Abstract: We report on the magnetic and structural properties of La2Ni(Ni1/3Sb2/3)O6 in polycrystal, single crystal and thin film samples. We found that this material is a ferrimagnet ( T c ≈ 100 K ) which possesses a very distinctive and uncommon feature in its virgin curve of the hysteresis loops. We observe that bellow 20 K it lies outside the hysteresis cycle, and this feature was found to be an indication of a microscopically irreversible process possibly involving the interplay of competing antiferromagnetic interactions that hinder the initial movement of domain walls. This initial magnetic state is overcome by applying a temperature dependent characteristic field. Above this field, an isothermal magnetic demagnetization of the samples yield a ground state different from the initial thermally demagnetized one.

Abstract: New double perovskites LaPbMSbO₆, where M²⁺ = Mn²⁺, Co²⁺, and Ni²⁺, were synthesized as polycrystals by an aqueous synthetic route at temperatures below 1000 ^°C. All samples are monoclinic, space group P2₁/n, as it is observed from Rietveld analysis of X-ray powder diffraction patterns. The distribution of M²⁺ and Sb⁵⁺ among the two octahedral sites have 3% of disorder for M²⁺ = Ni²⁺, whereas for M²⁺ = Mn²⁺ and Co²⁺ less disorder is found. The three samples have an antiferromagnetic transition, due to the antiferromagnetic coupling between M^{2 +} through super-superexchange paths M²⁺-O^2--Sb⁵⁺-O^2--M²⁺. Transition temperatures are low: 8, 10 and 17 K for Mn²⁺, Co²⁺, and Ni^{2 +} respectively, as a consequence of the relatively long distances between the magnetic ions M²⁺. Besides, for LaPbMnSbO₆ a small transition at 45 K was found, with ferrimagnetic characteristics, possibly as a consequence of a small disorder between Mn²⁺ and Sb⁵⁺. This disorder would give additional and shorter interaction paths: superexchange Mn²⁺-O^2--Mn²⁺.

Abstract: The synthesis, structural characterization, and magnetic properties of La3Co2SbO9 double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P21/n. Co2+ and Sb5+ have the maximum order allowed for the La3Co2SbO9 stoichiometry. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a=5.6274(2) Å, b=5.6842(2) Å, c=7.9748(2) Å and β=89.999(3)°. Magnetization measurements indicate the presence of ferromagnetic correlations with TC=55 K attributed to the exchange interactions for non-linear Co2+–O–Sb5+–O–Co2+ paths. The effective magnetic moment obtained experimentally is μexp=4.38 μB (per mol Co2+), between the theoretical one for spin only (3.87 μB) and spin-orbit value (6.63 μB), indicating partially unquenched contribution. The low magnetization value at high magnetic field and low temperature (1 μB/f.u., 5 T and 5 K) and the difference between ZFC and FC magnetization curves (at 5 kOe) indicate that the ferromagnetism do not reach a long range order and that the material has an important magnetic frustration.