Spin ordering and physical properties of NaPrFeWO6 and NaSmFeWO6 with polar double perovskite structure

Abstract NaPrFeWO 6 and NaSmFeWO 6 have been synthesized as polycrystalline powders by means of a sol-gel route. Both samples adopt a noncentrosymmetric structure derived from the simple perovskite ABO 3 and characterized by a layered ordering of A-site atoms (Na and rare earth) and a rock-salt ordering of B-site atoms (Fe and W) as deduced from x-ray and neutron powder diffraction techniques. Magnetization measurements revealed antiferromagnetic ordering below T N ∼23 K in both compounds but weak spontaneous magnetization is observed in the hysteresis loops of the samples above and below T N revealing a parasitic ferromagnetic component that comes from minor ferromagnetic impurities. The magnetic structure of NaPrFeWO 6 is formed by two antiferromagnetic sublattices (rare earth and Fe) that follow the same propagation vector k = (½, 0, ½). The magnetic structure is purely collinear with the moments oriented along the b -axis. It presents competitive magnetic interactions and reduced ordered moments at low temperatures. The rare earth moments in the zig-zag chains running along a (parallel to the ab plane) are ordered following a sequence typical of an E-type arrangement (up-up-down-down). Each Fe 2+ atom has four nearest Pr 3+ neighbors and it is coupled antiferromagnetically to three of them and ferromagnetically to the fourth one. Strong neutron absorption from Sm atoms hinders obtaining reliable neutron patterns of NaSmFeWO 6 for Rietveld analysis but the comparison of neutron patterns above and below T N detected the same magnetic peaks indicating a similar magnetic structure. Dielectric measurements are compatible with a paraelectric system with a strong Maxwell-Wagner contribution from depletion layers and electrical properties are dominated by leakage currents.