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The phase formation sequence was studied in the preparation of solid solutions of RE2O2S: Ln’ (RE = La, Y; Ln’ = Ce, Eu, Dy, Er) by the reduction of the match co-precipitated sulfates, followed by sulfidization of the reduction products. For uniform distribution of cations in the matrix, a method of chemical homogenization was used, consisting in the preparation of an aqueous solution containing all the necessary cations and their subsequent precipitation in the form of sulfates. The use of sulfates as precursors facilitates the process of obtaining solid solutions of oxysulfides, since sulfates already contain SO42--ions. The phase and morphological certification of the obtained solid solutions was carried out. The study of steady state luminescent properties demonstrated characteristic bands which are assigned to 4f-4f and 5d-4f transition. The obtained results showed the possibility of applying the method to synthesize optical ceramics based on solid solutionsRE2O2S: Ln (RE = La, Y; Ln = Ce, Eu, Dy, Er).

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ ZFC , from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ ZFC ( T ) at T b in fields below the anisotropy field B K ∼4 kG. T b ( B ) obeys the law [ T b ( B )/ T b (0)] 1/2 =1– B / B K with T b (0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M s . The coercive field is much smaller then B K and displays anisotropy inverted with respect to that of M s . Such magnetic behavior is expected for spheroidal Ni-rich Ni 1− x Sb x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.

Magnetic susceptibility, χ ( T ) , is investigated in ceramic La 1− x Ba x MnO 3 (LBMO) with x = 0.02 – 0.25 in the range of fields B = 10 – 80 G and temperatures T = 5 – 310 K . All samples exhibit a paramagnetic (PM) to ferromagnetic (FM) transition with T C increasing with x from 177 K ( x = 0.02 ) to 295 K ( x = 0.25 ) and magnetic irreversibility decreasing below T C with increasing x . In the PM phase an interval of the Curie–Weiss behavior of χ ( T ) with an effective Bohr magneton number, p eff ∼ 30 – 40 , is observed above T 1 ∼ 260 – 290 K . Below T 1 and down to T cr ∼ 190 – 220 K the susceptibility follows the scaling law χ - 1 ( T ) - χ - 1 ( T C ) ∼ ( T / T C - 1 ) γ with γ = γ 1 ≈ 1.4 corresponding to a three-dimensional (3D) Heisenberg spin system. Below T cr , χ ( T ) obeys the same scaling law as for T > T cr , but with another value of γ = γ 2 ≈ 1.7 – 1.8 , characteristic of a 3D percolation system. The temperature dependence of the susceptibility observed in the PM phase is explained by small FM particles embedded in the host LBMO matrix above T 1 . The size of these particles increases between T 1 and T cr up to nanometer scale, forming critical percolation clusters below T cr . The magnetic irreversibility is connected to a mixed, FM and cluster-glass, phase.

Reliable correlation of simultaneously measured resistively, susceptibility and specific heat critical parameters of hole-underdoped YBCO permitted to establish a phenomenological model of normal-superconductor transition. After the model, fluctuations of quasifree electrons drives at T * ≫ T C the appearance of paired charge carriers (pseudogap-PG) characterized by partial coherence of the order parameter’s phase. The evolution of PG state from T * ∼ 140 K to the critical temperature T C ∼ 92 K is connected to the average increase of the statistically distributed partial coherence lifetime (from τ ∼ ps to ∞) and coherence length (from λ ∼ nm to ∞) of the fluctuating precursor islands. In this dynamic process the specific heat’s maximum signals (at temperature T CC ∼ 93.6 K) the appearance of the first mesoscopic coherent islands. T C marks the percolation of these islands. We believe that the presented concept of dynamical evolution (in coherence time and space from 0 to ∞) of the paired charge structures is a promising scenario while describing the properties of hole doped high- T C superconductors between the temperatures T * and T C .

In a weak (B=2 G ) magnetic field LaMnO3+δ exhibits at δ=0.065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, TC, a mixed (spin-glass and FM) phase followed by a frustrated FM phase at δ between 0.100 and 0.154. The same behavior is observed in La1−xCaxMnO3 with x between 0 and 0.3. This can be understood by the similar variation of the Mn4+ concentration, c between ≈0.13 and 0.34, in both materials when x or δ is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of TC(c) and the magnetic susceptibility, χ(c), as well as in the critical behavior of χ(T) near TC. These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO3+δ than in La1−xCaxMnO3.

The effects of the temperature gradient ( p ) and heat transfer’s velocity ( q ) parameters on the shape of AC susceptibility curves were experimentally explored. At homogeneous Y 1.01 Ba 1.95 Cu 2.97 O 6.86 ceramic samples only broadening with negligible horizontal translation and shape distortions of the χ ′ and χ ″ susceptibility curves were observed. At samples with local inhomogeneity—beside broadening—a proportional peak-deep-hump type distortion and horizontal translation of the χ ″, and rotational translation of the χ ′ curves appeared. The results—leading to a clearer understanding of how inhomogeneities and the scanning parameters influence the shape of the AC susceptibility curves—allow separating the effects of local inhomogeneities from the ones produced by structural modifications. It is confirmed that the spatial resolution of the method for local inhomogeneities is proportional to p and inversely proportional to q until a limit is determined mainly by the relaxation time of the studied phenomenon.

Low-field (B=2−80 G ) dc magnetic susceptibility, χ, investigated in hole-doped La1−xCaxMn1−yFeyO3 (LCMFO) with x=0.3 and y=0.05−0.10, exhibits non-uniform critical behavior near the paramagnetic (PM)-to-ferromagnetic (FM) transition temperature, TC. We observe the scaling law, χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ, with γ≈1.4 corresponding to a 3D Heisenberg spin system below τcr∼0.1–0.2, and with γ≈1.7 characterizing a 3D percolation system, above τcr. The non-universal scaling results from strongly inhomogeneous distribution of holes leading to formation of percolation clusters from the hole-rich nanoscale FM particles, embedded in the host PM matrix. The resistivity, ρ, of LCMFO exhibits between a temperature T v ≈310–330 K and TC the Shklovskii–Efros-type variable-range hopping conductivity law, ρ(T)=ρ0(T)exp[(T0/T)1/2], governed by generation of a complex gap in the density of states. The prefactor follows the law ρ0(T)∼Tm, where m changes from 9/2 at y=0 to 5/2 at y=0.03, 0.07 and 0.09, reflecting an additional fluctuating short-range potential induced by doping with Fe.

Thermopower and conductivity are measured for the perovskite-like manganite La1−xBaxMnO3 (0.02⩽x⩽0.35) in the temperature range 200–950°C and oxygen pressure varying between 10−11 and 0.5 atm. The data for thermopower and electrical conductivity indicate a transport mechanism dominated by adiabatic hopes of small polarons. The high-temperature limit for the thermopower is mainly defined by the spin contribution while configurational contribution does not depend essentially on the doping level. Changes in the conductivity with doping are shown to reflect variations in the mobility of charge carriers.

The ac response of a slab of a clean type-II superconductor in the mixed state is theoretically investigated taking into account the bulk and surface pinning. In extremely clean superconductors sharp resonances of the ac response are predicted. Pinning-induced resonance is found only for an asymmetric ac magnetic field h(ω) pattern with the direction of h(ω) being opposite on opposite sides of the superconducting slab. The contribution of the surface pinning to the resonance frequency depends on the tilt modulus of the flux line lattice. A resonance at the cyclotronic frequency is found only in samples thinner than the London penetration depth. In the universal flux-flow regime, predicted for d-wave superconductors, wide maxima are found in the frequency dependence of the ac absorption.

Low-field ( B =2–80 G) DC magnetic susceptibility, χ , is investigated in ceramic La 1− x Ca x Mn 1− y Fe y O 3 (LCMFO) samples with x =0.3. When y is increased from 0 to 0.1 the ferromagnetic (FM) Curie temperature, T C , decreases from 259 to 107 K, indicating growth of disorder by iron doping. A transformation of LCMFO from a weakly frustrated FM phase between y ≈0–0.05 to a strongly frustrated mixed, FM and glassy, phase between y ≈0.07–0.10 is revealed by measurements of magnetic irreversibility and long-time (up to t =10 5 s) relaxation of thermoremanent magnetization. Critical behavior of χ ( T ) obeying the scaling law χ −1 ( T )− χ −1 ( T C )∼( T / T C −1) γ ≡ τ γ is observed below τ cr ∼0.05–0.2 with γ = γ 1 ≈1.4 corresponding to a three-dimensional (3D) Heisenberg spin system. Above τ cr , χ ( T ) can be fitted with the same scaling law as for τ τ cr , but with another value of γ = γ 2 ≈1.7, which characterizes a 3D percolation system. The change of γ upon lowering the temperature can be explained by the formation of percolation clusters from an assembly of hole-rich nanoscale particles of the second FM phase embedded into the host LCMFO matrix at T well above T C . The magnetic moments, radii, and concentration of these particles are determined.

Using a variant of the AC susceptibility method, we have demonstrated that under non-equilibrium thermal conditions, spontaneous incoherent local diamagnetic fluctuations could lead to large coherent oscillatory relaxation in YBCO ceramics above Tc. The probability and the amplitude of giant relaxation become maximum at ∼50% volume fraction of the diamagnetic state. Using the size of the largest synchronous cluster (determined from the amplitude of susceptibility relaxation) as coherence length (ξ VG ∼1.6±0.1 mm ) together with the directly measured repetition period of the relaxation (τ∼10±1 s ), the critical dynamic exponent has been calculated to be z=4.9±1. This value is in agreement with published vortex-glass exponent data.

A high number of normal-superconductor transitions of sintered YBCO samples have been monitored by AC susceptibility measurements simultaneously in three different regions of samples, in the presence of strong thermal gradient. With this method, different accidental mesoscopic vortex structure instabilities have not only been observed but also located. Even if local magnetothermal instabilities of oscillatory nature (f=0.1–0.02 Hz ) appear in strong thermal gradient fields, their evolution remains localized. However, when such instabilities appear in superposed temperature and electric (DC) potential gradient fields, they travel together with the mixed-state zone (v=0.06–0.1 mm s −1 ) along the macroscopic specimen. The interaction of transport currents with shielding current instabilities of the mixed-state — modifying some percolative network paths — can be considered as a possible macroscopic scenario for the traveling instabilities.

Effects of deformation of flux lines on electrodynamics is investigated near the surface of a type-II superconductor in the mixed state when dc magnetic field is perpendicular to the surface. One- and two-mode approximations to the problem are presented. It is shown that the surface effects can be described by an effective boundary condition for a dense flux line lattice ( B ≫ H c1 , where H c1 is the lower critical field) in the two-mode approximation. This condition determines the relative value of the amplitudes of the modes.

AC susceptibility has been measured simultaneously in three different regions of YBCO ceramic samples, in the presence of a strong thermal gradient, in order to pinpoint local perturbation. A local step-like perturbation, applied in addition to the monotonic temperature decrease, produces a jump-like increase of the coherent region. The superconducting regions enlarge much faster than the propagation of the thermal waves. We suggest that the adiabatic magnetic flux jump reflect the creation of new coherent domains over Tc by stimulated correlation of the order parameter phase.

Magnetic relaxation measurements are made in the remanent state of pure and Zn-doped submicron YBCO powders. A characteristic feature of these samples is dipolar interaction between single vortices trapped in different particles. Similar to other systems with dipolar interactions, such as powders of fine ferromagnetic particles, our samples show glassy behaviour: different metastable states with various pinning properties can be prepared by varying the strength of the magnetizing field, i.e. history of the sample. It is found also that the relation between the pinning energy and the magnetic moment can be described by an expression similar to the vortex glass interpolation formula.

Magnetic properties of cubic La 1− x Ca x MnO 3 (0⩽ x ⩽0.4) are investigated between T =5 and 310 K in the external field of 2 G. The dependence of the ferromagnetic transition temperature on x is analyzed with the model of spin polarons associated with electronic localization and electron–electron interaction. The bandwidth of the localized electrons is found to be W =1.90±0.05 eV. In the samples with x 4+ concentration c T 1 , on c can be well described by a model including the Mn 3+ –Mn 3+ superexchange and the Mn 3+ –Mn 4+ double-exchange interactions. In the samples with x ⩽0.2 and c ⩽0.22 the value of the critical exponent, γ =1.21±0.05, is obtained for the temperature dependence of the zero-field-cooled susceptibility function χ −1 ( T )∼( T − T C ) γ , for T > T C .

Reversible properties of the remanent magnetic moment, M rem , in superconducting submicron- and micron-size Y–Ba–Cu–O particles and thin films are investigated with a SQUID magnetometer. In micron-size particles and thin films a monotonic increase of M rem was observed when decreasing the temperature. In submicron particles, the same monotonic behaviour is observed at first but at low temperatures an unusual reentrant behaviour was found: M rem decreased when cooling the sample below 6 K. This phenomenon is independent of the direction of the thermal cycle. Possible mechanisms of this effect, such as midgap states in the quasiparticle excitation spectrum and defect-induced localization of Cooper pairs, are discussed.

The structure of trapped magnetic flux is investigated in small superconducting Y–Ba–Cu–O particles, attributing the trapped moment Mpol to an ensemble of vortex polarons which consists of vortices surrounded by diamagnetic matter. To distinguish between the contributions of the vortices and their diamagnetic surrounding to the magnetic moment, measurements of the field dependence of Mpol are made with a SQUID magnetometer around zero magnetic field H. It is found that the value of Mpol grows with increasing field and has a narrow dip at H=0. A similar line shape but with opposite sign is found in the field dependence of the zero field cooled susceptibility confirming the polaron nature of the trapped flux.

During normal-superconducting transition AC susceptibility was measured simultaneously in three different regions of YBCO ceramic samples. The experimental data obtained under quasi-one-dimensional temperature gradient conditions reveal—beyond the conventional flux-line lattice expulsion governed by the Meissner effect—both flux-exchanges between different macroscopic regions of the sample and localized oscillatory-relaxation (spatial inhomogeneity) as well as temperature fluctuations connected to these processes. These phenomena prove that the flux can enter the sample leading to instability of the superconducting state. The measured δ t ∼1 s characteristic time of the relaxation corresponds to a linear diffusion regime.

As a common effect of a large and variable temperature gradient and a temperature pulse, a thermal instability of a new type has been observed in high- T c superconductors. The primary effect is the temporal change of local energy density, that causes a dynamic state of simultaneously coexisting normal and superconducting domains, establishing a spatial and temporal variation of superconducting and normal states. The fluctuation of the ratio of the superconducting and normal volume fractions can be detected by the oscillation of DC and AC voltages measured on the samples. Supposing that every jump of the AC voltage represents the transformation of a single coherent domain, it leads to the average value of the linear dimension of domains as 2.1 mm, characteristic time of the transformation as 7.5 s and velocity of the propagation of this effect as 0.3 mm/s in the case of Y 1 Ba 2 Cu 3 O 7−δ specimens. The macroscopic consequence of the thermal disturbance introduced into the sample is the appearance of a giant flux creep. Its existence is confirmed by comparison of our measurements to others, by analytical and numerical calculations of the forces acting on the vortices and by evaluation of the effect of the huge fluctuation created in the samples by our experiments.

We describe two new memory effects detected as additional changes in signals of field-modulated microwave absorption of YBa 2 Cu 3 O 7−δ powders. They appear when the scanning direction of the field is reversed or when the sample is cooled. The observed effects can be attributed to vortex distribution in superconducting YBa 2 Cu 3 O 7−δ particles.

To test the applicability of the modified London equation (MLE) the method based on comparison of the temperature dependencies of the penetration length λab in the Meissner and the vortex states is proposed. For experimental testing we investigate the magnetic properties of submicron superconducting YBaCuO particles prepared by a citrate gel modification of the sol-gel technique. It is found that the dependence λab2(0)λab2(T) obeys a power law with the exponent n = 2.0 ± 0.2 and n = 2.5 ± 0.05 in the Meissner and the vortex states, respectively. This difference indicates that MLE is not valid in the vortex state at least in the scale of the used particles i.e. of some hundred A. Possible reasons for this are discussed.

Investigations of the Shubnikov-de Haas (SdH) effect have been performed on single crystals of the II-V group diluted magnetic semiconductor (Cd 1 − x − y Zn x Mn y ) 3 As 2 with x + y = 0.3, y = 0.04, 0.06 and 0.08, subjected to long-time thermal annealing. The presence of the first and the second harmonics in the spectrum of SdH oscillations is established. The strong magnetic field dependence of the cyclotron mass, observed earlier in the as-grown samples, is conserved after heat treatment. This dependence coexists with the influence of the s-d interaction to the effective g -factor. A comparison between the properties of as-grown and annealed crystals is made.

A symmetry analysis based on line-group formalism is given for the YBa2Cu3O7 − xhigh temperature superconductor with 7 − x = 6.55, which gives rise to a superlattice phase. The diffuse x-ray scattering intensity curves are calculated for each allowed supersymmetry. An analysis is given for comparison of the experimental and theoretical intensity profiles.

In the thermometric mapping method a large thermal gradient is created along a high-Tcsuperconducting sample. By keeping the lower end of the specimen near liquid nitrogen temperature and by varying the top temperature from above the onset temperature down to the critical temperature, normal and superconducting domains come into being, depending on the microstructure of the specimen and actual values of the local temperature. This operation results in a spatial and temporal change of superconducting and normal states. If a thermal pulse is also applied, it leads to a thermodynamically unstable state, characterized by an oscillation of d.c. and a.c. voltages detectable on samples. The speed of propagation of this effect can be determined on the basis of average transformation times of different experiments, providing values of 0.25–0.33 mm s−1for Y–Ba–Cu–O specimens.

Different methods of deviating efficiency exist for preparation of Hg–Ba–Ca–Cu–O high- T c superconductors. They may be divided into such fundamental groups, which lead to different kinds of Hg-based compounds having unlike macroscopic and microscopic characteristic features. This paper analyses the relationship between some efficient preparation procedures and superconducting properties of different compounds resulting from them. Not only are the effects of practical methods, which produce high-quality single-phase or nearly single-phase materials of the homologous series Hg 1 Ba 2 Ca n -1 Cu n O 2 n +2+δ investigated but the deviations from them and their influence on the developing characteristics of samples are also evaluated.

This paper reports low-temperature ac magnetic susceptibility measurements on the Pd x Co 50 Al 50− x alloy system with 5≤ x ≤ 10. The temperature dependence of the in-phase susceptibility component χ ′( ω , T ) of specimens with x ≤ 8 cooled in a zero magnetic field shows a cusp which is rounded in a magnetic field, indicating rather pure spin-glass behaviour. The positions of the cusps (freezing temperatures, T f ) and the frequencies of the ac field are scaled according to a conventional critical slowing down law. The alloy with x = 10 indicates re-entrant behaviour in the range 18–139 K. The thermoremanent magnetization obeys a logarithmic time dependence.

Strong suppression of the ratio of the thermoremanent (MTR) and the field-cooled (MFC) magnetizations MTR/MFC, exceeding two orders of magnitude, when decreasing the size of superconducting YBCO particles from micron to submicron scale has been observed. A high saturation field (Hs > 8 kOe) of MTR (H) in submicron particles excludes the applicability of critical state models for interpretation of the results. A model of trapping and relaxation of isolated single vortices in particles having size D ⪅ λ (λ is the magnetic field penetration depth) is introduced showing that interaction between a vortex and its mirror image results in a fast relaxation of MTR with size-dependent relaxation time τ ∝ D2. The reduction of the remanence and its temperature and field dependencies in submicron particles are explained by the fast relaxation resulting in vortex-free particles and threshold fields of vortex trapping.

The influence of oxygen ordering on the temperature dependence of the magnetic field penetration depth λ is investigated in submicron YBa 2 Cu 3 O 6+ x particles obtained by low-temperature ( T s = 800 − 850°C) synthesis from a sol-gel precursor. Additional OO bonds created at low temperature synthesis hinder the oxygen ordering in Cu(1)O plane despite of its high content ( x > 0.82). Oxygen exchange as well as additional annealing stimulate tetra-ortho transition and oxygen ordering. A wide linear range is observed in λ ab −2 ( T ) for samples with highly disordered oxygen. This is attributed to strong phase fluctuations caused by local strains. A gauge-glass model with random distribution of order parameter phases describes the behavior of λ ab −2 ( T ), revealed at initial stage of the hindered tetra-ortho transition. Transformation of the linear λ ab −2 ( T ) fuction to a quadratic one with oxygen ordering is related to decreasing of local deformations and to transformation from a glass state to the usual superconducting orthophase. Two essentially different ways of oxygen ordering, reoxidation and additional annealing, result in the same final state with a similar power law of λ ab −2 ( T ).

Comparing our experimental data collected on intermediate-states instabilities in YBCO superconductors with similar data published in the literature for type-II superconductors we suggest the following: the occurrence of oscillations and limited flux jumps (initiated by external perturbations) in ceramic YBCO superconductors are in no obvious way different from that initiated by magnetic field perturbations in type-II superconductors. For this reason the validity of the adiabatic-stability conditions of the critical state determined for type-II superconductors could be extended to the instabilities in Y based superconductors.

The temperature dependence of the magnetic susceptibility in Co 0.5 Al 0.5− x Pd x when x is increased from 0.015 to 0.11 is similar to that of Co 0.5 Al 0.5− x Co x and demonstrates subsequently cluster-glass, mictomagnetic and re-entrant ferromagnetic behaviour. At the same time, its low-field and low-temperature magnetic properties are retarded with respect to the non-stoichiometric AlCo alloy. These features are analyzed from the point of view of the remanent lattice disorder in these alloys. Specific properties of magnetic clusters in Co 0.5 Al 0.5− x Pd x such as relatively low cluster cohesive energy and rapid growth of the cluster moment when the Pd concentration is increased are used to interpret the observed magnetic field evolution of susceptibility versus temperature and the magnetic field dependence of the freezing temperature.

Using the thermal scanning susceptometry, completed with the thermal pulse method, a series of hills has been observed on the susceptibility (real component) curves of Y-based ceramic superconductors. Supposing that these are the manifestations of cooperative vortices (superconducting domains promoted by the granular and defect structure of the material), the medium coherence length (ξ VG ~ 2 mm), medium correlation time (τ ~ 51.2 s), a dynamic ( z ~ 5.7) and a static ( v ~ 1.1) scaling exponents have been determined. Because the determined exponents of the instability are very similar to those predicted theoretically and verified experimentally for vortex-glass structures, we suggest that the observed fluctuations are produced by vortex-glass-like domains.

Ac surface impedances of screen printed thick films and bulk YBCO samples having a wide range of geometrical aspect-ratio are compared. It is shown that very thin (d ∼ 45 μm) samples cut from bulk pellets behave in ac magnetic fields as bulk materials with low aspect-ratio. This demonstrates that the widely accepted explanation for the observation that in ac magnetic fields applied normal to surface of a superconductor film vortex depinning occurs much more easily than in bulk superconductors is a consequence of a high aspect-ratio of the film geometry is not correct. We also show that the ac surface impedance measurement on films is equivalent to the complex susceptibility measurement on bulk samples.

It is shown by electrical modelling that the appearance and type of the resistivity or conductivity peak depends only on the character (or role) of the energy source used during the normal-superconducting or superconducting-normal transition. Treating the substituting circuit as series resistors (source of constant voltage) or parallel conductors (source of constant current), the response function has a maximum in time (and accordingly by the temperature) giving the possibility for the appearance of a resistivity or conductivity peak in both the case of cooling and heating. It is also demonstrated that the existence of the spatial distribution of local inhomogeneities in the sample is equivalent to the non-uniform temporal distribution of the transition.

Low-temperature, low-field magnetic measurements have been performed for the Co 50 Al 50− x Cu x system. On basis of the results a magnetic phase diagram is proposed for this compound. The observed time dependence of the remanent magnetization is discussed according to the theory of ultrametric organization of metastable states. Antistructure cobalt is considered to be the reason for the local anisotropy of the cobalt clusters.

Relaxation of the isothermoremanent magnetization has been investigated in ceramic Y-Ba-Cu-O samples with different grain sizes. It is found that the effective pinning energy U eff determined at different temperatures by using relaxation data of the remanent magnetic moment M IR and the relationship dln M IR / dln t =- kT / U eff has a maximum at a value of the magnetizing field H 0 corresponding to the most rapid increase of M IR with increasing H 0 . A phenomenological theory based on a critical- state model has been developed to describe these phenomena. The main conclusion of our analysis is that the dependence of U eff on H 0 can be attributed to the evolution of the distribution of the trapped flux (as H 0 is varied) without involving any assumptions about the dependence of the pinning barrier on the field.

Low-field, low-temperature static susceptibility measurements are reported for some CoAl1-xTx alloys with small concentrations of T = Cr, Fe, Co, Ni or Cu. All these alloys show field cooling effects. With increasing amount of T atoms spin-glass, mictomagnetic or ferromagnetic behaviour is observed. The Cr, Co and Cu alloys are in a spin-glass regime and pass subsequently through a cluster glass and reentrant regimes at the examined concentrations whereas the Fe and Ni alloys are mictomagnetic.

We have investigated magnetic properties of porous silicon prepared by anodical etching of n- and p-type Si wafers. Measurements with a SQUID magnetometer have revealed in our samples ferromagnetism characterized by hysteretic behaviour, saturation in fields above 1.3 T and a remanent moment persisting up to Tc ⋍ 570 K. Electron paramagnetic resonance investigations have shown the presence of paramagnetic centers related with silicon dangling bonds.

Low field magnetic measurements have been made on AlCo alloys between 4.2–180 K. Using magnetic susceptibility and small angle X-ray scattering data simultaneous appearance of a spin glass phase and growth of clusters in the samples could be observed. With increasing amount of Co a phase with mixed spin glass and ferromagnetic properties could be found before transition to the ferromagnetic phase. Influence of thermal treatment on mictomagnetic behaviour of AlCo alloys is briefly discussed.

Temperature dependence of the thermoremanent magnetization (MR(T)) has been investigated in YBa2Cu3O7-δ powders and ceramics after cooling the sample in a magnetic field. When the density of vortices in the grains in high, MR(T) can be expressed with an exponential law by using an effective activation energy Ueff related to interactions between vortices. In powders, the value of Ueff does not depend on the size of particles. Measurements with ceramics give different values of Ueff for samples prepared by using different heat treatment times. Finally, the temperature dependence of MR(T) is discussed within the framework of a vortex glass model. This approach is supported by a memory effect observed in MR(T).

Isothermoremanent magnetization and its relaxation with time have been investigated in ceramics and powders of YBa 2 Cu 3 O 7−δ at different temperatures after magnetizing the sample in a field of H ext eff ≈0.7 eV, independent of H ext . In the ceramic material U eff was found to depend strongly on the applied field reaching a value of 5 eV at 10 Oe and decreasing steeply when the magnetizing field was increased. The field dependence is discussed by using a model which takes into account the interaction between vortices.

It is shown that below T f ≈ 200 K the d.c. susceptibility of the semimagnetic semiconductor (Cd 1- x Mn x ) 3 As 2 cooled in zero magnetic field differs from that observed after cooling the sample in a field. This phenomenon was found only in the range of H ≤ 300 Oe. We attribute it to freezing of magnetic clusters according to local anisotropy fields in the sample. The high value of the freezing temperature indicates relatively strong intracluster magnetic interactions.

Photoinduced superconductivity has been investigated in YBa 2 Cu 3 O 6.4 films near to the semiconductor-metal transition. Light-induced changes in the resistance and growth of the diamagnetic moment were observed. The data revealed a dependence between the photoinduced reduction of the normal state resistance and the increase of the onset temperature of superconductivity in the films. Prolonged irradiation led to a seemingly complete loss of resistivity below 5 K. These phenomena are attributed to photogeneration of additional mobile holes in CuO 2 layers resulting in superconductivity.

Using different batches of starting materials (samples I, TINO 3 , CaO, Ba 2 Cu 3 O 5 ; II, TINO 3 , CaO, BaO, CuO; III, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO; IV, TINO 3 , CaO, BaCuO 2 ; V, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO, O 2 ) and the same heat treatment process, high T c superconducting systems of nominal composition TICaBaCuO 4.5 were prepared. Different superconducting phases were formed depending on the starting compounds used. Specimens I and IV had a single-phase (2,1,2,2) character. In samples II and III the dominant (2,1,2,2) phase was accompanied by (2,0,2,1) and by (1,2,2,3) and (2,2,2,3) phases in small quantity, respectively, while specimens V can be characterized by the existence of CuO and Cu 2 O beside the chief (2,1,2,2) phase. The superconducting properties were affected by starting materials, which manifests itself in the difference in onset and critical temperatures, lower and upper critical magnetic fields, paramagnetic limits, coherence length, penetration depth and Ginzburg - Landau parameter of specimens of different kinds. Due to the weak coupling created by interstitial material, different intergrain, intragrain and direct transport critical currents were determined, depending on the type of samples. The main aim of this work was to compare the experimental data with the corresponding theories to draw conclusions on the role of the starting compounds.

The phenomenon of persistent photoconductivity in YBa2Cu3O6+x (x ≈ 0.4) films near the semiconductor-metal transition has been studied. Transport and magnetic measurements of the illuminated films were performed. The transition to the metallic state and photoinduced superconductivity have been observed. The mechanism of persistent photoconductivity in reduced YBa2Cu3O6+x films is also discussed.

Low-field magnetization has been investigated in powders of YBa 2 Cu 3 O 7- δ using samples with different maximal characteristic particle sizes d max of between 2.4 and 7.4 μm. Measurements of the thermoremanent magnetization reveal the existence of a threshold field H th corresponding to trapping of a single flux quantum Φ 0 in microcrystal: H th =(1.1±0.2) Φ 0 / d 2 max . Flux creep was found to depend on the grain size, giving evidence for the presence of a surface barrier of flux trapping. A reversible temperature dependence of the thermoremanent magnetization has been observed. This effect is attributed to a local field created by particles containing trapped flux.

The structural, electrical and magnetic properties of screen-printed superconducting BiPbSrCaCuO thick films have been investigated. The films were either solid state sintered or first melt processed and then annealed over a long period of time. The films were primarily made up of the 2223 phase. The solid state sintered films had randomly arranged flaky crystals. The melt processed films were strongly textured with the (001) planes of the large plate-like grains parallel with the film surface. A pulse method was utilized for critical current measurements, which had the advantage of excluding the effect of local heating of the samples while at the same time increasing the sensitivity of measurement. The magnetic properties of both the solid state sintered and melt processed films were similar, but the transport critical parameters of the melt processed films were superior; the best films having their T c at 105 K and a critical current density of ≈ 2000 A cm −2 at 77 K in a zero external magnetic field. Variation of annealing procedures resulted in clearly different microstructures, which in turn manifested itself in differing superconducting properties. From the point of view of applications, the melt texturing process seems to be promising.

Vortex structures in the ferromagnet/type-II superconductor bilayer are investigated when the ferromagnet has domain structure and perpendicular magnetic anisotropy. It is found that two equilibrium vortex structures can be realized: straight vortices with alternating directions corresponding to the direction of the magnetization in the ferromagnetic domains and vortex semi-loops connecting the ferromagnetic domains with opposite direction of the magnetization. The values of the critical magnetization for the formation of these vortex structures are determined.