Magnetic and transport properties of electron-doped Ca3−x Bi x Mn2O7

Ca_3?x Bi _x Mn₂O₇ with the nominal composition x=0.05, 0.1, 0.2 and 0.3 is synthesized by solid-state reaction. The refined X-ray diffraction pattern of Ca_2.807Bi_0.193Mn₂O₇ with the nominal Bi³⁺ content x=0.2 indicates that about 71 % of the Bi³⁺ ion enters into the Ca²⁺ (2a) site and the remaining 29 % is in the Ca²⁺ (4e) site. The doped Bi³⁺ ion produces a ferromagnetic component in the antiferromagnetic matrix. Below the transition temperature, at about 110 K, the ferromagnetic and antiferromagnetic interactions coexist. The alignment of the magnetic moment is canted at 5 K. The electric transport shows insulating behavior. Around the magnetic transition, at about 110 K, the resistance sharply drops like a well. A model proposed by Glazman and Matveev (GM model) is applied to the thermal variation of the resistance from 40 K to 138 K. Above this temperature, it is due to thermally activated hopping of small polarons with the activation energy of 50 meV. A negative magnetoresistance, 17 %, is observed with the doping content as low as 0.05. The magnetoresistance is due to the spin-polarized inelastic tunneling through nonmagnetic localized states embedded in an insulating barrier.     

Correlation between the magnetic and electrical properties of the (VS)x(Fe2O3)2−x oxysulfide system

The correlation between the magnetic and electrical properties of the (VS)_x(Fe₂O₃)_2?x (0.9<x<1.25) oxysulfide solid solutions has been studied. The crossover of conductivity from the semimetallic to semiconducting type is accompanied by changes in the magnetic susceptibility, which are characteristic of the transition from delocalized to localized electrons. For x=1.25, a region of the ferromagnetic ordering has been established in the temperature range 90–120 K.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Magnetic and transport properties of CuCr1−x VxS2 compounds

A comparison of experimental and calculated effective magnetic moments shows that the sulfur in CuCr_1−x V_xS₂ compounds has two different oxidation states, S¹⁻ and S²⁻, while the vanadium has a higher degree of oxidation (V³⁺) than the chromium (Cr²⁺). A model is proposed to explain the reduction in the activation energy for defect formation in the Cu-sublattice and the rise in the Cu⁺-cation conductivity when chromium is replaced by vanadium. Fiz. Tverd. Tela (St. Petersburg) 41, 1450–1451 (August 1999)     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

Dielectric and magnetic properties of Bi1−xYxFeO3 ceramics

Y doped BiFeO₃ polycrystalline ceramics were prepared by sol–gel method. Crystal structure examined by X-ray diffraction indicates that the samples were single-phase and crystallize in rhombohedral structure. An anomaly in the dielectric constant and dielectric loss in the vicinity of the antiferromagnetic Neel temperature (T_N) was observed. Saturated magnetization loops were observed for all sample with saturated magnetization M_s=0.678 emu/g and remnant magnetization M_r=0.084 emu/g for x=0.3.     

Magnetic properties of GdBa2Cu3O7−x in the normal and superconducting states

The magnetic properties of superconducting GdBa₂Cu₃O_7–x withT _c95 K are measured. In the normal state, its susceptibility closely follows the Curie law with an effective moment of 7.77 _B .H _c1 is determined to be 650 Oe. No superconducting fluctuation can be delineated. Flux pinning indicated behavior of type II superconductors. A 25–405 diamagnetic state indicating bulk superconductivity in GdBa₂Cu₃O_7–x .     

Optical properties and structural phase transitions in Mg x Zn1−x O under hydrostatic pressure

This article reports on the optical properties under pressure of Mg _x Zn_1?x O (x?<?0.13?±?0.2) thin films deposited on mica and fluorite substrates by pulsed-laser deposition. The absorption edge of the semiconductor alloy is measured in both the ambient pressure wurtzite phase and the high-pressure rock-salt phase for several Mg contents. In wurtzite Mg _x Zn_1?x O, a larger value of the band-gap is clearly correlated to a larger pressure coefficient of the band-gap. This effect is shown to be consistent with the decreasing contribution of cation d-levels to the valence band maximum states as the Mg content increases. The wurtzite-to-rock-salt transition pressure is observed to decrease from 9.5?±?0.2 (pure ZnO) to 7.0?±?0.2?GPa (for x?=?0.13), with an almost linear dependence on the Mg content. The same linear dependence on x, with virtually the same slope, is also found for the rock-salt-to-wurtzite reverse transition in the pressure down-stroke. For x?>?0.13, the rock-salt phase is observed to be metastable at room pressure, after a pressure cycle up to 15?GPa.     

Transport and magnetic properties of bulk polycrystalline (YBa2Cu3O7)1−x(Nd0.7Sr0.3MnO3)x nanocomposites

We have prepared a series of bulk polycrystalline samples with the nominal compositions (YBa₂Cu₃O₇)_1?x(Nd_0.7Sr_0.3MnO₃)_x (x = 0–1) by a conventional solid-state reaction method using the larger difference in sintering temperature of the two constituent oxides and a well conceived sintering sequence. XRD patterns show that the samples are composites consisting of YBa₂Cu₃O₇ and Nd_0.7Sr_0.3MnO₃ particles with average grain size of ～65 nm. For x ? 0.55, with increasing x, the zero-resistance superconducting transition temperature, T_C0, measured at zero magnetic field decreases and the normal state resistivity increases rapidly. The T_C0 for the sample with x ～ 0.48 is estimated to be 0 K. The M–H hysteresis loops indicate the coexistence of ferromagnetism and superconductivity in the samples. The depression of T_C0 can be attributed to the proximity effect between ferromagnetism and superconductivity.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Temperature hysteresis of magnetic phase transitions in Tb1 − x Ce x Mn2O5 (x = 0, 0.20, 0.25)

The temperature evolution of the magnetic structure of multiferroics Tb_{1 ? x} Ce _x Mn₂O₅ (x = 0, 0.20, 0.25) has been investigated using the neutron scattering methods. It has been found that, despite the qualitative similarity of the magnetic states and the series of phase transitions for pure TbMn₂O₅ (TMO) and doped crystals Tb_{1 ? x} Ce _x Mn₂O₅ (TCMO, x = 0.20 and 0.25), there are significant differences in their properties. In contrast to TMO, where there are three magnetic phases, TCMO can include two magnetic phases that coexist in a wide temperature range and exhibit a rather wide temperature hysteresis. One of these phases with wave vector k ₁ = (0.5, 0, k _z1), k _z1 = 0.25, is commensurate and arises at temperatures below T _N ～ 39 K (for x = 0.2) and T _N ～ 38 K (x = 0.25). The second phase is incommensurate with wave vector k ₂ = (1/2, 0, k _z2), k _z2 = 0.256(2), and appears upon cooling at T = 21 K (x = 0.2) and T = 19 K (x = 0.25). Upon further cooling to 16 K, the component k _z2 increases to 0.292(2) and then remains constant. The component k _z1 increases to the value of 0.280(2) upon cooling in the range from 15 to 10 K and then remains constant down to 1.5 K. With an increase in the temperature, the components k _z1 and k _z2 undergo reverse changes to their initial values, but these changes occur at temperatures 7 K higher than those observed with a decrease in the temperature. For TMO, two phases also coexist, but the temperature hysteresis in this case is considerably smaller than for TCMO. This is explained by different densities of domain walls and different sizes of domains in pure and doped crystals.     

A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe1−xCox)2As2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe_1?xCo_x)₂As₂ superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Phase transitions in the ferroelectrics (Sn1−x In(2/3)x )2P2S6 at high pressures

New ferroelectric solid solutions (Sn_1−x In_(2/3)x )₂P₂S₆ were investigated at high hydrostatic pressures. The range in which the incommensurate structure exists was determined. A dynamic shift of the incommensurate-ferroelectric phase transition temperature with increasing rate of change of temperature and the appearance of “reverse hysteresis” were observed. The characteristic features in the appearance of the latter effect in these crystals are investigated. Fiz. Tverd. Telsa (St. Petersburg) 41, 1276–1278 (July 1999)