Properties of manganite/ruthenate superlattices with ultrathin layers

The properties of manganite/ruthenate superlattices are reviewed with a specific focus on the manganite/ruthenate interface. La_0.7Sr_0.3MnO₃/SrRuO₃ and Pr_0.7Ca_0.3MnO₃/SrRuO₃ superlattices grow with a high crystalline perfection as illustrated in the figure to the right: at the interface the individual cation species can be clearly identified, interdiffusion is marginal. The superlattices show magnetization processes with an intricate interplay between magnetocrystalline anisotropy, size of the layer magnetization, spin confinement and interfacial antiferromagnetic interlayer coupling. There is further an unprecedented Curie temperature stabilization at room temperature values of the La_0.7Sr_0.3MnO₃ layers in the superlattices down to layer thicknesses of one unit cell. The magnetotransport properties, especially the Hall effect, indicate the existence of a quasi‐two‐dimensional hole gas at the La_0.7Sr_0.3MnO₃/SrRuO₃ interface; this is further supported by an analysis of cation displacements as determined from scanning transmission electron microscopy. The manganite/ruthenate interface might be considered as a model system for the study of interfacial reconstruction and charge transfer in a highly correlated ferromagnetic system.

     

Growth of single crystals of the new double perovskite Ba2HoRu1−xCuxO6 from high temperature solutions

Single crystals of Ba₂HoRu_1−xCu_xO₆ have been grown from high temperature solutions using PbO–PbF₂ as solvent in the temperature range 1150–1250 °C. Crystals with a six sided plate like morphology measuring up to 3 mm across and 0.5 mm thick and polyhedral habit measuring up to 2 and 1 mm in thick mass were obtained. Powder X-ray diffraction patterns obtained on the crystals were indexed to give a monoclinic space group P2₁/n with lattice parameters a=5.875(2), b=5.874(3), c=8.960(1) and β=89.995(2)°. The crystals with x=0 show a single anomaly at 6.5 K corresponding to an antiferromagnetic phase with . The crystals containing Cu show additional anomalies at 18 and 48 K. The SEM and EDS analysis reveals a 2116 phase.     

Growth of big single crystals of Sr2YRu1‐xCuxO6 from high temperature solutions

Single crystals of Sr₂YRu_1‐xCu_xO₆ (x = 0 ‐ 0.4) have been grown from PbO‐PbF₂ based solutions in the temperature range 1150 – 1350°C. A silicon carbide heating element furnace (with a recrystallized alumina tube lining) in a vertical configuration was used to grow the crystals in platinum crucibles. Conditions for the stable growth of big crystals have been investigated. The morphology of the crystals containing Cu was found to change from octahedral to cube octahedral as the growth temperature is increased from 1150 to 1350°C. Crystals measuring up to 4.5 mm across and 2.5 mm thick have been grown from 1250°C. The incorporation of Cu into the crystals was ascertained by EDS and x‐ray diffraction analysis. A diamagnetic transition which increased in magnitude and temperature with x was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and magnetic properties of pyrochlores Gd2−xMxRu2O7 (M=Ho,Y)

In this paper we investigated the structural and magnetic properties of the Gd_2−xM_xRu₂O₇ (M=Ho and Y, x  =0.0, 0.1, 0.2, 1.0 and 2.0) pyrochlores. The samples were prepared by solid state reaction method, characterized structurally by X-ray diffraction with Rietveld refinement and magnetically by measuring the temperature dependence of the magnetization. X-ray diffractograms of all samples studied presented a cubic pyrochlore type crystal structure with lattice parameters varying linearly in accordance with Vegards law. It is shown that by substituting Gd by Ho one can tune the magnetic order of this system moving from antiferromagnetic (in the absence of Ho) to ferromagnetic (in the absence of Gd). On the other hand by replacing Gd for Y the antiferromagnetic order is decreased.©$\copyright$ 2010 Elsevier Science. All rights reserved     

Thermal properties of the magnetic RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Subscript> superconductor

Summary A thermal study was performed on the RuSr₂GdCu₂O₈ (Ru-1212) magnetic superconductor phase to investigate the effect of the annealing treatments in flowing O₂ on the stability limit of the phase and on its structural, magnetic and transport properties. DTA-TG measurements were utilized to determine the decomposition process and the dependence of the decomposition temperature on the annealing atmosphere. The decomposition of the Ru-1212 phase was found sensitive to the oxygen partial pressure and increases with P_O2 and the annealing time. The annealing treatments exert a depressing effect on the strength of the magnetic interaction, an enhancement on the superconductive properties and the vanishing of the magnetostriction. A decomposition reaction of Ru-1212 phase was proposed and discussed.     

Thermodynamic properties of the ternary oxides in the Eu–Ru–O system by using solid-state electrochemical cells

The Gibbs free energies of formation of Eu₃RuO₇(s) and Eu₂Ru₂O₇(s) have been determined using solid-state electrochemical technique employing oxide ion conducting electrolyte. The reversible electromotive force (e.m.f.) of the following solid-state electrochemical cells have been measured:

Thermodynamic properties of BaRuO3

We have investigated the specific heat, thermal expansion and thermodynamic properties of BaRuO₃ in rhombohedral, hexagonal and cubic phase using the modified rigid ion model (MRIM) by incorporating the effect of lattice distortions. The calculated Debye temperature, specific heat and other thermal properties reproduce well with the available experimental data, implying that MRIM represents properly the nature of the perovskite-type BaRuO₃. Besides, we have reported the thermal expansion (α), bulk modulus (B), cohesive energy (?), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ _D), and Gruneisen parameter (γ).     

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca₃(Ru_1-xM_x)₂O₇ (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T², in the localized region. The C ∝ T² term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.     

Structural Investigations of ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd)—A Comparison between XRD-Rietveld and EXAFS Results

The structures of five perovskite-related oxides with the general composition ACu₃Ru₄O₁₂ with A=Na, Ca, Sr, La and Nd, have been examined both by XRD-Rietveld refinements and Ru-K EXAFS-spectroscopy. In addition, the behavior on reduction was investigated by thermogravimetry (TG). The TG measurements revealed that the composition was almost exactly A₁Cu₃Ru₄O₁₂ for all samples. The inter-atomic distances derived from EXAFS- and XRD-Rietveld fits show an excellent agreement with differences smaller than 8 mÅ even for R>5 Å. All inter-atomic distances increase in the order Na<Ca<Sr<Nd<La and were found to depend linearly on the product of charge and ionic radius of the A-cation. The experimentally found distances are compared with the corresponding values expected from bond-valence calculations.