The magnetic instability in the heavy fermion compounds Ce1−x La x Ru2Si2

The magnetization and the specific heat of Ce_1–x La _x Ru₂Si₂ with x0.13 are reported with special attention to the effect of magnetic field and the role of lanthanum doping. Evidence is given of differences between the undoped (x=0) and the solid solution (x0) cases. A common feature is the occurrence of well-defined anomalies in the magnetization at the metamagnetic fieldH _M independently of whether the ground state is one of long-range order or Pauli paramagnetism. Forx=0, the ground state appears to be a Pauli paramagnet for any strength of the magnetic field; quantum fluctuations or deviations from an ideal lattice may prevent the occurrence of a true static magnetic transition.Laboratoire associé à l'Université Joseph Fourier, Grenoble.     

Magnetocaloric Effect in La1−x Cd x MnO3

Calculations of the magnetocaloric effect for La_1?x Cd _x MnO₃ (LSCM) upon 0.05 T magnetic field variation have been carried out. It is found that magnetic entropy change distribution of the LSCM is much more uniform than that of gadolinium. This feature is desirable for an Ericson-cycle magnetic refrigerator. Furthermore, at different concentrations of Cd, the temperature range between 150 K and room temperature can be covered using the La_1?x Cd _x MnO₃ system. Therefore, the LSCM system is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperatures.     

Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+y Te1−x S x

Transport and structural properties of Fe_1+y Te_1?x S _x were studied in pulsed magnetic fields. Application of high magnetic fields induces first order transitions showing positive magnetoresistance effects in the antiferromagnetic phase. Polarizing microscope images taken at high magnetic fields revealed the concomitant melting of the orbital order. These results indicate the importance of crossed coupling between spin and lattice or orbital degrees of freedom in this compound.     

Phase coexistence in Bi1−x Pr x FeO3 ceramics

Bi_1?x Pr _x FeO₃ ceramics across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi_1?x Pr _x FeO₃ driven by doping concentration and temperature are significantly different from those in BiFeO₃ compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 °C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO₃-based compounds near the morphotropic phase boundary.     

Homogeneous Coexistence in CaFe1−x Co x AsF and Phase Segregation in LaFeAsO1−x F x Studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of ⁷⁵As NMR measurements in LaFeAsO_1?x F _x (x=0.026) (La1111), and CaFe_1?x Co _x AsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe_1?x Co _x )₂As₂ and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T _c , suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S _±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S ₊₊-wave superconductivity.     

Phase relation in the system La1−xBaxFeO3−y (0⩽x⩽0.70)

Samples of the system La_1–xBa_xFeO_3–y, (0x0.70) have been synthesized and investigated by means of X-ray diffraction and Mössbauer effect. Two different phases, i.e. a barium-poor orthorhombic perovskite phase and a barium-rich cubic perovskite phase, were found to exist in the system. The phase relation at room temperature is presented.     

Piezoelectric Properties of TlIn1−x Nd x Se2 Crystals

The piezoelectric properties of TlIn_1–x Nd _x Se₂ crystals (0 x 0.05) have been investigated. It has been found that these crystals possess high coefficients of tensosensitivity which change on partial substitution of the indium atoms by neodymiun atoms and strongly depend on the intensity of the spectral composition of optical illumination.     

Magnetic Properties and Metamagnetic Transition in SmCo1−x Fe x AsO (0≤x≤0.2)

Polycrystalline SmCo_1?x Fe _x AsO (x=0, 0.05, 0.1, 0.2) materials with single phase have been synthesized by solid-state reaction. According to the X-ray diffraction patterns, the a-lattice parameter shrinks, but the c-lattice parameter expends with the increased amount of Fe doping. Complicated magnetisms consist of antiferromagnetic, ferromagnetic, and paramagnetic have been observed between 5 K and 300 K in the present system. Magnetic measurement shows that with increasing Fe content, antiferromagnetic order of SmCoAsO is suppressed and ferromagnetic order is enhanced. Below their antiferromagnetic transition temperature, metamagnetic transition can be detected in samples with x<0.2, and the transition field decreases with increasing Fe content. When Fe content reaches 0.2, no metamagnetic transition is observed down to 5 K. A canting of the spins plays a crucial role in metamagnetic behavior of the present system.     

Microscopic Scenario for x = 1/8 Anomaly in La2 − x Sr x CuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 ? x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (π, π) at intermediate temperatures and near (π, π/2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for “stripe” picture is proposed.     

Lattice-matched epitaxial Pb1 − x Mn x Se/PbSe1 − x S x heterojunctions

This paper examines the possibility of producing lattice-matched p-n heterojunctions based on epitaxial n-Pb_{1 ? x} Mn _x Se (x = 0.02) and p-PbSe_{1 ? x} S _x (x = 0.04) films. The heterojunctions have been grown by molecular beam epitaxy in a single processing cycle, without breaking the vacuum, using a compensating Se vapor source in the growth process. Optimal conditions have been found for the growth of structurally perfect (W _1/2 = 90″-100″) epitaxial films and fabrication of lattice-matched heterojunctions based on such films, photosensitive in the IR spectral region.     

Magnetization reversal and ferrimagnetism in Pr1−x Nd x MnO3

Detailed magnetic properties of Pr_{1? x} Nd _x MnO₃ (x = 0.3, 0.5 and 0.7) have been reported. All the samples crystallize in orthorhombic perovskite structure with Pnma space group. Magnetization measurements under field cooled (FC) protocal reveal magnetization reversal at low temperatures and low magnetic field. This indicates clear evidence of two magnetic sublattices aligned opposite to each other. There is a well-defined maximum around 48 K in the x = 0.7 sample (i.e. Pr_0.3Nd_0.7MnO₃) in the χ′ value which is identified as paramagnetic to ferrimagnetic transition. The peak value shifts to higher temperature with decrease of x and width of the maximum broadened. It is also observable that with decrease of Nd, both the value of χ′ and χ″ decrease. An attempt is made to explain the magnetization reversal within the framework of available models.     

Magnetic Phase Transition and Electronic Anomalies in La1.96−x Y0.04Sr x CuO4 around x∼1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of T _c and the structural transformation around x1/8 in La _1.96–x Y _0.04 Sr _x CuO ₄. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x0.115 below temperatures T _s and T _N, respectively. T _s vs x shows a bell shaped curve with the maximum value of 65 K at x00115 where most prominent suppression of T _c appears. T _N shows similar x dependence to T _s with the maximum value of 40 K at x0.115. The change of the electronic state below T _s and the suppression of T _c become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.     

Raman Studies of Anisotropic Magnetic Excitations in Fluctuating Nematic Striped La2−x Sr x CuO4 and the Comparison to Uniform Nd2−x Ce x CuO4

The mechanism of the high temperature hole-doped superconductivity was investigated by Raman scattering. The Raman selection rule is unique, so that anisotropic magnetic excitations in a fluctuating spin-charge stripe can be detected as if it is static. We use different Raman selection rules for two kinds of magnetic Raman scattering processes, two-magnon scattering and high-energy electronic scattering. In order to confirm the stripe effect, the Raman spectra of striped La_2?x Sr _x CuO₄ (LSCO) and nonstriped Nd_2?x Ce _x CuO₄ (NCCO) were compared. The main results in LSCO are (1) magnetic excitations are presented by individual energy dispersions for the k∥ stripe and the k⊥ stripe, (2) the charge transfer is allowed only in the direction perpendicular to the stripe. The direction is the same as the Burgers vector of an edge dislocation. Hence, we assume that a charge moves together with the edge dislocation of the charge stripe. The superconducting coherence length is close to the intercharge stripe distance at x<0.2. Therefore, we propose a model that superconducting pairs are formed in the edge dislocations. The binding energy is related to the stripe formation energy.     

Composition range and physicochemical properties of La1 − x Ba x Mn1 − y Fe y O3 solid solutions

We have determined the extent of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ solid solutions with orthorhombically and rhombohedrally distorted perovskite structures. A partial phase diagram of the LaMnO_{3 + δ}-BaMnO₃-BaFeO_2.5-LaFeO₃ system in air at a temperature of 1373 K has been proposed for the first time. We have measured the relative length change of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ samples and calculated their thermal expansion coefficients.     

Magnetic and Magnetocaloric Properties of Laves Phase Dy1−x Gd x (Co1−x Ni x )2 Solid Solutions

We report magnetic and magnetocaloric properties of the polycrystalline series of Dy_1?x Gd _x (Co_1?x Ni _x )₂ (x=0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions. The samples were characterized by powder X-ray diffraction patterns taken at room temperature and revealed that all the Dy_1?x Gd _x (Co_1?x Ni _x )₂ solid solutions consist of the C15 cubic Laves phase MgCu₂ type structure and a small amount of DyCo₃ and Dy₂O₃ impurity phases. Magnetic measurements showed that the samples undergoes a second-order type phase transition at T _C<130 K, from paramagnetic to ferromagnetic state. Heat capacity measurements have been performed for all solid solutions and allowed us to determine the Debye temperature. The magnetocaloric effect has been studied by means of specific heat measurements in magnetic field 0.42, 1 and 2 T. The GdNi₂ substitution effect on magnetic and magnetocaloric properties will be discussed.     

Pressure effect on anomalous suppression ofT c around x=1/8 in La2−xBaxCuO4 and La1.8−xx Nd0.2BaxCuO4

In La_2–xBa_xCuO₄ (LBCO) and La_1.8–xNd_0.2Ba_xCuO₄ (LNBCO), the low-temperature structural change at T₁ (=60–110K), resulting from tilt of the CuO₆ octahedra, is easily suppressed at high pressures. However, the structural change of LNBCO (x=0.125) remains up to 2GPa though it jumps from T₁(110K) to T* (60K), where the electric resistivity begins to upturn, at 0-1.5GPa. The upturn of is also suppressed at high pressures, and the superconducting critical temperature T_c is fully restored except in a very narrow x-range around x=0.125. The T_c for x 0.125 is appreciably lower at 2GPa than those for other Ba-contents.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.