Reported here are measurements and analysis of the magnetization ($M$) versus temperature (1.9–400 K) in magnetic fields $H$ up to 90 kOe for a polycrystalline sample of Ising chain ferromagnet $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ with $T_C=2.9\mathrm{K}$. For $T>T_C$, the fit of magnetic susceptibility $\chi =M/H$ ($H=300\mathrm{Oe}$) to $\chi ={\chi }_0+C/\left(T-\mathrm{\Theta }\right)$ with ${\chi }_0=0.0009\mathrm{emu}\mathrm{mo}{\mathrm{l}}^{–1}{\mathrm{Oe}}^{–1}$ determined from high-$T$ data yields $\mathrm{\Theta }=11\mathrm{K}$ and magnetic moment $\mu =4.994{\mu }_B$ per ${\mathrm{Co}}^{2+}$ ion calculated from experimental $C=3.12\mathrm{emu}\mathrm{K}\mathrm{mo}{\mathrm{l}}^{–1}{\mathrm{Oe}}^{–1}$. Values of $g$ obtained from ${\mu }^2/{{\mu }_B}^2=g^{2}S\left(S+1\right)$ for spin $S=1/2$ and 3/2 are used to determine ${\mu }_Z=gS{\mu }_B$ and compared with ${\mu }_Z$ obtained from saturation magnetization and neutron diffraction for $T\ll T_C$. This analysis of the data for both above and below $T_C$ shows that the ground state of ${\mathrm{Co}}^{2+}$ in $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ has the effective spin $S=1/2$ and not $S=3/2$ expected from Hund's rules, the $S=1/2$ ground state resulting from the combined effects of a noncubic crystalline field and spin-orbit coupling. The fit of the data for $T>T_C$ to $\chi ={\chi }_0+\left(C/T\right)\exp \left(J_0/2k_{B}T\right)$ valid for an Ising chain with $S=1/2$ yields the intrachain ferromagnetic exchange constant $J_0/k_B=6.2\mathrm{K}$, whereas the $g$ value with $S=1/2$ and the experimental critical fields for spin flips yields interchain antiferromagnetic exchange constants $J_1/k_B=-0.42\mathrm{K}$ and $J_2/k_B=-0.67\mathrm{K}$.

• Received 20 November 2020
• Accepted 25 January 2021

DOI:https://doi.org/10.1103/PhysRevB.103.064415