Search

Substitutional studies have been carried out on sintered bulk samples of YBCO to determine the effect of doping it with Pr, Ca (at Y site) and Ni (at Cu site) in small concentrations on the formation of effective pinning centres for jc enhancement. The results show Pr and Ca substitutions to be far more effective than Ni substitution. The former two give rise to formation of rows of oxygen vacancies with pronounced strain fields in CuO2 planes where superconductivity resides. The defects thus formed seem to be analogous to arrays of normal nanodots in CuO2 planes which are expected to have strong intragrain pinning interactions with pancake vortices of CuO2 planes forming the mixed state structure. Both these substitutions tend to marginalize the adverse effects of grain boundaries. The superconducting volume fraction of the pristine sample is significantly raised by both these substitutions. These special features are, however, absent with Ni substitution. This paper brings out the importance of Pr substitution in significantly enhancing jc(B).

We report the synthesis, high-resolution micro-structure, magneto-transport and magnetization of nano-diamond doped MgB2?nDx with x = 0.0?0.1. The superconducting transition temperature (Tc) is not affected by x up to x = 0.05, indicating that the added nano-diamond (1) does not decompose to C and (2) does not partially substitute for B in MgB2. R(T) versus H measurements show higher Tc values under the same applied magnetic field for the nano-diamond added samples, resulting in higher estimated Hc2 values. Isothermal magnetization measurements show that above 2?T, the critical current density (jc) is of the order of 105?A?cm?2 for the pristine sample. jc is further increased to three times for 3% nano-diamond doped samples. High-resolution transmission electron microscopy (HRTEM) observations clearly show the dispersion of nano-diamond particles, with an average particle size of 8?10?nm, in the MgB2 matrix. It seems likely that the dispersed nano-diamond particles of below 10 nm in size are acting as effective pinning centres responsible for improving the superconducting performance of the parent MgB2.

The temperature dependence of the resistivity, ρ, of ceramic LaMnO3+δ with δ = 0.065 and 0.154 (or simply # 065 and # 154), is investigated under hydrostatic pressure, p, between the temperatures T = 80 and 320 K in magnetic fields up to 10 T. The data for ρ(T) are analysed within a Shklovskii–Efros-like variable-range hopping conductivity model, governed by a soft Coulomb gap, Δ, and a rigid gap, γ, in the density of localized states. The validity of this approach to the resistivity of LaMnO3+δ under ambient pressure was established in recent investigations (Laiho et al 2005 J. Phys: Condens. Matter 17, 105). In the present work it is shown that upon increasing p up to 11 kbar (# 154) and 13 kbar (# 065) the magnetic Curie temperature, TC, is increased by ~12–13%. Correspondingly, Δ(p) is decreased by ~7–8% and γ(p) by ~8% in # 154 and 12% in # 065. Similarly, the electron localization radius a(p) is increased by ~4% in # 154 and 8% in # 065, respectively. The influence of pressure on TC, Δ, γ and a is interpreted by increasing the electron bandwidth and decreasing the polaron potential well, which stimulates the delocalization of electrons when p is increased.

The Hall effect, electrical conductivity and electron mobility are investigated at temperatures between 55 and 500 K in n-ZnSe crystals doped with Cu, Ag or Au. The presence of a small amount of Cu atoms leads to an inversion of the sign of the Hall coefficient at temperatures above 300 K. Anomalous temperature dependence of the electron mobility is observed in the samples with low Cu concentration (

YBCO films were prepared by pulsed laser deposition from nanocrystalline targets doped with different concentrations of BaZrO3 ranging from c = 0.9 to 9.0 wt%. The critical temperature of the films decreases almost linearly with increasing BaZrO3 content whereas the critical current density shows a maximum near 3.9 wt%. In comparison with undoped YBCO films the accommodation field B* is considerably enhanced and the critical current density is improved in high fields in the films doped with BaZrO3, e.g. for c = 3.9 wt% by factor of 4.5 in a field of 5 T at 5 K. In the doped films the BaZrO3 particles grow epiaxially with YBCO. Transmission electron microscopy results show that the density of the BaZrO3 particles increases with increasing doping but the particle size remains practically the same (5–10 nm).

The Hall effect in the II–V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to B = 25 T and temperatures between T = 2 and 77 K for samples oriented along the [0 1 0] and [0 0 1] crystallographic axes. The Hall coefficient, R (B, T), exhibits a sequence of an almost flat region followed by a descending interval and an upturn when B is increased. The decrease of R (B, T) is interpreted by the presence of two groups of electrons with concentrations n2(T) > n1(T) and mobilities μ2(T) < μ1(T). Analysis of n1(T) and n2(T) demonstrates that the high-mobility carriers n1 are the conduction band (CB) electrons, whereas the low-mobility carriers n2 are itinerant electrons of a lower resonant impurity band (IB), having at low T energies of Ei ~ 3–4 meV above the CB edge. In addition, near the CB edge lies a higher IB containing only localized electron states. The IBs are split by spin to states differing by an energy ΔEi ≈ 0.9 meV. The upturn of R (B, T) in the high-field region is explained by the redistribution of the electrons between the IBs due to the decrease of ΔEi when B is increased. The mobility of the CB electrons is determined presumably by strong anisotropic scattering on neutral impurity centres, accompanied at high T by isotropic scattering on acoustic phonons, whereas scattering from ionized impurities is small.

The Hall coefficient, electrical conductivity and electron mobility are investigated in Ag-doped n-ZnSe single crystals at temperatures between 8 and 500 K immediately after doping the samples and after storing them for 5 years under normal room conditions in darkness. The formation of donor-type Agi interstitial defects stimulated by time is found for the first time. After 5 years storage, the samples show a dramatic increase in the electrical conductivity, free electron concentration and concentration of shallow donors. A model that explains these changes by the displacement of Ag ions into interstitial sites due to lattice deformation forces is proposed. The formation of an Ag-donor impurity band in n-ZnSe:Zn:Ag crystals stored at room temperature is also studied.

Magnetic susceptibility, M-H plot, magnetoresistance and thermoelectric power of the RuSr2Eu1.5Ce0.5Cu2O10 superconductor are measured. Values of the magnetic transition temperature Tmag, superconductivity transition temperature Tc, upper critical field Hc2, chemical potential mu, and energy width for electric conduction W(sigma) are obtained from these measurements. It has been found that Tmag = 140 K, Tc = 25 K (33 K) from susceptibility (magnetoresistance) measurements, Hc2 (0) > 32 T, mu = 8 meV, and W(sigma) = 58.5 meV. These values are compared with other ruthenate superconductors, and resulting physical information is discussed., 18 pages of TEXT + FIGS. To appear in - J. PHYS. COND. MATT. (2006)

The magnetic properties of oriented CdSb single crystals doped with 2 at% of Ni are investigated. From measurements of magnetic irreversibility defined by deviation of the zero-field-cooled (ZFC) susceptibility from the field-cooled (FC) susceptibility, the value of the mean anisotropy field BK ~ 4 kG is obtained. The ZFC susceptibility displays a broad maximum at a blocking temperature, Tb, depending on B according to the law [Tb(B)/Tb(0)]1/2 = 1 ? B/BK with Tb(0) ~ 100 K. The field dependence of the magnetization exhibits saturation above ~20?30 kG with values of Ms different for B along the [1?0?0], [0?1?0] and [0?0?1] axes. The temperature dependence of Ms is weak, increasing slightly upon cooling the sample below ~100 K. The temperature dependence of the coercive field, Bc(T), is weak above Tb but is enhanced strongly with decreasing temperature below Tb. The anisotropy of Bc is inverted with respect to the anisotropy of Ms. Such behaviour can be attributed to spheroidal Ni-rich Ni1?xSbx nanoparticles with a high aspect ratio, broad size distribution and distribution of the orientation of the major axis around a preferred direction. The relation Bc BK and the anisotropies of Ms and Bc are consistent with reversal of the magnetization by the curling mode, whereas the Tb(B) dependence is typical of the coherent rotation mode. This difference is connected to the proximity of the average transversal cluster radius to a critical value for transition between the two magnetization reversal modes within a wide crossover interval, due to broad distribution of the cluster sizes.

The resistivity, ?, of ceramic La1?xBaxMnO3 with x = 0.02?0.10 corresponding to the concentrations of holes c?0.15?0.17 displays an activated behaviour both above and below the paramagnetic to ferromagnetic transition temperature TC = 175?209?K, obtained from measurements of the magnetization. Above T~310?390?K ?(T,x) is determined by nearest-neighbour hopping of small polarons with activation energy Ea = 0.20?0.22?eV. Below the onset temperature Tv = 250?280?K, depending on x, a Shklovskii?Efros-like variable-range hopping conductivity mechanism, governed by a soft temperature independent Coulomb gap, ??0.44?0.46?eV, and a rigid gap, ?(T), is found. For the range T~50?120?K, ?(T) is connected to the formation of small lattice polarons in conditions of strong electron?phonon interaction and lattice disorder. The rigid gap obeys a law ?(T)~T1/2 within two temperature intervals above and below TC, exhibits an inflection at TC and reaches at Tv a value of ?v?0.14?0.18?eV. Such behaviour suggests a spin dependent contribution to ?(T). The localization radius of the charge carriers, a, has different constant values within the temperature intervals where ?(T)~T1/2. With further decrease of T, a increases according to the law expected for small lattice polarons.

Structural and transport properties of ceramic LaMnO3+δ are investigated for δ = 0–0.154. According to x-ray diffraction measurements at room temperature the crystal structure of this compound varies from orthorhombic (Pbnm )f orδ = 0t or hombohedrally distorted cubic (Pm3m )f or δ = 0.065–0.112 and to rhombohedral (R ¯ 3c )c rystal symmetry for δ = 0.125–0.154. These structural modifications are confirmed by the Raman micro-spectroscopy measurements. The resistivity displays in the range δ = 0–0.154 an activated behaviour both above and below the paramagnetic (PM) to ferromagnetic transition temperature, TC .I n the f ieldof 8 T the relative magnetoresistance, �ρ( B)/ρ(0) ,r eaches at δ = 0.154 the values of −88% near TC and −98% at T ≈ 20 K. The resistivity of the PM phase of LaMnO3+δ with δ = 0.100–0.154 satisfies the Shklovskii–Efros-like variablerange hopping (VRH) conductivity law between TC ∼ 130–160 K and the VRH onset temperature Tv ∼ 250–270 K. The resistivity is governed by a complex energy dependence of the density of the localized states near the Fermi level, comprising a soft Coulomb gap � ≈ 0.43–0.48 eV and a rigid gap γ ≈ 0.14–0.17 eV, the latter being connected to formation of small polarons. (Some figures in this article are in colour only in the electronic version)

Magnetophonon resonance in quantum wells in a tilted magnetic field B is investigated. Measurements of the Hall coefficient and correspondingly of the carrier concentration as a functions of magnetic field and temperature are simultaneously performed. It is shown that the experimental data can be interpreted in terms of a great sensitivity to the effects of varying the two dimensional carrier concentration ns in a certain concentration interval. In other words, the observed angular dependence of the MPR amplitudes is a manifestation of dependence of ns on the magnitude of the magnetic field B. We believe that such a dependence can be relevant in general for the interpretation of magnetotransport in nondegenerate 2D electron gas.

The Hall effect in the anisotropic II–V group semiconductor p-CdSb is investigated at temperatures between T = 3.6 and 200 K and pulsed magnetic fields up to B = 25 T in unintentionally doped samples oriented along the crystallographic axes [100] and [010]. The Hall coefficient, R(B, T), with B ∥ [001] exhibits in low fields a flat region followed by a descending interval when B is increased. This behaviour is attributed to the presence of two groups of holes with concentrations p2(T) > p1(T) and mobilities μ2(T) Tcr p1 and p2 are related to the holes activated to the light- and heavy-hole bands, respectively. The analysis of μ1(T) and μ2(T) confirms the existence of the heavy-hole band or a non-equivalent maximum and two equivalent maxima of the light-hole valence band.

The resistivity of the anisotropic semiconductor p-CdSb is investigated in a wide temperature range of T = 1.9–300 K in pulsed magnetic fields up to B = 25 T. Two not intentionally doped single-crystalline samples oriented along the crystallographic axes [100] (no. 1) and [010] (no. 2) are used for measurements of the resistivity, ρ, in transversal magnetic field configuration. Below the resistivity follows the laws for B Bc, where the coefficients C and S do not depend on T, and . These are characteristics of nearest-neighbour hopping conductivity. The coefficients C and S depend on the direction of B, and their ratios agree completely with the values calculated with the components of the hole effective mass. The acceptor concentrations, and for no. 1 and no. 2, respectively, are relatively close to the critical value of the metal–insulator transition (MIT), . This leads to enhancement of the mean localization radii, in no. 1 and in no. 2, with respect to the value of far from the MIT determined by an asymptote of the wavefunction at a large distance from the impurity centre.

YBa2Cu3O6+x nanopowder, prepared by the citrate sol–gel method, is segregated by sedimentation in ethanol into three size groups with average particle heights of 0.7, 1.6 and 2.3 nm. The structural properties and composition of the powders, investigated by x-ray diffraction, atomic force microscopy, Auger electron spectroscopy and EPR-spectroscopy, show no clear differences, except the size. According to investigations by magnetometry and by non-resonant microwave absorption the as-prepared powder contains weak links which, however, disappear during the segregation. The magnetic susceptibility of the samples decreases with the decreasing particle size, in agreement with the susceptibility values calculated from the London equations for cylindrical particles smaller than the London penetration depth. In all three size groups the critical temperature of superconductivity is 92 K.

The resistivity, ρ, of ceramic La1−xCaxMn1−yFeyO3 with x = 0.3 and y = 0.0–0.09 is found to obey, between a temperature Tv ≈ 310–330 K and the ferromagnetic-to-paramagnetic transition temperature, TC = 259–119 K (decreasing with y), the Shklovskii–Efros-type variable-range hopping conductivity law, ρ(T) = ρ0 (T) exp [(T0 /T)1/2 ]. This behaviour is governed by generation of a soft Coulomb gap Δ ≈ 0.42 eV in the density of localized states and a rigid gap δ(T) ≈ δ(Tv)(T/Tv)1/2 with δ(Tv) ≈ 0.16, 0.13 and 0.12 eV at y = 0.03, 0.07 and 0.09, respectively. Deviations from the square root dependence of δ(T), decreasing when y is increased, are observed as T → TC. The prefactor of the resistivity follows the law ρ0 (T) ~ Tm, where m changes from 9/2 at y = 0 to 5/2 in the investigated samples with y = 0.03, 0.07 and 0.09, which is connected to introduction of an additional fluctuating short-range potential by doping with Fe.

In ceramic La1-xCaxMnO3 samples with 0≤x≤0.15 and the concentration of holes c = 0.18-0.22 between T = 77 and 340 K the resistivity, ρ, is shown to have an activated character both above and below the ferromagnetic Curie temperature, TC. The temperature dependence of the local activation energy, Eloc (T) = dln ρ(T)/d(kT)-1 gives evidence for a variable-range hopping conductivity over states of the Coulomb gap. The width of the gap Δ≈0.24 eV, the effective dielectric permeability κ≈3.6 and the carrier localization radius ξ≈5.1 A in the paramagnetic phase are determined. The value of ξ suggests hopping of small lattice polarons at T>TC. With decreasing temperature two successive transitions, at temperatures TC' and TC'', associated with divergence of the correlation length, are observed in the vicinity of TC. They can be interpreted in terms of critical increase (as T approaches TC' from higher temperatures) and decrease (when T shifts from TC'' towards lower temperatures) of the radius of magnetic polarons, generated by a small fraction of the lattice polarons as T→TC. The difference between TC and the temperature of the first divergence of ξ, TC'

Low-field magnetic properties of ceramic La1-xCaxMnO3 (0≤x≤0.4) are investigated between T = 5 and 310 K. The paramagnetic-ferromagnetic transition is observed in all the samples. The dependence of the Curie temperature, TC, on x is described within a model of spin polarons associated with electronic localization. Critical behaviour of the susceptibility χ-1(T ) ~ (T-TC)γ is observed for T>TC, with the critical exponents γ = 1.20±0.05 and γ* = 1.64±0.06 below and above the composition x≈0.18 corresponding to the Mn4+ ion concentration c ≈ 0.23, respectively. For the compound with x = 0.3 no temperature hysteresis of the resistivity is observed in magnetic fields between 0 and 8 T. In all the samples the field-cooled and zero-field-cooled magnetizations deviate below TC, the difference being approximately equal to the thermoremanent magnetization (TRM). Long-time relaxation of TRM in LCMO is observed for time scales up to 104 s. The relaxation rate reaches a maximum near a wait time tW ~ 103 s. The time dependence of TRM can be described with a stretched exponential law, as in spin or cluster glasses in conditions where the observation time is comparable with tW .

Magnetic properties of the II-V diluted magnetic semiconductor .Zn1 x Mnx/3As2 are investigated for xD 0:08, 0.10 and 0.13, betweenT D 3 and 500 K and in fields up to 60 kG. All samples show a steep decrease of the magnetization above 280 K. In low fields (5-80 G) the temperature dependence of the magnetic susceptibility is strongly irreversible below the blocking temperatureTb 250 K. Above 15 kG the difference between the zero-field-cooled and the field- cooled susceptibilities disappears. These features give evidence for the presence of two magnetic subsystems: (i) paramagnetic centres including a single Mn ion and an open or closed triple antiferromagnetic cluster of Mn 2+ and (ii) ferromagnetic MnAs nanoclusters. The size distribution of the MnAs clusters is described by two overlapping Gaussian functions with the maxima at R1D 2:6-3.1 nm andR2D 3:3-3.8 nm, depending onx.

Random-exchange metallic spin glass Ho5Co50Al45 is investigated by static and dynamic magnetization measurements. The specimen is initially characterized as spin-glass-like below the freezing point Tf~14 K. The freezing phenomenon at Tf is examined by static and dynamic scaling analysis. The results support the existence of a paramagnetic-spin-glass phase transition at Tg14.2 K, with critical exponents of the transition as = 6.4, = 3.3 and z = 11.8.

The preparation and magnetic properties of the diluted magnetic semiconductor , opening a novel group of compounds alloyed with transition metal elements, are reported for the first time. Single crystals of are obtained with a modified Bridgman method. For their structure is isomorphic to the parent compound. Magnetic properties of are investigated for x = 0.01, 0.05 and 0.1 between T = 2 and 500 K in fields up to 60 kG. All the samples show a steep decrease of magnetization well above 300 K. In low fields (2-200 G) the temperature dependence of the magnetic susceptibility is strongly irreversible below K. The magnetization displays nonlinear field dependence starting from -2 kG and reaching full saturation above 40 kG. Its temperature dependence is weak between 2 and 300 K. The magnetic properties are explained by the presence of MnAs clusters. The distribution of cluster sizes is described by three overlapping Gaussian functions with the maxima at 1.7, 2.4 and 3.3 nm. At 300 K the corresponding magnetic moments are found to be 1.5, 4.1 and 10.4 in units of .

A new phenomenon of periodic oscillations is observed in I-V characteristics of porous silicon under illumination by visible light. The measurements are performed at room temperature using a scanning tunnelling microscope. The heights of the oscillation peaks appear to be a linear function of the oscillation number. The experimental value of the Coulomb energy determined from the oscillation period is much smaller than . The oscillations are attributed to a Coulomb effect, i.e. to the periodic trapping of a multi-electron level in a quantum well within a Si nanocrystal under the combined influence of the Coulomb interaction among the carriers and the variation of the potential difference between the STM tip and the semiconductor surface.

Nanocrystalline powders have been prepared by a citrate gel modification of the sol - gel technique. Using a relatively low temperature, , and (i) a process of extracting oxygen and reoxidizing the precursor powder or (ii) a sequence of vacuum annealing and prolonged heating in air, it was possible to avoid sintering of the particles. According to x-ray powder diffraction analysis the particles have orthorhombic structure pertinent to the phase. Measurements made by atomic force microscopy show that they have flat shape with two maxima in the thickness distribution, one at 1.2 nm and the other at 3.8 nm , corresponding to one or three orthorhombic unit cell lengths of the crystal. The average width of the particles is 40 nm. A similar value could be obtained from XRD measurements and the width of microwave absorption line around zero applied field. Superconducting properties of the powders were investigated with magnetization measurements.

A variant of scanning tunnelling microscopy with spin-resolving properties, based on observation of circularly polarized luminescence from a semiconductor tip, is suggested. The method is analysed using a three-step model of electron transport, including tunnelling from a ferromagnetic sample to the tip through the vacuum gap, ballistic transport through the band-bending region of the tip subsurface and radiative recombination with holes in the semiconductor bulk. The tunnelling of spin-polarized electrons is treated with the transfer-Hamiltonian method. Then the model reveals a close connection between the degree of circular polarization of the light and the local spin polarization of the sample surface: namely, by making measurements with photons propagating in three non-collinear directions it would be possible to obtain a complete map of the distribution of surface spins with high spatial resolution without affecting the magnetization of the surface. The possibility of practical realization of such experiments is discussed.

Disorder of stacking of vacant sites in the crystal structure of II3V2 compounds, including the diluted magnetic semiconductor (Zn1-xMnx)3As2 is investigated. Special attention is paid to the remanent disorder at temperatures much below the order-disorder phase transition temperature Tph. Dependences of Tph and the activation energy of hopping conductivity on composition are analysed. Good agreement is obtained between theory and experimental results for the phase transition and the hopping conductivity, occurring in two quite different temperature regions. It is suggested that the remanent structural disorder leads to generation of complex centres with a large magnetic moment and a large amount of internal spin degrees of freedom. We call these centres clusters of polarization. Conditions of freezing-in of a system of interacting clusters of polarization is analysed. A way to obtain the parameters describing these centres and their interaction is discussed.

The electrical conductivity of (Zn1-xMnx)3As2 solid solutions with 0

Magnetostatic interaction between the tip of a magnetic force microscope (MFM) and an Abrikosov vortex line is theoretically analysed in isotropic superconductors and in high-Tc materials with the magnetic field oriented along the c axis. The predicted forces are of the order of 10-9-10-10 N suggesting that it is possible to observe the distribution of the magnetic field of a single vortex by existing MFM techniques. The influence of the surrounding vortex lattice on measurements is also discussed.

(Cd1-xMnx)3As2 (CMA) is a semimagnetic n-type semiconductor with Eg approximately=-0.095 eV. Magnetic susceptibility measurements in single crystals of CMA show that in weak applied fields (H or=0.04 and is attributed to presence of large magnetic clusters in the sample. From the data of the thermoremanent magnetization it is suggested that some of them can remain up to temperatures much higher than Tf.

Low-field (B = 10 G – 1 kG) magnetic properties are investigated in ceramic samples of La1–xSrxMn1−yFeyO3 (LSMFO) with x = 0.3 and y = 0.15, 0.20 and 0.25. A substantial decrease of the ferromagnetic (FM) Curie temperature, TC, with increasing y is observed, which is connected to breaking of the FM double-exchange interactions by doping with Fe. Strong magnetic irreversibility at B = 10 G gives evidence for a frustrated magnetic state of LSMFO. The asymptotic Curie-Weiss behaviour of the magnetic susceptibility, χ (T), observed well above TC, yields the values of the effective Bohr magneton per magnetic ion, exceeding considerably those of single ions. Critical behaviour of χ (T) ~ (T/TC – 1) −γ is influenced at y = 0.15 and 0.20 by a percolative (γ ≈ 1.8) and non-percolative or Heisenberg (γ ≈ 1.4) spin systems. At y = 0.25 the percolative contribution to χ (T) is not observed. The low-field magnetic properties above can be explained by the phase separation effect or generation of nanosize FM particles in the paramagnetic host matrix of LSMFO.

The resistivity, , is investigated in ceramic La1-xSrxMn1-yFeyO3 samples with x = 0.3 and y = 0.03  0.25 at temperatures T ~ 5  310 K in magnetic fields B up to 8 T. The metallic conductivity at y = 0.03 and the variable-range hopping (VRH) conductivity between y = 0.15  0.25 are observed. The Mott VRH conductivity takes place in all investigated samples with y ≥ 0.15 in different temperature intervals. In addition, at y = 0.25 the interval of the Shklovskii- Efros VRH conductivity, characteristic of the Coulomb gap in the density of the localized states (DOS), is observed. Analysis of the VRH conductivity yields the values of the microscopic parameters, including the DOS features and the localization radii of charge carriers.

The temperature dependence of the resistivity, ?, of ceramic La1 ? xSrxMn1 ? yFeyO3 (LSMFO) samples with x = 0.3 and y = 0.03, 0.15, 0.20 and 0.25 (or simply #03, #15, #20 and #25, respectively) is investigated between temperatures T ~ 5 and 310?K in magnetic fields B up to 8?T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behavior of ?(T) is more complicated, comprising of two extremes, divided by an interval of metallic behavior in #15, and two inflections of ?(T) in #20 within similar intervals ?T below ~ 100?K. Mott variable-range hopping (VRH) conductivity is observed in #15 above the ferromagnetic Curie temperature, TC. In #20 the Mott VRH conductivity takes place in three different temperature intervals at T > TC, T close to TC and T < TC. In #25, the Mott VRH conductivity is observed in two different intervals, above and below TC, divided by an intermediate interval of the Shklovskii?Efros VRH conduction regime. Analysis of the VRH conductivity yielded the values of the localization radius, ?, and the dependence of ? and of the density of the localized states, g, near the Fermi level, on B. Above TC the localization radius in all samples at B = 0 has similar values, ??1.0?1.2??, which is enhanced to ??3.3?? (#20) and 2.0?? (#25) below TC. The sensitivity of ? and g to B depend on y and T. The complicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopic parameters ? and g, is attributable to different electronic and magnetic phases of LSMFO varying with temperature and Fe doping.

Magnetoresistance (MR) of single crystals of the group II–V semiconductor n-CdSb doped with In is investigated in magnetic fields B up to 7 T and temperatures T between 1.2 and 2.4 K under hydrostatic pressure p = 0–10 kbar. Shubnikov–de Haas (SdH) oscillations of MR observed up to B ~ 4 T are characterized by a single period, giving evidence for one participating group of conduction band electrons. Pressure dependence of the SdH period and concentration reveal a second group of charge carriers, connected to a resonance impurity band with delocalized states. The SdH amplitude, A, exhibits non-universal behavior attributable to its sensitivity to scattering mechanism at low quantum numbers. The parameters characterizing the non-universality of A(T) and the pressure dependence of the cyclotron effective mass, mc(p), and of the concentration of conduction electrons, nSdH, are given. The dependences of mc and the Fermi energy on pressure are consistent with Kane-type non-parabolicity of the conduction band.

The influence of thermal annealings on La(0.9)Ca(0.1)MnO(3) (LCMO) films in oxygen and in vacuum with low hole doping is investigated in the phase separation region where competition between AFM and FM phases is high. Measurements by x-ray diffractometry, atomic force microscopy and magnetometry reveal changes in the lattice parameters and magnetic properties of the films, depending on the oxygen content. All films show magnetic cluster glass properties with similar freezing temperatures of around 45 K. Clearly the highest increase of the magnetization is observed in the films annealed in vacuum. We attribute this effect to trapping of unpaired electrons at oxygen vacancies where they can form rigid self-trapped magnetic polarons in potential wells of local moments. As a result long-range spin distortions with local ferromagnetic order may be realized. In conformity with these results, photoinduced persistent magnetization showing different mechanisms of generation, depending on the method of thermal annealing, is observed.

The resistivity of YBa2Cu3O7−δ (YBCO) films deposited by a pulsed laser from nanograined targets doped with various amounts of BaZrO3 (BZO) is investigated in the thermally activated flux-flow (TAFF) regime, where the activation energy, U, has a linear temperature dependence. It is found that the magnetic field dependence of the irreversibility temperature, Tirr(B), has a maximum at some optimal BZO concentration, which depends on the applied magnetic field. Our results show that the superconducting materials used in different magnetic fields need optimization of the dopant concentration to achieve the best properties for various applications.

The phonon echo excited by radio-frequency pulses in superconducting MgB2 in external magnetic field was studied. Using the echo technique two contributions to ultrasound attenuation are observed: low-temperature relaxation, which depends on the magnetic field, and a decay connected to the superconducting energy gap at temperatures close to Tc. The value of about 7kBTc for the energy gap was extracted from the experimental data.

A series of studies has been carried out on thin films which were prepared by pulsed laser deposition from micro-grained targets of Y1-xPrxBa2Cu3O7-? (Y1-xPrxBCO) to determine the doping effect of Pr (at Y site) in concentrations between x = 0 ?0.20. Earlier results on bulk materials show that the Pr substitution can significantly enhance the value of Jc(B) at cost of Tc. Our X-ray diffraction results confirmed the good quality of the films without any impurity phases. Magnetometric studies show the systematic decrease of Jc(B) when the Pr concentration is increased.

We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La0.67Sr0.33MnO3 (LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.

The Hall effect in single crystals of the group II–V semiconductor p-CdSb doped with 2 at% of Ni is investigated between T = 1.5 and 300 K in pulsed magnetic fields up to B = 25 T. The Hall resistivity, ρH, exhibits a nonlinear dependence on B, which is strongly pronounced below ~10 K but is still observed even up to 300 K. The analysis of ρH(B) gives evidence for the presence of a positive normal and a negative anomalous contribution, ρN = R0B and ρA, respectively. The temperature dependence of the (normal) Hall coefficient R0 is determined by the activation of holes into the valence band with a small contribution of the itinerant holes from the acceptor band at lowest T. The dependence of ρA on T is quite different within two temperature intervals, being weak between ~50 and 300 K and very strong below ~50 K, the latter resembling that of the resistivity, ρ. Analysis of ρA below ~77 K demonstrates that it scales approximately as ρn with n = 1.6 ± 0.1 within four decades of ρA and more than two decades of ρ. The anomalous Hall effect in p-CdSb:Ni is attributable to the presence of magnetic Ni-rich nanoclusters, whose properties have previously been investigated by the analysis of the magnetization data.

Homogeneous crystals of diluted magnetic semiconductor solid solution Zn0.9Cd0.1GeAs2 doped with 0, 1.13 and 2.65 mass% of Mn were synthesized. The grown crystals were characterized by atomic absorption and synchrotron x-ray powder diffraction analyses showing that the lattice parameter of the sample crystals are closely related to GaAs. Measurements of magnetic and transport properties were performed in the temperature range of 4–400 K using steady and pulsed magnetic fields up to B = 50 and 150 kG, respectively. The ferromagnetic Curie temperature of the sample crystals was found to be about 350 K. The magnetic ordering is attributed to the presence of MnAs clusters with mean size ~3.8 nm in the ferromagnetic phase. Influence of paramagnetic Mn2+ ions on magnetic properties of the crystals was observed only at low temperatures and was explained by the p–d interaction between charge-carrying holes and localized Mn moments.

The oscillation of a photocurrent produced by illumination of a mesoscopic metal film as a function of external magnetic field is considered. The orientation of the magnetic field B is assumed to be arbitrary. The thickness of the film is supposed to be much larger than the penetration depth of light but much smaller than the electron mean free path due to the scattering in the bulk of the sample. The oscillation of the current in the plane of light incidence is analyzed. Our theory predicts oscillations of two types. The first type can exist for any constant energy surface, including spheres. The second type can exist only for nonspherical constant energy surfaces. The amplitudes of the oscillations of the first and second type are proportional, respectively, to B−3 and B−3/2.

The dependence of the 139La nuclear magnetic resonance (NMR) spin-echo signal on temperature and on the power of the applied radio-frequency pulse is investigated in zero applied magnetic field in La0.7Ca0.3Fe0.09Mn0.91O3 (LCFMO) with Curie temperature (TC) of about 125 K. Plots of the spin-echo signal versus the NMR pulse power reveal two maxima, out of which the high-power component is observed up to temperatures above TC. It is also found that while the temperature dependence of the NMR signal amplitude follows the value of static magnetization M(T), the NMR frequency, which is related to the local field at La positions, decreases only slowly and the 139La NMR signal is preserved at least to 220 K. These results confirm the existence of large ferromagnetic clusters with long life-times above TC in LCFMO.

Photoluminescence (PL) spectra of n-ZnSe single crystals doped with Au are investigated at temperatures between 4.4 and 300 K immediately after doping and after storage for four years at room temperature in darkness in air. The formation of donor-type Aui interstitial defects stimulated by time is found in ZnSe for the first time. Changes in the structure of the edge and long-wavelength PL spectra with the course of time are observed. The long-wavelength band attributed to (AuZn-DIII,VII) acceptors is not found in the PL spectra of the stored samples. At the same time, a green PL band with maximum at 527 nm (2.35 eV) appears. This band is ascribed to (VZn–Aui) acceptors formed as a result of displacement of the AuZn atoms into interstitial sites due to deformation forces. When the Au concentration in the Zn + Au melt exceeds 1 at.%, an Aui-donor impurity band is formed and the intensities of all the PL bands are reduced.

Ferromagnetic resonance (FMR) was investigated in K2CuF4 by observing scattering of light from the crystal under microwave excitation at frequencies around 35 GHz. The spectrum of the scattered light contains satellites whose frequency differs from that of the incident light by the amount of the FMR frequency. The intensity of the satellites is used to estimate the number of the uniform precession magnons and to study their relaxation in a wide range of pumping powers. The power dependencies of the different magnetostatic modes are separately measured and show marked non-linearities. Investigations with light propagating along the c and a axes of the crystal are used to obtain information on the spin wave energy gap.

Structural phase transitions of tetramethyl ammonium manganese trichloride (TMMC) are investigated by using the Brillouin scattering method. The temperature dependences of the elastic coefficients C11 and C33 extracted from the experimental data are analysed by using the Landau theory of phase transitions. The step anomaly of C33 observed at the order-disorder transition temperature Tc can be attributed to a coupling between the order parameter and totally symmetric strains. An anomalous softening C33 above Tc indicates an interaction also between tetramethyl ammonium (TMA) molecules and the MnCl3 chains. A new lattice anomaly, which the authors attribute to a change in the rotational degree of freedom of the TMA molecules, is observed at about 395K. Doping of TMMC with 2% of Cu2+ creates a sequence of phases with transition temperatures at 92, 124 and 130K. An incommensurate phase stabilised by impurities may be involved in this sequence. Strong elastic scattering of light indicating a formation of needle-like domains along the c axis is observed in the doped crystals between 90 and 130K.

Brillouin scattering measurements were made in the temperature range from 80K to 600K. From the data the velocities of the phonon modes corresponding to the elastic stiffness coefficients C11, C22 and C33 were determined. In the incommensurate phase between T1=297K and T2=280K, C22 and more clearly C33 shows an anomalous decrease with decreasing temperature. Between T1 and T=260K the Brillouin peaks are broadened attaining their maximum widths just above T2. These effects are interpreted by assuming a coupling between phonons and modulations of the incommensurate phase. The results suggest that the dominant contribution comes from the phase mode-acoustic mode interactions.

Resonant Brillouin scattering from longitudinal and transverse phonons has been investigated in alpha -ZnP2 by tuning thermally the absorption edge through the 632.8 nm line of a He-Ne laser. The observed cancellation of scattering cross section has been discussed by a destructive interference between the resonant and the non-resonant transitions near the indirect band edge.