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Abstract: Magnetic hysteresis displacement, thermal inversion of the magnetization, hysteresis loops jumps and crossing branches of hysteresis loops at low magnetic fields are reviewed. Most of these phenomena have been observed in magnetic oxide systems, particularly in perovskite-type manganites and chromites. The paper takes into account structural considerations and different geometrical parameters, such as volume or thin layers. [Copyright &y& Elsevier]

Abstract: The magnetic phase transition and the magnetic entropy change (−ΔSM ) in the La0.67Ba0.33Mn0.9Cr0,1O3 manganite were investigated by measuring the magnetization as a function of temperature. The maximum magnetic entropy change (−ΔSM ) and the relative cooling power (RCP) are found to be, respectively, 4.20Jkg−1 K−1 and 238Jkg−1 for a 5-T field change, making of this material a promising candidate for magnetic refrigeration near room temperature. To investigate the nature of the paramagnetic (PM) to ferromagnetic (FM) phase transition, found to be of second-order, we performed a critical exponents study by dc-magnetization M(H,T) measurements around the Curie temperature T C, in a temperature range including the critical region |ɛ|=|T − T C|/T C ⩽0.05. From the derived values of the critical exponents (β =0.380, γ =1.345), we conclude that La0.67Ba0.33Mn0.9Cr0.1O3 belongs to the three-dimensional Heisenberg class with short-range interaction. Scaling relations are obeyed, indicating renormalized interactions around T C. By investigating the field dependence of RCP and ΔSM , it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott method. [Copyright &y& Elsevier]

Abstract: A systematic investigation of the La0,67Ba0,33Mn0,9Cr0,1O3 perovskite has been undertaken, mainly to understand the impact of the sintering temperature on the structural, microstructural, magnetic and electrical transport properties in these materials. The La0,67Ba0,33Mn0,9Cr0,1O3 manganite was sintered by a solid-state route at 1000°C, 1200°C and 1400°C. The X-ray diffraction studies show that all samples crystallize with the rombohedral symmetry within the space group RC, regardless of the sintering temperatures employed in this work. The MnOMn bond angle decreases and the MnO bond length increases with the increase of the grain size. All samples undergo a paramagnetic (PM)–ferromagnetic (FM) phase transition at T=T C. Both the magnetization and the Curie temperature T C decrease with increasing grain size mainly due to the increase of the MnO bond length d MnO. The electrical resistivity (ρ) increases with decreasing grain size because of the enhancement of the grain boundary effect. Low temperature resistivity data below T C was fitted by the relation ρ=ρ 0+ρ 2 T 2+ρ 4.5 T 4.5, indicating the importance of the grain/domain boundary, the electron–electron scattering effects and, to a lesser extent, the electron–(magnon,phonon) scattering effects in the mechanism of conduction. On the other hand, the high temperature resistivity above the metal–insulator (M–I) transition T p for the sample sintered at 1000°C was explained using variable range hopping (VRH) and small polaron hopping (SPH) models. IR studies revealed that the vibration mode at 411cm−1, associated with the internal bending of MnO6 octahedra, becomes softer, indicating an increase of the distortion. [Copyright &y& Elsevier]

Abstract: We have investigated the structural, magnetic and electrical transport properties of a series of ABO3-type perovskite compounds, La0.67Ba0.33Mn1−x Cr x O 3 (0≤ x ≤0.15), which strongly depend on the doping level x. The slight difference between the ionic radii of Cr3+ and Mn3+ causes no change in the structure when x ≤0.1, remaining rhombohedral (space group R-3C), while for x =0.15 the structure becomes cubic (space group Pm-3m). Energy dispersive X-ray analysis (EDAX) confirms the expected stoichiometry of all samples. Upon Cr doping on the Mn site, the lattice parameters, the unit cell volume and the Bh name="sbnd" />B bond angle are reduced. All samples present a single magnetic transition from ferromagnetic to paramagnetic phase, showing a decrease of the Curie temperature T c and the magnetization M when x increases (x ≤0.15). However, Cr doping makes the saturation magnetization at 5K to decrease, which indicates that the Cr3+ moments tend to be antiparallel to the Mn3+ moments at low temperature. The Cr-doped manganites exhibit a large variation in resistivity values. The increase of Cr doping (x ≤0.15) leads to an increase of the electrical resistivity. Below 10at.% of Cr3+, the electrical resistivity shows a metallic behavior, which is well fitted by the relation ρ = ρ 0 + ρ 2T2 + ρ 4.5T4.5, indicating the importance of the grain/domain boundary, the electron–electron scattering effects and, to a lesser extent, the electron–(magnon, phonon) scattering effects in the mechanism of conduction. On the other hand, the 15at.% of Cr3+ doping makes the material to exhibit a semiconductor behavior, for which the electronic transport can be explained by a variable range hopping (VRH) and small polaron hopping (SPH) models. Results are consistent with a reduction of the number of available hopping sites for the Mn e g (↑) electron due to the substitution of Mn3+ by Cr3+, which suppresses the double exchange (DE) interactions. [Copyright &y& Elsevier]

In the last ten years, the study and the search for new multiferroic materials have been a major challenge due to their potential applications in electronic technology. In this way, bismuth-containing perovskites (BiMO3), and particularly those in which the metal M position is occupied by a magnetically active cation, have been extensively investigated as possible multiferroic materials. From the point of view of synthesis, only a few of the possible bismuth-containing perovskites can be prepared by conventional methods but at high pressures. Herein, the preparation of one of these potential multiferroic systems, the solid solution xBiMnO3-(1− x)PbTiO3 by mechanosynthesis is reported. Note that this synthetic method allows the oxides with high x values, and more particularly the BiMnO3 phase, to be obtained as nanocrystalline phases, in a single step and at room temperature without the application of external pressure. These results confirm that, in the case of Bi perovskites, mechanosynthesis is a good alternative to high-pressure synthesis. These materials have been studied from the point of view of their structural characteristics by precession electron diffraction and magnetic property measurements. [ABSTRACT FROM AUTHOR]

Abstract: The critical properties of the manganese perovskite La0.67Ba0.33Mn0.98Ti0.02O3 around the paramagnetic-ferromagnetic phase transition were investigated through various techniques such as modified Arrott plot, Kouvel–Fisher method and critical isotherm analysis based on the data of static magnetic measurements recorded around the Curie temperature T c. The magnetic data analyzed in the critical region using the above methods yield the critical exponents β =0.551±0.008 with T c =310.47K±0.10 (from the temperature dependence of the spontaneous magnetization below T C) and γ =1.020±0.024 with T C =310.11K±0.14 (from the temperature dependence of the inverse initial susceptibility above T C) and δ =2.826, determined separately from the isothermal magnetization at T C. These critical exponents fulfill the Widom scaling relation δ =1+ γ/β, implying that the obtained values of β and γ are reliable. Based on these critical exponents, the magnetization–field–temperature (M–H–T) data around T c collapse into two curves obeying the single scaling equation . The values deduced for the critical exponents in La0.67Ba0.33Mn0.98Ti0.02O3 are close to the theoretical prediction of the mean-field model rather than the universal theory of 3D-Heisenberg, 3D-Ising and tricritical mean-field models. Moreover we have investigated the validity and usefulness of the theoretical modeling in our compound La0.67Ba0.33Mn0.98Ti0.02O3 based on the mean-field analysis of the magnetic entropy change (‑∆S M) versus the magnetization data. Results obtained through this approach are compared to those obtained from extrapolation of the Arrott curves. An excellent agreement was obtained between this approach with the one obtained from the extrapolation of the Arrott curves. [Copyright &y& Elsevier]

Abstract: Rare-earth-based manganites ABO3 may present interesting properties when the lanthanide (A-site) and/or the manganese (B-site) are partially substituted by divalent elements. Heavy lanthanides are particularly appealing because of the expected interplay between the intrinsic magnetic properties of the rare-earth element (Ln) and those of the ferromagnetic manganese sublattice. As such, a spin reorientation has been observed during magnetization-versus-temperature cycles due to a negative exchange interaction between Mn and Ln. We present herein high-quality epitaxial thin films (∼200nm thick) of Gd0.67Ca0.33MnO3 deposited onto (100) SrTiO3 substrates by pulsed-laser deposition. Enhanced properties were observed in comparison with bulk samples. The magnetic transition temperature T c of the as-grown films is much higher than the corresponding bulk values. Most interesting, magnetization measurements performed under small applied fields, exhibit magnetization reversals below T c, no matter whether the film is field-cooled (FC) or zero-field-cooled (ZFC). The reversal mechanism is discussed in terms of a negative exchange f–d interaction and magnetic anisotropy, this latter enhanced by strain effects induced by the lattice mismatch between the film and the substrate. [Copyright &y& Elsevier]

The preparation and crystalline features of perovskite solid solution Y(Co[sub x]Mn[sub 1-x])O[sub 3], x = 0.6 and 0.7, belonging to the Co-rich region of the pseudobinary system YMnO[sub 3]-YCoO[sub 3] have been studied. Dilatometric experiments performed under different atmospheres have been carried out for determining the best conditions to obtain single-phase samples. The influence of the thermal cycle and of the sintering density on the appearance of secondary phases has also been established. Firing in O[sub 2] atmosphere favors the formation of the pure perovskite. The cooling rate plays an important role for inhibiting the formation of secondary phases. All of the parameters depend ultimately on the final density of the polycrystalline bodies. [ABSTRACT FROM AUTHOR]

Two polymorphic forms of a mixed zinc/copper biquinoline dihydrogenphosphate are presented, showing almost identical monomeric units, viz. (2,2′-biquinoline-κ2 N, N′)bis(dihydrogenphosphato-κ O)copper(II)/zinc(II), formulated as [Zn xCu1− x(H2PO4)2(C18H12N2)], with x = 0.88 (1) and 0.90 (2). The cation is tetrahedrally coordinated to a chelating biquinoline system and two diprotonated phosphate anions. The structures differ mainly in their intermolecular hydrogen-bonding interactions, leading to different packing schemes. No significant evidence of stress due to the Zn/Cu solid solution formation was detected. [ABSTRACT FROM AUTHOR]

In the title monomer, [Cu(NO3)2(C24H16N2)2], the copper(II) cation is eight-coordinate within an octa­hedral-like polyhedron. The coordination polyhedron is formed by two chelating diphenyl­phenanthroline groups that define the highly distorted CuN4 equatorial plane and two weakly bound bidentate (chelating) nitrate groups in the apical or axial positions. The complex crystallizes in the monoclinic space group C2/ c; a twofold axis passes through the copper(II) cation and bis­ects the two nitrate ligands. This gives the mol­ecule C2 point-group symmetry, rendering only half of the mol­ecule independent. [ABSTRACT FROM AUTHOR]

Highlights: [•] La0.67Ba0.33x□ x MnO3 manganites crystallize in the rhombohedral structure. [•] Barium vacancies modify the structural parameters such as the volume, the Mn–O bond length and the Mn–O–Mn angles. [•] La0.67Ba0.33x□ x MnO3 exhibit a PM-semiconducting state at high temperatures and a FM-metallic state at low temperatures. [•] The resistivity is described in terms of percolation phase theory in the whole range of temperatures. [•] The Curie temperature TC and the electrical transition temperature T M-SC decrease with increasing barium deficiencies. [ABSTRACT FROM AUTHOR]

Abstract: RECo0.50Mn0.50O3 perovskites (RE: La, Y, Er) have been prepared by an auto-combustion method and calcined at 700°C, 850°C and 950°C. YCo0.50Mn0.50O3 forms at temperatures as low as 700°C, much lower than those required in solid-state ceramic synthesis. All materials show adequate specific surface when calcined at 700°C. ErCo0.50Mn0.50O3, which has the largest surface area, shows the lowest activity due to the presence of segregated phases, while YCo0.50Mn0.50O3, which reaches the perovskite structure at lower temperatures, exhibits the highest catalytic activity. The catalytic activity, evaluated from the total combustion of methane, can be related to the presence of the perovskite structure, an adequate surface area and the reducibility of the material. At the calcination temperatures of 850°C and 950°C, at which the surface area and crystalline degrees are almost the same for the La-, Y- and Er-systems, no differences in the catalytic activity were detected. The catalytic activity, higher for materials calcined at lower temperatures, is progressively lost due to sintering processes occurring at higher calcinations temperatures, as confirmed by SEM observations. [Copyright &y& Elsevier]

Abstract: The magnetic phase transition and magnetic entropy change (−ΔS M ) in the La0.40Nd0.30Sr0.30Mn0.70Cr0.30O3 perovskite were investigated by measuring the magnetization as a function of temperature. The ferromagnetic transition temperature T C and the critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T C =191K, β =0.433, γ =1.053 and δ =3.486. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M s (T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (−ΔS M ) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change ((−ΔS M ) vs. M 2) and the classical extrapolation from the Arrott curves (μ 0 H/M vs. M 2), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well. [Copyright &y& Elsevier]

Abstract: The effect of Ti substitution on the magnetocaloric effect of the La0.70Sr0.30MnO3 perovskite was investigated. The magnetic entropy change (−ΔS M ) was deduced by two methods: a Maxwell relation and the Landau theory. The magnetocaloric data displays a large value of the magnetic entropy change (−ΔS M ) near the Curie temperature (T C =210K), which increases when increasing the applied magnetic field. A good agreement is found between the experimental (−ΔS M ) and the one estimated by Landau theory. The relative cooling power values vary from 49 to 288Jkg−1 upon variation of the applied magnetic field at 1 and 5T. Under 5T, the relative cooling power value for La0.70Sr0.30Mn0.90Ti0.10O3 is about 70% of the conventional refrigerant Gd material. As a result, the herein reported compound can be considered as a promising material in magnetic refrigeration technology. [Copyright &y& Elsevier]

Abstract: We have successfully prepared a single-phase Ce-doped sample (La0.56Ce0.14)Sr0.30MnO3 by a solid-state route, without CeO2 oxide segregation. Compared to the undoped sample La0.70Sr0.30MnO3, the structure remains hexagonal with space group without a large variation of the cell parameters and volume. The thermal variation of the magnetization shows that the Ce-doped sample exhibits a ferromagnetic to paramagnetic transition at 357K and an anti-ferromagnetic coupling below 100K between the ferromagnetic Mn moments and the Ce moments and thus form a ferrimagnetic ground state. Comparing our results to those of the literature, we can suggest that the cerium ions are in a trivalent state (Ce3+) and the anomalous behaviour attributed to a Kondo-like effect. [Copyright &y& Elsevier]

Abstract: Thermal and reduction–oxidation stability of substituted LaMn1−y Co y O3 perovskite-type oxides (0.0≤ y Co ≤1.0) prepared by the citrate route have been studied by means of surface area, X-ray diffraction, FTIR spectroscopy and magnetic properties. The perovskite orthorhombic structure is found for y Co ≤0.5, with the exception of y Co =0.1, which corresponds better to rhombohedral LaMnO3.15. For y Co >0.5 the diffraction profiles are quite similar to the cobaltite’s rhombohedral structure. Magnetic iso-field studies (ZFC-FC) reveal that, for y Co ≤0.50, the system presents an antiferromagnetic canted-like ordering of the Mn/Co sublattice, in which the presence of divalent Co ion creates Mn3+–Mn4+ pairs that interact ferromagnetically through the oxygen orbital. This interpretation is confirmed by the magnetization loops, in which the magnetic moment increases when substituting Mn for Co. Therefore, the general trend is: for y Co ≤0.5, the Co ions are inserted in the manganite structure and for y Co >0.5, the Mn ions are inserted in cobaltite structure. The enhancement of the ferromagnetic properties and the thermal stability against reduction for y Co =0.5 is attributed to optimized Co2+–Mn4+ interactions. [Copyright &y& Elsevier]

Abstract: Substituted LaMn1−y Co y O3 perovskite-type oxides (0.0≤ y ≤1.0), prepared by the citrate method, have been investigated as catalysts in the total combustion of acetylacetate. The characterization results indicate variation in specific surface area, crystal structure, infrared spectra and reducibility. The crystal phase transformation was found to occur for y Co values above 0.5. The role of Mn is to stabilize the cobaltite structure, which does not tolerate more than 30% substitution of Co for Mn. The catalytic activity in the total combustion of acetylacetate can be related with changes in the crystal structure and specific surface area. The higher intrinsic activity of the cobaltite doped with 10wt% Mn, y Co =0.9, can be explained by the changes in crystal and electronic properties. Magnetic properties (susceptibility, ZFC/FC and M-loops) show strong ferromagnetic interactions in the range (0.0< y Co ≤0.5) while antiferromagnetism is progressively settled at higher cobalt concentrations. [Copyright &y& Elsevier]

Abstract: Nanocrystalline single-phase samples of Zn1− x Ni x Fe2O4 ferrites () have been obtained via a soft-chemistry method based on citrate–ethylene glycol precursors, at a relatively low temperature (650°C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20m2/g) being obtained for NiFe2O4 annealed at 650°C for 15h. For all compositions, the surface area decreases for prolonged annealing at 650°C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50nm) observed in the NiFe2O4 sample (650°C, 15h) steadily increase as Ni ions were replaced by Zn, reaching ∼100nm in the ZnFe2O4 sample (650°C, 15h). For all the Zn1− x Ni x Fe2O4 samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650–400cm−1, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest -tetrahedral stretching, observed at ∼615cm−1 in NiFe2O4, shifts towards lower values with increasing Zn, whereas the -octahedral vibration, observed at 408cm−1 in NiFe2O4, moves towards higher wavenumbers, reaching 453cm−1 in ZnFe2O4. [Copyright &y& Elsevier]