Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Observation of long-range magnetic ordering in the proposed spin-singlet state of Hg2Ru2O7

The magnetic ground-state of the pyrochlore material Hg₂Ru₂O₇, which undergoes a metal to Mott insulator transition (MIT) at T=107 K, was investigated using muon spin relaxation and neutron powder diffraction (both non-polarized and polarized) measurements. Unlike the expected behavior associated with a spin-singlet ground-state, these surprisingly revealed the onset of long-range magnetic ordering below the MIT.     

A re-evaluation of the heat capacity of cerium zirconate (Ce2Zr2O7)

The heat capacity of cerium zirconate pyrochlore, Ce₂Zr₂O₇, was measured from 0.4 to 305 K by hybrid adiabatic relaxation method for various magnetic field strengths. Magnetisation measurements were performed on the sample also. The results revealed a low-temperature anomaly that showed Schottky-type characteristics with increasing magnetic field strength. The estimated entropy due to the magnetic ordering of the two Ce³⁺ moments is 1.37R, close to the theoretical value for a doublet ground state (1.39R). The enthalpy increments relative to 298.15 K were measured by drop calorimetry from 531 to 1556 K. The obtained results significantly differ from those reported in the literature; the origin of the discrepancy is due to the probable oxidation of the pyrochlore structure into fluorite.     

Study of the Ru sublattice magnetic structure in the magnetic superconductor RuSr2GdCu2O8

In order to investigate the Ru sublattice magnetic structure, a study of the field dependence of the ^99,101Ru nuclear magnetic resonance (NMR) has been carried out on the magnetic superconductor RuSr₂GdCu₂O₈. It is found that the ^99,101Ru NMR signal intensity increases significantly with applied magnetic field up to ≈3 kOe, beyond which, it progressively decreases. In addition, a shift of the NMR peaks to lower frequency is observed to begin at ≈1.3 kOe. These behaviors are shown to be accompanied by a field-induced Ru moment spin-flop in the ab planes, and are understood in terms of a previously proposed type-I antiferromagnetic ordering for the Ru sublattice. Based on this model, the inter-plane antiferromagnetic exchange coupling is determined to be ≈1.8 kOe along with a reversible in-plane spin-flop which is characterized by a field ≈0.6 kOe.     

Structural and magnetic properties and hyperfine interaction in La3.5Ru4O13 compound

Structural, magnetic and hyperfine interaction measurements have been carried out on the novel compound La_3.5Ru₄O₁₃ prepared under two different atmospheres (air and oxygen flow). This compound is formed in the orthorhombic structure (space group Pmmm, # 47). The coexistence of the triple-layered perovskite-type planes (quasi-2D structure) and the rutile-like slabs (1D structure) leads to interesting magnetic and electronic properties in this compound. The magnetic susceptibility of this system shows a peak at T~47 K associated with antiferromagnetic interactions. The Curie-Weiss behaviour of the susceptibility provides an effective magnetic moment consistent with Ru ions in low-spin state. Perturbed angular correlation measurements carried out with ¹¹¹Cd probe in the temperature range 10-60 K reveal only quadrupole interactions and indicate the occurrence of structural distortions for T<40 K.     

Thermoelectric properties of the Bi2−xYxRu2O7 (x=0-2) pyrochlores

Electrical conductivity and Seebeck coefficient for the Bi_2−xY_xRu₂O₇ pyrochlores with x=0.0,0.5,1.0,1.5,2.0 were measured in the temperature range of 473-1073 K in air. With increasing Bi content, the temperature dependence of the electrical conductivity changed from semiconducting to metallic. The signs of the Seebeck coefficient were positive in the measured temperature range for all the samples, indicating that the major carriers were holes. The temperature dependence of the Seebeck coefficient for the Y₂Ru₂O₇ indicated the thermal activation-type behavior of the holes, while that for the Bi_2−xY_xRu₂O₇ with x=0.0-1.5 indicated the itinerant behavior of the holes. The change in the conduction behavior from semiconductor to metal with increasing Bi content is consistent with the increase in the overlap between the Ru4d t_2g and O2p orbitals, but the mixing of Bi6s, 6p states at E_F may not be ruled out. The thermoelectric power factors for the Bi_2−xY_xRu₂O₇ with x=1.5 and 2.0 were lower than 10⁻⁵ W m⁻¹ K⁻² and those with x=0.0,0.5,1.0 were around 1-3×10⁻⁵ W m⁻¹ K⁻².     

Heat capacity and magnetic properties of pyrochlore Hg2Os2O7

Hg₂Os₂O₇, which has the cubic pyrochlore structure, remains metallic down to the liquid helium temperature unlike its isostructural counterpart Cd₂Os₂O₇, which shows metal-insulator transition at 226 K. Magnetization and heat capacity data for Hg₂Os₂O₇ are presented. The magnetic anomaly at T_N=88 K shares many characteristics in common with the metal-insulator transition in Cd₂Os₂O₇, though Hg₂Os₂O₇ remains metallic below T_N. The heat capacity C_p shows no or very little change in the magnetic entropy around T_N, supporting the view that there is no long-range ordering of localized spins. The measured value of electronic heat-capacity coefficient γ=21 mJ K⁻²mol⁻¹ is comparable to the value obtained from band-structure calculation on Cd₂Os₂O₇, suggesting that mass-enhancement is small in Hg₂Os₂O₇. There is a pronounced peak in C_p/T³ at 13.1 K, which corresponds to a peak in the phonon density of states at 40 cm⁻¹.     

Anomalous field dependence of the inverse magnetocaloric effect in Ce(Fe0.93Ru0.07)2

A summary of detailed isothermal magnetization along with the ac susceptibility measurements on Ce(Fe_0.93Ru_0.07)₂ is presented. Near the first-order transition, i.e. near 110 K, Ce(Fe_0.93Ru_0.07)₂ displays a large positive magnetic entropy change, ΔSm, of 7.8 J/kg K in fields of only 2 T, i.e. it displays a strong inverse magnetocaloric (MCE) effect, as expected, on entering an antiferromagnetic (AF) state. However, the variation of the magnitude and width of this entropy change with field are anomalous when compared with model predictions, the former increasing with applied field below 2 T, while the latter exceeds 60 K in a field of 8 T.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.     

The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7

The Ruddlesden–Popper (RP) phase compounds (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu, Sm, Nd and La) were prepared, and their transport and thermoelectric properties were investigated. The results indicate that high-T electrical resistivity ρ (300 K<T<1000 K) increases monotonically with temperature and basically has a relation ρ∝T^M, with M varying from 0.91 to 1.92 at temperatures T>~650 K, suggesting acoustic phonon scattering is dominant. At low temperatures (5 K<T<300 K), ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Nd and La) decreases monotonously with decreasing temperature, whereas ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu and Sm) decreases first, and then increases instead as T decreases to a critical temperature T_c. Moreover, electrical conductivity σ∝T^1/2 holds at lower temperatures, indicating that the electron–electron interaction caused by the presence of disorder dominates the transport process at the low temperatures. Besides, experiments show that at T<~400 K the lattice thermal conductivity of the doped compounds basically decreases with increase of the atomic mass of dopants. Generally, the figure of merit (ZT) at 1000 K increases first, and then decreases with the increase of the dopants' ionic radius, and the largest ZT is achieved in (Sr_0.95Gd_0.05)₃Ti₂O₇ mainly owing to its lower lattice thermal conductivity.     

Magnetoelectric behavior in the complex CaMn7O12 perovskite

We report a magnetoelectric effect in the double perovskite CaMn₇O₁₂, that shows a complex magnetic behavior below 90 K with two magnetic phases coexisting (one ferrimagnetic and the other modulated). A second magnetic transition, associated with changes in the magnetic modulation and magnetic ordering coherence lengths of the two magnetic phases occurs at 50 K (T_N2). A detailed structural characterization of this compound, that we have carried out by means of high-resolution X-ray powder diffraction, reveals an anisotropic thermal expansion of its lattice parameters at 50 K (T_N2). In addition, our study of the complex permittivity of this sample as a function of temperature, frequency and magnetic field shows very interesting results below 90 K and specially below 50 K: the dielectric constant ε′_r that was decreasing continuously on cooling experiences an upturn, and even more, on application of a magnetic field it shows a moderate magnetoelectric response. We attribute such dielectric behavior to the formation of electric dipoles by magnetostriction in this charge and spin ordered system, that are sensible to the presence of an external magnetic field.     

Al2O3 supported Ru catalysts prepared by thermolysis of Ru3(CO)12 for catalytic wet air oxidation

Low loading catalysts Ru/γ-Al₂O₃ and Ru-Ce/γ-Al₂O₃ were prepared by thermolysis of Ru₃(CO)₁₂ on γ-Al₂O_3. The catalysts were characterized by XPS, XRD and SEM. Two new Ru species (Ru_A and Ru_B) were detected during the Ru₃(CO)₁₂ decomposition process due to chemical interaction with the active OH groups on the surface of Al₂O₃ support, and the reduction of them can lead to more dispersed metallic phases. The sample was completely decomposed at 673 K in H₂, and RuO₂ was formed with minor amounts of Ru⁰. When the temperature was increased to 773 K to heat the sample, the ratio of Ru⁰ to RuO₂ increased. However, after the addition of CeO₂, only RuO₂ was detected on surface. The catalysts exhibited high activities in Catalytic Wet Air Oxidation (CWAO) of different organic compounds at high concentration such as isopropyl alcohol, phenol, acetic acids and N,N-dimethylformamide, which is attributed to the better dispersion of Ru particles and the addition of CeO₂ further enhanced number of effectively active sites on the cluster-derived catalyst surface.     

Static susceptibility and heat capacity studies on V3O7·H2O7 nanobelts

V₃O₇·H₂O nanobelts were prepared by a hydrothermal method at 190 °C using V₂O₅·nH₂O gel and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 40 to 70 nm with lengths up to several micrometers. Measurements of the static magnetic susceptibility and the specific heat show a discontinuous phase transition at around T=145 K, which separates two regions of paramagnetic behavior.     

Successive magnetic transitions and low temperature magnetocaloric effect in RE2Ni7 (RE=Dy, Ho)

Magnetic properties of rare-earth intermetallics RE₂Ni₇ (RE=Dy, Ho) are reported. Both the samples undergo two successive magnetic transitions at T_h (paramagnetic to ferromagnetic) and T_l (spin reorientation) below 100 K. The transitions are found to be second order in nature as evident from the Arrot plot analysis. Large reversible magnetocaloric effect (MCE) was observed at low temperature in the studied samples. The maximum value of the magnetic entropy change in Ho₂Ni₇ is found to be −12.5 J/kg K (for 0 to 50 kOe of field change) around 25 K with a high relative cooling power (RCP) of 534 J/kg. The Dy counterpart also shows moderately large values of MCE (−7.3 J/kg K) and RCP (475 J/kg) around the magnetic transition region for similar change in the magnetic field. RE₂Ni₇ compounds can be promising materials for magnetic refrigeration in the temperature range of helium and hydrogen liquefaction.     

Flat bands and enigma of metamagnetic quantum critical regime in Sr3Ru2O7

Understanding the nature of field-tuned metamagnetic quantum criticality in the ruthenate Sr₃Ru₂O₇ has presented a significant challenge within condensed matter physics. It is known from experiments that the entropy within the ordered phase forms a peak, and is unexpectedly higher than that outside, while the magnetoresistivity experiences steep jumps near the ordered phase. We find a challenging connection between Sr₃Ru₂O₇ and heavy-fermion metals expressing universal physics that transcends microscopic details. Our construction of the T–B$T\text{–}B$ phase diagram of Sr₃Ru₂O₇ permits us to explain main features of the experimental one, and unambiguously implies an interpretation of its extraordinary low-temperature thermodynamic in terms of fermion condensation quantum phase transition leading to the formation of a flat band at the restricted range of magnetic fields B. We show that it is the flat band that generates both the entropy peak and the resistivity jumps at the QCPs.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Magnetocaloric effect in R2Fe17 (R=Sm,Gd, Tb,Dy, Er)

A series of R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) have been synthesized. The magnetocaloric effect (MCE) of these compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. The Curie temperature of Er₂Fe₁₇ is 294 K. The maximum magnetic entropy change of Er₂Fe₁₇ under 5 T magnetic field is ∼3.68 J/kg K. In the R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) system, the maximum magnetic entropy change under 1.5 T magnetic field is 1.72, 0.89, 1.32, 1.59, 1.68 J/kg K corresponding to their Curie temperature (400, 472, 415, 364, 294 K), respectively.     

EuSr2Ru1-xTaxCu2O8中Ta替代效应

通过对EuSr₂Ru_1-xTa_xCu₂O₈ (x=0.0, 0.1, 0.2, 0.5和1.0)体系的结构、电阻和磁化强度的观测，发现EuSr₂RuCu₂O₈(x=0.0)样品在130.2K以下呈现铁磁有序，在35K时发生了超导转变，并呈现典型的欠掺杂高温超导体特征；随着Ta对Ru替代浓度x值的增加，铁磁相变温度和超导临界温度均下降 关键词： 高温超导电性 铁磁有序 Ru-Cu氧化物     

Influence of Dy addition on the magnetocaloric effect of La0.67Ca0.33Mn0.9V0.1O3 ceramics

The influence of partial substitution of La by Dy on the magnetocaloric response of (La_1−xDy_x)_0.67Ca_0.33Mn_0.9V_0.1O₃, where x=0.03, 0.15 and 0.25 is studied. Rietveld refinement of X-ray diffraction pattern using GSAS method shows that the compounds adopt the orthorhombic structure with Pnma space group. The systematic change in lattice parameters and magnetic phase transition indicates the substitution effect of Dy. From the magnetization isotherms at different temperatures, magnetic entropy change close to their respective transition temperatures (T_C) has been evaluated. The maximum value of entropy change near T_C is found to be about 4.8 J/kg K at 187.5 K for LCMVDy_0.03, 2.45 J/kg K at 107.5 K for LCMVDy_0.15 and 2.15 J/kg K at 92.5 K for LCMVDy_0.25 at 4 T. Dy addition produces a reduction in T_C and in magnitude of the magnetic entropy change. Even though the entropy change decreases with increasing Dy substitution the refrigerant temperature range, ΔT, is found to be 10 K for LCMVDy_0.03, 31 K for LCMVDy_0.15 and 35 K for LCMVDy_0.25 compounds [90%] at 4 T. The field dependence of the magnetic entropy change is also analyzed showing the power law dependence, ΔS_M∞Hⁿ where n=0.75(2) for LCMVDy_0.03, n=0.80(4) for LCMVDy_0.15 and n=0.92(8) for LCMVDy_0.25 compounds at their respective transition temperatures. The relative cooling power and its field dependance are also analyzed.     

Thermal stability of A-site ordered PrBaMn2O6 manganites

The crystal structure and electromagnetic properties as well as thermal stability of the A-site ordered PrBaMn₂O₆ manganites have been investigated. These samples have been prepared by using ‘two-steps’ synthesis mode. They have tetragonal structure with no tilt of MnO₆ octahedra and show ferromagnetic metal to paramagnetic semiconductor transition. The most significant structural feature of the A-site ordered manganites is that the MnO₂ sublattice is sandwiched by two types of rock-salt layers PrO and BaO. The different degree of Pr and Ba ions in the A-sublattice is revealed. The A-site ordered PrBaMn₂O₆ sample with maximum degree of the A-site order demonstrates ferromagnetic metallic to paramagnetic insulating transition with the Curie point ∼320 K. The A-site disordered Pr_0.50Ba_0.50MnO₃ sample is ferromagnetic metal below T_C≈140 K. The cation order in these compounds is stable in air up to 1300 °C. For the partly A-site ordered samples the magnetic and electronic phase separation is observed. The magnetotransport properties of the A-site ordered manganites treated under different conditions are discussed in terms of the superexchange interactions and A-site order degree.