Spin structure and magnetic anisotropy of Cr5S6 and rhombohedral Cr2S3

The magnetic properties of single crystals of rhombohedral Cr₂S₃ have been measured. The data indicate a preferred orientation of the spins of the Cr atoms in a plane perpendicular to the trigonal axis. New neutron diffraction data for powdered samples which confirm this conclusion are presented.     

Structural and magnetic properties of Cr1+x(Se1−yTey)2 compounds

The structural and magnetic properties of Cr_1+x(Se_1−yTe_y)₂ having a NiAs structure has been studied for (1+x)=1.27, 1.32 and 1.36 and y=0.75 by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The sub-stoichiometry and the disorder on the chalcogenide sub-lattice has been treated by means of the coherent potential approximation (CPA) alloy theory. From total energy calculations a preferential site occupation on the Cr sub-lattice was found together with an antiparallel alignment of the magnetic moments on the two inequivalent Cr layers. The magnetic properties at finite temperature has been studied by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed to determine the critical temperature in good agreement with experiment.     

The pressure effect on the Curie temperature and exchange striction of Cr2S3 and Cr2Te3

The pressure derivative of T_c was found to be 1.0 deg/kbar for Cr₂S₃ and -1.78 deg/kbar for Cr₂Te₃. The volume exchange striction was found to be negative for Cr₂S₃ and positive for Cr₂Te₃. From these results, it was found that the exchange striction was dominant in the a-axis direction.     

Electric and magnetic properties of the system between Cr3S4 and VCr2S4

The electrical resistivity and magnetization of V_xCr_3?xS₄ (x = 0.0 - 1.0) have been measured. Weak ferromagnetic moments are observed at low temperatures for x ≠ 0.0. Curie temperatures are determined from magnetization curves and resistivity anomalies.     

Magnetic properties of stuffed copper chromium tellurides Cu1+xCr2+yTe4 (x=0-1, y<0.3)

We present a detail study of the effect of excess metal atoms on the magnetic properties of Cu_1+xCr_2+yTe₄ at 2-400 K. With the increase in x=0-1 and y<0.3, these compounds retain metallic behavior, while ferromagnetic ordering temperature reduces from 325 to 160 K. Our low field susceptibility χ_ac measurements reveal a second transition on cooling below the ferromagnetic ordering; the transition at around 160-180 K intensifies with the excess amount of copper and chromium atoms. The value of spontaneous magnetization at 2 K remains between 2.6 and 2.9μ_B across all the compositions and it reduces with temperature as M(T)∼A₀T^3/2+A₁T^5/2, as expected for the excitation of Bloch's spin waves in a model of the Heisenberg ferromagnet. Our terminal composition Cu_1.9Cr_2.25Te₄ showed only second transition at 160 K with short range magnetic order much above the transition temperature and in the absence of the specific heat jump at this temperature. The magnetic properties are explained as a result of random magnetic anisotropy in the excess-metal compositions induced by the interstitial atomic defects in their parent spinel structure. The large stuffing of cations has been made possible in the telluride compounds because of the large size of tellurium and also by the covalent bonding that stabilizes the defect structure.     

Photoluminescence of chromium doped CdIn2S4

Well-resolved photoluminescence spectra of CdIn₂S₄: Cr³⁺ recorded at 77K and 2K have been analysed. The R-lines from the doublet ²E-⁴A₂ transition and their associated vibronic spectra have been assigned. These results yield a revised value for the crystal field parameter of B = 618 cm^-1     

Absence of hydrogen in superconducting molybdenum sulfide,Mo3S4

Superconducting Mo₃S₄(T_c=1.8 K) was prepared from the ternary sulfides MMo₃S₄ (M=Cu, Ni) by the acid extraction method, and was studied by neutron diffraction and NMR spectroscopy. In contrast to earlier reports no significant amounts of hydrogen could be found in the rhombohedral structure. However, large proton concentrations were found in samples which had been exposed to moist air.     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

La solution solide Cr1−xRhxO2

A new solid solution Cr_1?x,Rh_xO₂ (0?x?1) has been prepared using oxidizing acids or ammonium perchlorate under pressure. The magnetic study suggests that chromium (IV) and rhodium (IV) are present in the structure at x ? 0.2 and chromium (V) and rhodium (III) at 0.2 < x ? 0.5. Forx > 0.5 increasing rhodium (IV) content gives rise to properties similar to those of RhO₂.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Electrical conduction in Fe2O3 and Cr2O3

The influence of some impurities on the conduction properties of Cr₂O₃ and Fe₂O₃ are examined and contrasted. A mechanism is proposed to account for the effect of Ti in Cr₂O₃.     

Conductivity of the chalcogenide semi-insulator system: As2S3Sb2S3

The conductivities of mixed As₂S₃Sb₂S₃ thin films are discussed. Temperature independent a.c. data are interpreted in terms of a hopping mechanism where correlation effects are important. Excepting As₂S₃, which exhibits extrinsic behaviour, the dielectric constant, a.c. conductivity and d.c. conductivity pre-exponential factor, all scale approximately linearly with composition.     

Photo-synthesis of As3S2 crystal from AsAs2S2 mixture

Existence of As₃S₂ compound which is stable in a certain temperature range higher than room temperature has been confirmed in AsAs₂S₂ system. Photo-synthesis effect of As₃S₂ crystal from As and As₂S₂ mixture, namely light induced conversion from a crystalline state to another crystalline state, has also been investigated.     

Magnetic and structural studies of Cr2O5 and Cr3O8

The infrared spectra, indexed X-ray powder diffraction patterns, magnetic susceptibilities between 80 and 300 K, and electron paramagnetic resonance spectra at 80 and 300 K are reported for Cr₂O₅ and Cr₃O₈. The results indicate that both oxides are Cr³⁺/Cr⁶⁺ mixed-valence compounds which contain CrO₆ octahedra and CrO₄ tetrahedra in different ratios. The valence formula for Cr₂O₅ is Cr³⁺₂Cr⁶⁺₄O₁₅ and that of Cr₃O₈ is Cr³⁺₂Cr⁶⁺₇O₂₄. The X-ray powder data for Cr₂O₅ and Cr₃O₈ could be indexed on the basis of a monoclinic unit cell $\text{(a = 12.01(2), b = 8.52(1), c = 9.39(1)}\text{A}\text{?}\text{β = 92.0(1)°)}$ and an orthorhombic unit cell $\text{(a = 12.01(7), b = 36.60(7) and c = 3.82(1)}\text{A}\text{?}\text{)}$, respectively.     

Magnetic properties in the series TlV5−yMyS8 (M = Ti,V, Cr,Mn, Fe; O ≤y ≤ 1)

Single phased ternary vanadium sulfides with various 3d transition metal substitutes were prepared by thermal treatment and characterized by X-ray diffraction. Magnetic susceptibility measurements on the homogeneous powders reveal that itinerant and localized magnetism in all samples co-exists, due to the inequivalent vanadium sites in the structure. While the substitution of V with Ti to Mn has only a small effect on the magnetic properties, the substitution with Fe brings out a localized magnetic moment. The behaviour of the series will be discussed.     

The S exchange interaction in Gd doped La3-x□xS4

In this letter we show that the ESR line widths of Gd³⁺ in La₂S₃ (an insulator) and La₃S₄ (a metal) are the same at 0° K, but that in the latter it has a linear temperature dependence. Because of the identical crystal structures of these compounds this result can unequivically be interpreted as arising from an exchange interaction between the localized moment and the conduction electrons. The system La_3-x□_xS₄ therefore is an excellent host for studying the effects of localized moment-conduction electron interactions.     

Behavior of the irreversibility line in the new superconductor La1.5+xBa1.5+x−yCayCu3Oz

The irreversibility properties of high-T_c superconductors are of major importance for technological applications. For example, a high irreversibility magnetic field is a more desirable quality for a superconductor [1]. The irreversibility line in the H–T plane is constituted by experimental points, which divides the irreversible and reversible behavior of the magnetization. The irreversibility lines for series of La_1.5+xBa_1.5+x−yCa_yCu₃O_z polycrystalline samples with different doping were investigated. The samples were synthesized using the usual solid estate reaction method. Rietveld-type refinement of x-ray diffraction patterns permitted to determine the crystallization of material in a tetragonal structure. Curves of magnetization ZFC–FC for the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z, were measured in magnetic fields of the 10–20,000 Oe, and allowed to obtain the values for the irreversibility and critical temperatures. The data of irreversibility temperature allowed demarcating the irreversibility line, T_irr(H). Two main lines are used for the interpretation of the irreversibility line: one of those which suppose that the vortexes are activated thermally and the other proposes that associated to T_irr a phase transition occurs. The irreversibility line is described by a power law. The obtained results allow concluding that in the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z a characteristic bend of the Almeida–Thouless (AT) tendency is dominant for low fields and a bend Gabay–Toulouse (GT) behavior for high magnetic fields. This feature of the irreversibility line has been reported as a characteristic of granular superconductors and it corroborates the topological effects of vortexes mentioned by several authors 1 and 2.     

Line strength measurements of carbonyl sulfide (OCS) in the 2v3, v1+2v2+v3, and 4v2+v3 bands

To support planetary studies of the Venus atmosphere, we measured line strengths of the 2v₃, v₁+2v₂+v₃, and 4v₂+v₃ bands of the primary isotopologue of carbonyl sulfide (¹⁶O¹²C³²S), whose band centers are located at 4101.387, 3937.427, and 4141.212 cm⁻¹, respectively. For this, infrared absorption spectra in normal carbonyl sulfide (OCS) sample gas were recorded at an unapodized resolution of 0.0033 cm⁻¹ at ambient room temperatures using a Bruker Fourier transform spectrometer (FTS) at the Jet Propulsion Laboratory. The FTS instrumental line shape (ILS) function was investigated, which revealed no significant instrumental line broadening or distortions. Various custom-made short cells and a multi-pass White cell were employed to achieve optical densities sufficient to observe the strong 2v₃ and the weaker bands in the region. Gas sample impurities and the isotopic abundances were determined from mass spectrum analysis. Line strengths were retrieved spectrum by spectrum using a non-linear curve fitting algorithm adopting a standard Voigt line profile, from which Herman–Wallis factors were derived for the three bands. The band strengths of 2v₃, v₁+2v₂+v₃, and 4v₂+v₃ of ¹⁶O¹²C³²S (normalized at 100% of isotopologue) are observed to be 6.315(13)×10⁻¹⁹, 1.570(2)×10⁻²⁰, and 7.949(20)×10⁻²¹ cm⁻¹/molecule cm⁻², respectively, at 296 K. These results are compared with earlier measurements and the HITRAN 2004 database.     

Structure and superconductivity in FexCu1−xBa2YCu2O7+y superconductors synthesized by high pressure

Comprehensive studies of X-ray diffraction, oxygen content, superconductivity and Mössbauer effect have been made on Fe_xCu_1−xBa₂YCu₂O_7+y superconductors (0.00≤x≤0.70) synthesized by ambient (AM) and high pressure (HP). Results indicate that all the HP-samples have tetragonal structure, smaller lattice parameter c and unit-cell volume than the AM-samples. The studies of oxygen content, and Mössbauer spectroscopy indicate that the HP-samples have higher oxygen content, carrier concentration and average valence of Fe than the AM-samples. Moreover, for the HP-samples more Fe atoms located in CuO_x chains have fivefold-oxygen coordination. These are important reasons for the enhancement of T_c in the HP-samples.     

Superconductivity in the RexMo6S8

The rare earth molybdenum sulfides RE_xMo₆S₈ (RE = rare earth) have been synthesized and are found to be superconducting with a few exceptions. This result, which is surprising in view of the high concentration of magnetic ions, is correlated with the particular structure of these compounds based on on units of Mo₆S₈.