Magnetothermodynamic properties of frustrated pyrochlore Gd2FeSbO7 in cooperative two-sublattice molecular-field model

Abstract

The pyrochlore, Gd2FeSbO7, having general formula unit A2B2O7, was chosen for magnetic and heat capacity study in this work due to presence of Fe3+ moments, whose 3d orbitals (3d5:3t2g3eg2) are half-filled and carry a large magnetic moment, at B site of the structure, and hence create additional Gd−O−Fe−O−Gd pathways, which result in strongly antiferromagnetic superexchange interactions among the Gd−Gd spins as well as for the Gd−Fe bonds. Gd2FeSbO7 behaves paramagnetic down to ~10 K with effective antiferromagnetic nearest-neighbour interactions resulting negative Curie-Weiss temperature, θCW = −12.62 K. Fe sublattice exhibits a weak ferromagnetic transition at TC ≈ 5 K. The frustration index f = |θCW|/TC becomes 2.5 indicative of moderate frustration, which may prevent Palmer-Chalker ground state configuration of Gd sublattice at lower temperature in the system. Heat capacity Cp exhibits a broad anomaly at ~3.1 K and calculated magnetic heat capacity Cm/T attains a peak at ~1 K, but is much weaker than for other pyrochlores Gd2B2O7 (B = Sn, Ti, Ge, Pt) which are all found to order at 1 K or lower, thereby reflecting a short-range second-order transition in Gd2FeSbO7. The nature of transition was also verified by Arrott plot of magnetization. Finally the magneto-thermodynamic data were analyzed on the basis of a cooperative two-sublattice model taking account of intra-sublattice (Gd−Gd and Fe−Fe) and inter-sublattice (Gd−Fe) magnetic interactions, defined by three molecular-field parameters, λGd−Gd = −0.71 T/μB, λFe−Fe = 0.87 T/μB, and λGd−Fe = −0.61 T/μB in presence of the easy-planer crystal-field anisotropy at Gd-site. Without Gd−Fe interaction, two independent interpenetrating Gd3+ and Fe3+Sb5+ sublattices can not precisely demonstrate the measured thermomagnetic properties of Gd2FeSbO7. This study thus revealed that 3d−4f magnetic interactions offer Gd2FeSbO7 a unique place in the series of gadolinium-based pyrochlores.