Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.     

Orthorhombic structure: a necessity in superconducting 1-2-3 compounds

X-ray and resistivity measurements on YBa₂Cu₃O_7−δ (1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase shows aT _c ∼ 31 K. The effect of oxygen concentration onT _c is isolated.T _c=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the superconducting mechanism to be operative.     

A correlation between the structural and magnetic phase transitions for the mixed systems: KMn1-xNixF3 and KMn1-xCoxF3. II. Magnetic properties

Susceptibility measurements at temperatures from 55 K to 620 K are reported for the mixed crystals KMn_1-xCo_xF₃ where x 0.10. All the compounds show antiferromagnetism and a transition to a weak ferromagnetic phase below T _N . The observed phase transition AF—WF is very sensitive to the presence of admixtures, which changes not only the phase transition temperature, but also it's character. A strong correlation between the structural and magnetic phase transition is evident from our study.     

39K quadrupole coupling and spin-lattice relaxation in the high temperature phases in KLiSO4

³⁹K quadrupole perturbed nuclear magnetic resonance spectra show that in KLiSO₄ atT _c =743 K a phase transition from a room temperature hexagonal to a high temperature orthorhombic phase takes place. The high temperature phase is definitely not incommensurately modulated. The huge shortening of the³⁹K spin-lattice relaxation time on approachingT _c from below demonstrates that KLiSO₄ becomes a superionic conductor above 743 K. The self-diffusion coefficient of the Li-ions is estimated asD=10^–6 cm²/s at 780 K.     

Metal-insulator transition in electron-doped Ba1−x La x MnO3 compounds

Electron-doped (Ba_1−x La _x )MnO₃ compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (T _c) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and T _c of M-I transition. The resistivity data below T _c could be fitted to the expression ρ=ρ ₁+ρ ₂ T ² and this shows that double exchange interaction plays a major role even in Mn⁴⁺-rich compound. Above T _c the resistivity data were fitted to variable range hopping and small polaron models.     

Superconductivity magnetism and low temperature specific heat in YBa2(Cu1−xFex)3O7−δ with δ0 and δ1

Magnetic and low temperature specific heat measurements have been performed on iron doped YBa₂(Cu_1−xFe_x)₃O_7−δ samples with different oxygen contents (δ0 and δ1). Iron doping induces an orthorhombic to tetragonal transition and a decrease of both T_c and diamagnetic signal. Low temperature specific heat measurements reveal a Schottky type anomaly for δ0 samples with x=0.01 (1.8 K) and x=0.02 (3 K). This anomaly is attributed to magnetic interactions within iron limited chains. A numerical analysis of this effect is proposed.     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

Structural transition and enhanced magnetization in Bi1−xYxFeO3

Bi_1−xY_xFeO₃ (x=0-0.2) powders were prepared to study the effect of Y substitution on their structural and magnetic properties. A structural symmetric breaking from the rhombohedral R3c to orthorhombic Pnma at around x=0.10 was identified across a ferroelectric-paraelectric phase. A parabolic dependence of the magnetization upon substitution was obtained with a maximum at the phase transition boundary and a switching behavior for x=0.20. The composition-driven magnetic structure evolution was proposed to account for the magnetic properties in Bi_1−xY_xFeO₃.     

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method ( ⁺SR) is applied on La_2–x Sr _x CuO₄ around forx=0.12 at which the high-T _c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105x0.120. The magnetic phase boundary is very similar to the one of the La_2–x Ba _x CuO₄ in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO₂ plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T _c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

Structural phase transition in superconducting BaPb0.75Bi0.25O3

The room temperature crystallographic properties of distorted perovskite-type system BaPb_1-xBi_xO₃ were examined by using X-ray diffraction analysis. In addition, structural temperature changes were investigated for superconducting BaPb_0.75Bi_0.25O₃ crystals with T_c=11.4K. It was found that the room temperature structure of BaPb_1-xBi_xO₃ has an orthorhombic symmetry in the range of O≤×?0.9 and a monoclinic one in 0.9?×≤1, and that superconducting BaPb_0.75Bi_0.25O₃ shows a structural transition from an orthorhombic to a monoclinic phase at about 160K.     

Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni_2+x Mn_1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T _M and magnetic T _C (Curie point) phase transitions to converge. In the region where T _C =T _M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)     

Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd₅Si_xGe_4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd₅(Si,Ge)₄-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm⁻¹) permittivity and dynamic conductivity of La_1−x Sr _x MnO₃ (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La_1−2x Sr _x MnO₃ is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 331–336 (25 August 1998)     

Hysteresis behavior in Raman intensities in YBa2Cu3O7-y -family superconductors between T c and T o = 234 K

The intensity of the 646 cm^-1 Raman line in YBa₂Cu_3-x V _x O_7-y has been measured for cooling and heating from 77 K to 234 K. The intensity exhibits intensity hysteresis of order 20% in the cooling versus heating values between T _c = 89 K and T _o = 234 K. This effect is unexplained but is extremely similar to that reported by Ledbetter et al. for the bulk modulus and C ₁₁ elastic coefficient. In each case the hysteresis vanishes below T _c , independent of the value of T _c , which may be adjusted from 60 K to 94 K by changing oxygen concentration y. Qualitative interpretation requires that the structural phase transition near 234 K be intimately related to the superconducting transition at T _c presumably through partial ordering of oxygens.     

Phase transformations and magnetotransport properties of the Pr0.5Sr0.5Co1 ? xMnxO3 system

The structural, magnetic, and magnetotransport properties of Pr_0.5Sr_0.5Co_{1 − x} Mn _x O₃ (x < 0.65) perovskites are studied by magnetization and electrical conductivity measurements in magnetic fields up to 14 T and by neutron diffraction. In the manganese concentration range x < 0.5 and T = 300 K, the crystal structure is described by monoclinic space group I2/a; at x > 0.5, it is described by orthorhombic space group Imma. When the temperature decreases, a structural transformation without changing the symmetry takes place in all compounds. This transformation is caused by an active role of the inner shells of the praseodymium ion in chemical bond formation. The substitution of manganese for cobalt breaks a long-range ferromagnetic order near x ≈ 0.25, and a metal-dielectric transition occurs at x ≈ 0.15. The negative magnetoresistance is found to be maximal near a critical manganese concentration, where a long-range magnetic order is broken; it reaches 95% in a field of 14 T at T = 10 K for x = 0.2. An unusual dielectric magnetic state with a small spontaneous magnetic moment and a sharp transition into a paramagnetic state at T > 200 K is revealed in the concentration range 0.30 ≤ x ≤ 0.65 in spite of the absence of coherent magnetic neutron scattering. A model is proposed to explain the behavior of the magnetic properties in this phase.