Electronic phase separation in a low-temperature tetragonal phase of lanthanum-strontium cuprates according to 139La NQR data

A nonuniform electron density distribution is observed in La_{1−x− y} Nd_ySr_xCuO₄ and La_1−x−y Eu_ySr_xCuO₄, and long-lived magnetic fluctuations in these compounds are studied. The dynamics of the magnetic fluctuations depends strongly on the magnetic properties of the rare-earth ions, which stabilize the low-temperature tetragonal phase. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 344–349 (10 March 1998)     

Low-temperature metal-insuslator transition and magnetic properties in the VxMn1−x S disordered system

A study of the structure and electrical and magnetic properties of the V_xMn_1−x S disordered system is reported. The existence of a low-temperature metal-insulator transition for Fermi-glass 0.4<x<0.5 compositions in paramagnetic phase, which is accompanied by a change in the structure and magnetic properties, has been established. An analysis of the magnetic properties permits a conjecture that current carriers become delocalized in these solid solutions at the metal-insulator transition temperature to form small ferromagnetically ordered regions (ferrons). Fiz. Tverd. Tela (St. Petersburg) 39, 1428–1431 (August 1997)     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

Structural and magnetic characterization of Bi1–xLaxFeO3 and BiFe1–yMnyO3 nanoparticles synthesized via a sol–gel method

Here, La-doped Bi_1–xLa_xFeO₃ and Mn-doped BiFe_1–yMn_yO₃ (x, y?=?0.1, 0.2, 0.3) nanoparticles were synthesized by a sol–gel process. For Bi_1–xLa_xFeO₃, structural analysis suggested that its structure changed from rhombohedral to tetragonal phase without secondary phase when x increased from 0 to 0.3. In addition, with the increasing doping concentration of La, transmission electron microscopy exhibited the doped nanoparticle size reduced, while the magnetic properties were correspondingly enhanced. However, for synthesis of BiFe_1–yMn_yO₃, phase analysis showed that there easily existed secondary phase for nanoparticles with 10% and 30% Mn-doped, while 20% Mn-doped nanoparticles indicated high crystallinity without any impure phases. Similarly, we observed the enhanced magnetic properties with the increase of Mn concentration.     

Aspects of magnetic ordering in the system (FexCr1−x )2O3

Anomalon in temperature dependence of the magnetic moment (FexCr_1−x )₂O₃ single crystal has been observed which can be explained by antiparallel orientation of the mean weakly-ferromagnetic moments of the d subsystems (weak ferromagnetism). The disappearance of the linear magneto-electric effect for x=0.1 below 45 K is explained by a spontaneous phase transition from a conical spiral to a cycloidal spin structure, which is found to agree with a thermodynamic treatment. Fiz. Tverd. Tela (St. Petersburg) 39, 112–114 (January 1997)     

Characteristics of the clustered structure of Pb(Li, La)(Zr1−y Tiy)O3 in the antiferroelectric-ferroelectric transition region

Mesoscopic inhomogeneities and their influence on the behavior of the physical properties (optical reflectivity, dielectric constant, and Curie temperature) of a system of lead zirconate-titanate Pb_0.85(Li_1/2La_1/2)_0.15(Zr_1−y Ti_y)O₃ solid solutions are investigated. It is established that the disruptions of long-range order have a mesoscopic scale and generate ion groups in the form of interlayers of elements of binary-oxide planes coherently embedded in the crystal structure of the host phase. It is shown that the clustering of the structure is manifested in nonlinearity of the physical properties of the lead zirconate-titanate solid solutions, where the most pronounced variations are characteristic of the coexistence region of the antiferroelectric and ferroelectric phases. Zh. Tekh. Fiz. 68, 43–47 (February 1998)     

Crystalline structural phase boundaries in (K,Na,Li)NbO 3 ceramics

Lead-free (K_xNa_1−x)_1−yLi_yNbO₃ ceramics (with x=0.50, y=0–0.08 and x=0.30–0.70, y=0.06, respectively) were synthesized using the conventional solid-state reaction technique. Compositional influences of K, Na and Li constituents on microstructures, crystalline structures, dielectric and piezoelectric properties were investigated. It has been known that microstructures change largely with the alkali constituents and that there exist three orthorhombic-tetragonal phase boundaries at room temperature. One occurs in (K_0.50Na_0.50)_1−yLi_yNbO₃ ceramics at y=0.05–0.06, which corresponds to the previously reported morphotropic phase boundary (MPB). The other two appear in (K_xNa_1−x)_0.94Li_0.06NbO₃ ceramics near x=0.40 and x=0.60, respectively. (K_0.50Na_0.50)_0.935Li_0.065NbO₃ and (K_0.45Na_0.55)_0.94Li_0.06NbO₃ ceramics show high piezoelectric properties with the d₃₃ values over 200 pC/N and the k_p values around 45% at room temperature. It is thought that the observed high piezoelectric properties are largely affected by the temperature-driven orthorhombic-tetragonal phase transition.     

Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

Interpretation of neutron scattering and μSR data on paramagnetic Gd in terms of magnetic clusters and coherent dissipative structures

Fullyc-axis oriented thin films of YBa₂(Cu_1−x ⁵⁷Fe _x )₃O_7−y were studied by varying the Fe- and O-content and the substrate. For the two substrates MgO and SrTiO₃ no difference in the CEMS spectra was observed. Measurements for differentx andy values and their comparison with results from powder samples proved that the solubility of iron is the same in the films and powders. The substitution of Cu(2) by Fe in the films is the highest ever reported. The CEMS spectrum of a reduced film shows that the direction of the magnetic hyperfine field for doubletC is perpendicular to thec-axis.     

Anomalies of the electronic properties of alloys in the presence of variation of the local magnetic moments

The concentration dependences of the electronic properties (residual electrical resistivity, diffusion thermoelectric power, normal Hall effect, and low-temperature specific heat) and the magnetic characteristics (magnetization and paramagnetic susceptibility) of quasibinary (Pd_xPt_1−x )₃Fe, Pt₃Mn_xFe_1−x , (Pd_xAu_1−x )₃Fe, and Sc_xTi_1−x Fe₂ alloys are investigated. A relationship is established between the anomalous behavior of the kinetic properties and the variation of the local magnetic moments. The absence of corresponding anomalies in the concentration dependence of the specific heat indicates that the density of states at the Fermi level does not change significantly and, therefore, that the conventional Mott two-band model cannot be used to describe the anomalies in the properties of the alloys in question. A single interpretation of the sum-total of the experimental results is given on the basis of the theory of local fluctuations of the electron spin density in metal magnets. Fiz. Tverd. Tela (St. Petersburg) 39, 1257–1262 (July 1997)     

Effect of next-nearest-neighbour interaction on dx2?y2-wave superconducting phase in 2D t-J model

An exact diagonalization calculation of the t-J model on 2D square cluster has been studied for the ground state properties of HTSC. Effect of next-nearest-neighbour hopping and magnetic (both antiferromagnetic and ferromagnetic) interaction on d _x ²−y ²-wave pairing has been shown. Relative strength of the next-nearest-neighbour interaction with respect to that of near-neighbour interaction for the strongest d _x ²−y ²-wave pairing has been estimated. A schematic phase diagram is shown. It is shown that a two-sublattice model with antiferromagnetic interaction between them and a small intra-ferromagnetictype interaction in one sublattice favours d _x ²−y ²-wave superconductivity and moderate negative type NNN hopping adds flavours to this phase.     

Nonequilibrium properties of re-entrant spin glasses in a magnetic field

In this paper the authors discuss how the re-entrant spin-glass state arises in the disordered alloys Ni_100−x Mn_x (x=19, 21, 23), along with the nonequilibrium magnetic properties of these alloys. It is shown that near the Gabay-Toulouse phase line the time it takes the system to reach equilibrium is comparable to times required to perform static experiments (10¹–10⁴ s); cooling the sample into the region of crossover with the de Almeida-Thouless line causes these times to increase to astronomical values of more than 10¹⁶ s. A method is proposed for constructing magnetic phase diagrams of systems of this type in “magnetic field-temperature” coordinates. Fiz. Tverd. Tela (St. Petersburg) 41, 2028–2033 (November 1999)     

Metals near a magnetic instability

Zero-temperature magnetic phase transitions exhibit an abundance of nearly critical magnetic fluctuations that allow to probe the traditional concepts of the metallic state. For the prototypical heavy-fermion compound, CeCu_6−x Au _x , a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field. The d-electron weak itinerant ferromagnet ZrZn₂, on the other hand, was recently found to display superconductivity in coexistence with ferromagnetism.     

Preparation and characterization of ternary magnetic semiconductors

Summary Crystal growth processes, the characterization of properties and the application prospects for ternary magnetic semiconductors (TMS) are reviewed. The regulation of the TMS properties can be achieved either by composition change while growing crystals or by further thermal treatment. The compounds CdCr₂Se₄, CdCr₂S₄, Cu _y Cr₂Se_4−z Br _x have been selected for discussion. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Effects of Pr-Al co-substitution on the magnetic and structural properties of M-type Ca-Sr hexaferrites

In this research, a series of Pr-Al co-substituted M-type hexaferrites with the chemical composition of Ca_0.4Sr_0.6-xPr_xFe_12.0-yAl_yO₁₉ (0.00 ≤ x ≤ 0.40, 0.00 ≤ y ≤ 0.60) were synthesized by the standard ceramic method. The phase identification of the samples was confirmed by X-ray diffraction analysis. A single magnetoplumbite phase is exhibited in the hexaferrites with the substitutiom of Pr (0.00 ≤ x ≤ 0.32) and Al (0.00 ≤ y ≤ 0.48) contents. For the hexaferrite containing Pr (x = 0.40) and Al (y = 0.60), an impurity phase α-Fe₂O₃ is observed in the structure. The morphology of the hexaferrites was analyzed by field emission scanning electron microscopy (FE-SEM). FE-SEM micrographs show that the hexaferrites with different Pr-Al contents have formed hexagonal structures, and the grain size of the magnets decreases with increasing Pr-Al content. A magnetic property measurement system was used to measure the magnetic properties of the hexaferrites. The remanence (B_r) and maximum energy product [(BH)_max] decrease with increasing Pr-Al content (0.00 ≤ x ≤ 0.40, (0.00 ≤ y ≤ 0.60). The intrinsic coercivity (H_cj) increases with increasing Pr-Al content (0.00 ≤ x ≤ 0.40, (0.00 ≤ y ≤ 0.60). The magnetic induction coercivity (H_cb) and H_k/H_cj ratio first increase with increasing Pr-Al content (0.00 ≤ x ≤ 0.24, 0.00 ≤ y ≤ 0.36) and then decrease with increasing Pr-Al content (0.24 ≤ x ≤ 0.40, 0.36 ≤ y ≤ 0.60).     

Magnetic and dielectric properties of orthorhombic and hexagonal multiferroics Tb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript>

The transition from a stable orthorhombic structure to a hexagonal structure has been revealed in Tb_1−x Y _x MnO₃ multiferroics at x = 0.2–0.4. It has been shown that almost single-phase crystals with an orthorhombic or hexagonal structure can be obtained by choosing the growth conditions. It has been found that the magnetic and dielectric properties of orthorhombic single crystals with x = 0.2–0.3 are similar to the properties of pure TbMnO₃ and are characterized by a strong anisotropy of the magnetic susceptibility at low temperatures and by the presence of a number of magnetic phase transitions, including those to the ferroelectric state. New spontaneous (T ≤ 15 K) and magnetic-field induced (H | C ₆) phase transitions accompanied by the appearance of an uncompensated rare-earth magnetic moment ∼1 μ_B/mole have been observed in hexagonal single crystals with x = 0.3–0.5.     

Characteristics of magnetic order in perovskite manganites La1−x CaxMnO3

Solid solutions of the system La_1−x Ca_xMnO₃ are synthesized, and their magnetic and electrical properties are investigated. As x is increased, the crystal lattice changes symmetrically from an orthorhombic (x=0 and x=0.8) to a cubic structure (0<x⩽0.6 and x=1). Nonstoichiometric LaMnO₃ and compositions with a low Ca content (0<x<0.4) are collinear ferromagnets. A metal-insulator transition is observed in them near the Curie temperature. Compositions with 0.6⩽x⩽0.9 exhibit a semiconductor-type conductivity and an unusual temperature behavior of the magnetization and the susceptibility with very distinct Curie and Néel temperatures. The magnetic properties of the solid solutions are explained on the basis of the model of ferro-antiferromagnetic phase separation. Zh. éksp. Teor. Fiz. 116, 1664–1675 (November 1999)     

Mesoscopic oscillations of the magnetization near Ising impurity centers in strong magnetic fields

The frequency of orientational quantum oscillations of the magnetization near impurity-ion clusters with Ising properties in a saturated magnetic crystal is calculated. It is noted that in compounds of the type Ho_xY_3−x Fe₅O₁₂, where magnetic phase transitions are observed, additional magnetization reversal and magnetic resonance features due to mesoscopic oscillations of the magnetization can be observed at low concentrations x<0.001 and cryogenic temperatures in fields comparable to the intersublattice exchange interaction field. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 445–448 (25 March 1997)