Thermal annealing effects on the magnetic behavior of Ce2NiSi3

We have investigated the crystallographic, magnetic and thermodynamic properties of the as-cast and annealed Ce₂NiSi₃ alloys, crystallizing in the AlB₂-type hexagonal structure. The DC-magnetic susceptibility data show that the as-cast sample exhibits an antiferromagnetic (AFM) ordering below T_N= 3.8 K, whereas the annealed sample shows, at 4.2 K, a magnetic transition of AFM nature and, around 2.5 K, an additional anomaly. The specific heat shows a peak with at 3.8 K for the as cast sample, which shifts to lower temperatures when the magnetic field increases, consistent with the antiferromagnetic nature of the transition. On the other hand, in the annealed alloy, the maximum of the specific heat peak reaches at 4.2 K, and no additional anomalies were observed. The different magnetic behavior between the as-cast and annealed samples is attributed to thermal effects on the structural disorder of nickel and silicon atoms, as already observed in other isotypic R₂TSi₃ alloys, where R=U or Ce, and T= transition metal.     

Low temperature magnetism of Cd-doped Ce2RhIn8 heavy fermion antiferromagnet

We report the low temperature magnetic properties of Cd-doped single crystals of Ce₂RhIn₈ grown from In-flux. Measurements of temperature-dependent magnetic susceptibility, heat capacity, electrical resistivity and X-ray magnetic scattering revealed that Cd-doping enhances the antiferromagnetic ordering temperature from for crystals with nominal Cd-concentration of ~20%. Similarly to the pure compound, Cd-doped Ce₂RhIn₈ presents just below T_N a commensurate antiferromagnetic structure with a propagation vector . Comparisons between our results and the general effects of Cd-doping on the single layer related family CeMIn₅ (M=Co, Rh and Ir) will also be given.     

Field suppression of the modulated phase of Ce2Pd2Sn

Low temperature magnetic (M) and thermal (C_P) properties of the intermetallic compound Ce₂Pd₂Sn have been investigated at zero and different magnetic fields. Two transitions were recognized at and , with latter nearly coinciding with the extrapolated Curie-Weiss temperature . The Curie factor evaluated from T≥T_M, is ≈2μ_B. The positive value of θ_P, the triangular coordination of the magnetic (Ce) atoms and the weak effect of applied magnetic field, reveal that T_M cannot be considered as a canonic antiferromagnetic transition like claimed in the literature. M(T) measurements under moderate magnetic fields () show T_C(B) increasing while T_M(B) is practically not affected. Both transition merge in a critical point at for , where the intermediate phase is suppressed. At , the cusp of a first order transition is observed in C_P(T). According to the proposed ferromagnetic ground state, it is followed by a C_P(T)∝T^3/2exp(-E_g/T) dependence, with a gap of anisotropy .     

Ultraviolet absorption spectrum and cross-sections of ethynyl (C2H) radicals

This work reports the ultraviolet absorption spectrum and cross-sections of ethynyl (C₂H) radicals in the wavelength range 235-260 nm, determined at T=298 K and at a total pressure of about 20 kPa (150 Torr). Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C₂HCF₃ or C₂H₂ in He. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples showed diacetylene (C₄H₂) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane, products of the reaction of ethynyl radicals with methane. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of a transient absorption centered at about 243 nm. The spectra obtained from the 193-nm photolysis of both C₂HCF₃/He and C₂H₂/He radical precursors were nearly identical, suggesting that the absorption feature can be attributed to the ethynyl radicals. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and an absorption peak at about 243.5 nm. The absorption cross-sections for ethynyl radicals have been determined in this work for the first time. The C₂H cross-section, at the maximum absorption, at 243.5 nm is . The stated uncertainty includes the random and systematic measurement errors. The UV absorption feature, detected in this work, can be assigned to the transitions from the ground electronic state and also possibly from coupled ground and lowest electronic states to the electronic state B (or 32A′) of C₂H. This assignment is based on previously reported high level ab initio molecular orbital calculations and results of recent laser-induced fluorescence studies of the ethynyl radical.     

Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden electronic transition of the cumulene carbene C₃H₂ have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm⁻¹ region is a result of vibronic interaction and is assigned to a combination of a₁ and b₂ vibrations with a frequency around 2250 cm⁻¹. Another vibronic band near 15 810 cm⁻¹ has an unusual rotational structure because the K_a = 0-1 subband is absent. It is assigned to a combination of a₁ and b₁ vibrations, ∼1850 cm⁻¹, which borrow intensity from the near lying state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the ùA₂ state, which were used for the determination of the geometry. The stronger transition of C₃H₂, measured in a neon matrix in the 16 161-24 802 cm⁻¹ range, was not detected. The reason for this is a short lifetime of the state, leading to line broadening.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

Detection of structural and magnetic transitions in La0.67Ba0.23Ca0.1MnO3 using the rf resonance technique

We report radio-frequency (rf) electrodynamics in polycrystalline La_0.67Ba_0.23Ca_0.1MnO₃ as a function of temperature and magnetic field using a home-built LC resonant circuit powered by an integrated chip oscillator. The resonance frequency (f_r) of the oscillator and the power (P) absorbed by the sample are measured simultaneously. The paramagnetic to ferromagnetic phase transition in the absence of an external magnetic field is accompanied by a rapid decrease in both P and f_r around the Curie temperature T_C=300 K. However, much below T_C, the f_r shows a step-like anomaly around 165 K (195 K) while cooling (warming), which we attribute to a structural phase transition from high temperature rhombohedral () to low temperature orthorhombic (Imma) phase. The step-like anomaly in f_r versus T disappears in a field of 300 G. Fractional changes as large as 19% in Δf_r/f_r and 10% in ΔP/P are observed under H=1 kG around T_C. Our study suggests that the rf resonance technique is a versatile tool to study the magnetization dynamics as well as to investigate the structural phase transition in manganites.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.     

Rotationally resolved electronic spectroscopy of a nonlinear carbon chain radical C6H4

Rotationally resolved electronic spectrum of the origin band in the 2A″-X2A″ transition of a nonlinear carbon chain radical C₆H₄⁺ has been recorded in the 604 nm region using cw cavity ring down spectroscopy. The radical was produced by a discharge through an acetylene-helium mixture in a supersonic planar expansion. The rotational structure has been analysed and precisely determined. A band having a-type prolate rotational structure has also been observed near 581 nm. By considering the results of ab initio calculations this band is assigned to a transition involving the excitation of the ν₁₂ fundamental in the upper 2A″ electronic state of the same C₆H₄⁺ isomer.     

Crystalline orientation of BiMnO3 thin films grown by rf-sputtering

BiMnO₃ thin films were deposited by means of rf-sputtering onto [1 0 0] oriented SrTiO₃ substrates. X-ray diffraction measurements revealed epitaxial growth with [0 0 l]_m orientation in the monoclinic structure representation, equivalent to the direction of the pseudocubic cationic lattice. This [0 0 l]_m orientation was obtained in a wide deposition parameters range. Preliminary magnetization measurements suggest that these films do not present ferromagnetism below the bulk Curie temperature, T_C=105 K.     

Luminescence and energy transfer of Ce and Tb in Y3Si2O8Cl

The luminescence properties of Ce³⁺ and Tb³⁺ in Y₃Si₂O₈Cl have been investigated. The Ce³⁺ excitation bands in the region from 220 to are attributed to the transitions from 4f level to the crystal-field splitting levels of 5d. The Tb³⁺ excitation bands in the region from 220 to are due to the 4f-5d transitions. The spectral energy distributions of Tb³⁺ emission strongly depend on the Tb³⁺ concentration, the emissions of Tb³⁺ decrease with increasing Tb³⁺ concentration, this phenomenon is due to the cross-relaxation between Tb³⁺ ions. The efficient energy transfer from Ce³⁺ to Tb³⁺ in the Y₃Si₂O₈Cl is observed and discussed.     

Thermal infrared cross-sections of C2F6 at atmospheric temperatures

Spectral absorption cross-sections, k_ν (), have been measured in the 8.0 and bands of C₂F₆. Temperature and total (N₂-broadening) pressure have been varied to represent the conditions specified in various models of the terrestrial atmosphere so that the absorption cross-sections can be applied directly to the optical remote-sensing of C₂F₆ in the atmosphere. The measured absolute intensities of the 8.0 and the bands are (1.636±0.003)×10⁻¹⁶ and (, respectively.     

Specific heat of EuIn2P2 at high magnetic fields

We have carried out specific heat measurements on EuIn₂P₂ at high magnetic fields perpendicular to the c-axis in the hexagonal crystal structure in order to understand its thermal properties. The temperature dependence of the specific heat exhibits a clear λ-type anomaly due to a magnetic transition at , indicating that the magnetic transition is of second-order. The λ-type anomaly becomes markedly broader with increasing the magnetic field. This remarkable field-dependence is consistent with the results of previous magnetization measurements which suggest that Eu²⁺ magnetic moments align ferromagnetically perpendicular to the c-axis below T_C. In addition, a hump in the specific heat is observed around 7 K, which can be ascribed to the Zeeman splitting of the Eu²⁺ multiplet by internal magnetic fields.     

Magnetic ordering and low-temperature thermodynamic properties of ErFe2Ge2

Polycrystalline sample of ErFe₂Ge₂ was investigated by means of magnetic susceptibility, heat capacity and electrical resistivity measurements, as well as by powder neutron diffraction. All these experiments yielded an evidence of magnetic ordering setting at about 3 K. The low-temperature neutron data revealed the formation of a sine-modulated commensurate antiferromagnetic structure characterized by the propagation vector k=(0, 0, ). The erbium magnetic moment is aligned parallel to the crystallographic a-axis. At T=1.55 K it is equal to 7.06(5) μ_B.