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

NaPrFeWO6 and NaSmFeWO6 have been synthesized as polycrystalline powders by means of a sol-gel route. Both samples adopt a noncentrosymmetric structure derived from the simple perovskite ABO3 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 TN ≈23 K in both compounds but weak spontaneous magnetization is observed in the hysteresis loops of the samples above and below TN revealing a parasitic ferromagnetic component that comes from minor ferromagnetic impurities. The magnetic structure of NaPrFeWO6 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 Fe2+ atom has four nearest Pr3+ 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 NaSmFeWO6 for Rietveld analysis but the comparison of neutron patterns above and below TN 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.