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: Recent studies show that the non-Fermi liquid (NFL) behavior of MnSi and Fe spans over an unexpectedly broad pressure range, between the critical pressure p and around 2p. In order to determine the extension of their NFL regions, we analyze the evolution of the resistivity ρ(T)˜A(p)T at higher pressures. We find that in MnSi the n=32 exponent holds below 4.8GPa≈3p, but it increases above that pressure. At 7.2 GPa we observe the low-temperature Fermi liquid exponent n=2 whereas for T>1.5K, n=53. Our measurements in Fe show that the NFL behavior ρ˜T extends at least up to 30.5 GPa, above the entire superconducting (SC) region. In the studied pressure range, the onset of the SC transition reduces by a factor 10 down to Tconset(30.5GPa)=0.23K, while the A—coefficient diminishes monotonically by around 50%.

Abstract: The La0.75Sr0.25MnO3 (LSMO)/Ba0.7Sr0.3TiO3 (BSTO) superlattices and bilayers, where LSMO is ferromagnetic and BSTO is ferroelectric, were grown by dc sputtering. X-ray diffraction indicates that the samples present a textured growth with the c axis perpendicular to the substrate. Magnetization measurements show a decrease of the sample’s magnetization for decreasing ferromagnetic thickness. This effect could be related to the presence of biaxial strain and a magnetic dead layer in the samples. Conductive atomic force microscopy indicates that the samples present a total covering of the ferromagnetic layer for a ferroelectric thickness higher than four unit cells. Transport tunneling of the carriers seems to be the preferred conduction mechanism through the ferroelectric layer. These are promising results for the development of multiferroic tunnel junctions.