromagnetic exchange interactions. High-temperature susceptibility and single-crystal neutron-diffraction measurements are used to deduce the dominant exchange paths and orbital configurations. The results suggest that at high temperatures ${\text{CaV}}_2{\text{O}}_4$ behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder. These two magnetic structures are explained by different orbital configurations and show how orbital ordering can drive a system from one exotic spin Hamiltonian to another.

Figure 1

(Color online) The double chain structure of ${\text{CaV}}_2{\text{O}}_4$ projected onto the (a) $b\text{−}c$ and (b) $a\text{−}b$ planes. The two inequivalent ${\text{VO}}_6$ octahedra are highlighted in pink (chain 1) and blue (chain 2). Arrows indicate the direction of the ordered moments and the intrachain exchange constants for the monoclinic phase are labeled. In the orthorhombic phase $J_{\text{zz}}^{n^{\prime }}=J_{\text{zz}}^{n^{″}}=J_{\text{zz}}^n$, $J_{\text{zz}}^1\approx J_{\text{zz}}^2$, and $J_{\text{leg}}^1\approx J_{\text{leg}}^2$.

Figure 2

(Color online) Single-crystal dc magnetic susceptibility of ${\text{CaV}}_2{\text{O}}_4$ for all three crystallographic directions. The inset shows the powder averaged data fitted to Curie-Weiss and exact diagonalization models.

Figure 3

(Color online) Magnetic intensity as a function of sample angle $\omega$ at the orthorhombic $\left(0,\frac{1}{2},\frac{\overline{1}}{2}\right)$ position for $T=6\text{K}$. The temperature dependence of the integrated magnetic Bragg intensity of twin 1 $\left(0,\frac{1}{2},\frac{\overline{1}}{2}\right)$ is shown in the inset.

Figure 4

(Color online) The orientation and occupancy of the vanadium $t_{2g}$ orbitals for an example chain in the (a) high-temperature $\left(Pnam\right)$ phase and (b) low-temperature $\left(P{2}_1/n11\right)$ phase. (Lower right) $t_{2g}$ energy-level diagram for both phases.