Magnetic ground state and exchange interactions in the Ising chain ferromagnet CoNb2O6

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 CoNb2O6 with T-C = 2.9 K. For T > T-C, the fit of magnetic susceptibility chi = M/H (H = 300 Oe) to chi = chi(0) + C/(T - Theta) with chi(0) = 0.0009 emu mol(-1) Oe(-1) determined from high-T data yields Theta = 11 K and magnetic moment mu = 4.994 mu(B) per Co2+ ion calculated from experimental C = 3.12 emu K mol(-1 )Oe(-1). Values of g obtained from mu.(2)/mu(2)(B) = g(2)S(S + 1) for spin S = 1/2 and 3/2 are used to determine mu(Z) = g S mu(B) and compared with mu(Z) obtained from saturation magnetization and neutron diffraction for T << T-C. This analysis of the data for both above and below T-C shows that the ground state of Co2+ in CoNb2O6 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) (C/T) exp(J(0)/2k(B)T) valid for an Ising chain with S = 1/2 yields the intrachain ferromagnetic exchange constant J(0)/k(B) = 6.2 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 K and J(2)/k(B) = - 0.67 K.

Automated summary

Measurements and analysis of the magnetization versus temperature in a polycrystalline sample of Ising chain ferromagnet CoNb2O6 reveal a ground state with effective spin S = 1/2, contrary to the expected S = 3/2 from Hund's rules. The fit of the data indicates the presence of noncubic crystalline field and spin-orbit coupling effects.