Role of orbital degrees of freedom in investigating the magnetic properties of geometrically frustrated vanadium spinels

Abstract: The inconsistency about the degree of geometrical frustration has been a long issue in AV${2}$O${4}$ (A $\equiv$ Zn, Cd and Mg) compounds, which arises from the two experimental results: (i) frustration indices and (ii) magnetic moments. In the present study, we try to understand such inconsistency by using {\it ab initio} electronic structure calculations. The orbital degrees of freedom are found to play an important role in understanding the geometrically frustrated magnetic behaviour of these compounds. The inclusion of the orbital and spin angular momenta for calculating the frustration indices improves the understanding about the degree of geometrical frustration in these compounds. The calculated values of the frustration indices ($f$${\it J}$) are largest for MgV${2}$O${4}$ and smallest for CdV${2}$O${4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the calculated values of $\Delta$M${2}$=M${\rm total}$-M${\rm exp}$ are largest for MgV${2}$O${4}$ and smallest for CdV${2}$O${4}$. Hence, the consistency about the degree of geometrical frustration is achieved. The absolute values of the nearest neighbour exchange coupling constant ({\it J${nn}$}) between V spins are found to be largest for MgV${2}$O${4}$ and smallest for CdV${2}$O${4}$, which indicate that the calculated absolute values of the Curie-Weiss temperature ($\varTheta$${CW}$)${\it J}$ are highest for MgV${2}$O${4}$ and smallest for CdV${2}$O${4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the magnetic transition temperature ($T$${N}$)${\it J}$ is found to be $\sim$150 K, $\sim$60 K and $\sim$22 K for MgV${2}$O${4}$, ZnV${2}$O${4}$ and CdV${2}$O${4}$, respectively, which shows that the order of ($T$${N}$)${\it J}$ is similar to that of ($T$${N}$)$_{\rm exp}$ for these compounds.