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

•Antiferromagnetic superexchange interaction through Gd−O−Fe−O−Gd pathways.•Fe sublattice exhibits a weak ferromagnetic transition at TC ≈ 5 K.•Moderately frustrated.•Cooperative two-sublattice model describes magnetic and heat capacity of Gd2FeSbO7. 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.