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We investigate the interface between Au catalysts and a Ge(1 1 1) substrate. This system is achieved by dewetting an Au layer above the Au–Ge eutectic temperature. We show by high resolution transmission electron microscopy that a large amount of Ge can be moved over a surface of Ge(1 1 1) by using assistance of eutectic Au–Ge droplets. This localization is achieved thanks to the ability of nano Au–Ge droplets to incorporate a large amount of Ge and to release it by cooling down the sample at room temperature. This makes the localization process irreversible with respect to annealing at a very high temperature. The extra Ge supplied by precipitation is in epitaxy with the Ge(1 1 1) substrate. This reflects in macroscopic I(V) measurements.

The electrochemical and structural characterization of thermally grown oxides formed on nickel based alloys (type Inconel 600 and 690) at 350 °C and during different time was performed by impedance measurements and near field microscopy combining atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM). The impedance results allow discussing the influence of the oxidation time on the capacitance and the resistance of the oxide films. The results obtained by near field microscopy technique show that the film formed during low oxidation time has a small grain size and also a small surface roughness. The values of grain sizes determined in both cases along x- and y-axis are practically the same indicating a spherical shape of oxide grains and are slightly different between oxides formed on type inconel 600 and 690.

The molecular-beam-epitaxy growth of Fe–Ge/Ge/Fe–Ge trilayers on Ge(111) wafers has been investigated as a function of three parameters: the Ge spacer coverage, the substrate temperature (TD) and the dynamic atomic hydrogen (H) exposure during the Ge spacer deposition. Morphology and crystal structure have been characterized in situ by means of scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron diffraction and ex situ with high-resolution transmission microscopy. Whatever the H flux, epitaxial growth of Ge spacer requires deposition temperature above ~ 220 °C. At the earliest stages of Ge deposition, the surface periodicity of the bottom Fe1.9Ge epilayer changes from p(2 × 2) to 3 × 3 R 30 ° for deposition temperature above ~ 220 °C, whatever the H dosing. It results from severe intermixing that modifies the stoichiometry of the whole Fe–Ge layer. However, this layer preserves its hexagonal B82 crystal structure and no additional Fe–Ge compounds formed at the interface. We found also that the H flux drastically modifies the Ge spacer growth mode for deposition temperature above ~ 220 °C. In particular, the Ge morphology evolves from 3D islands without H supply to flat and continuous film for H partial pressure above 10–3 Pa. Finally, the top Fe–Ge electrode crystallizes in the same structure as the bottom electrode, and the planar relations of the trilayer are: (111)Ge-wafer||(0001)bottom–Fe1.9Ge||(111)Ge-spacer||(0001)top–Fe1.9Ge and [− 110]Ge-wafer||[11–20]bottom–Fe1.9Ge||[1–21]Ge-spacer||[11–20]top–Fe1.9Ge. These fully epitaxial Fe–Ge/germanium hybrid heterostructures with single crystal Ge layer of diamond structure appear therefore as promising candidates for semiconductor spintronics.

Thermally oxidized AISI304 and AISI316 stainless steels are studied by Atomic Force Microscopy, Scanning Kelvin Probe Force Microscopy (SKPFM) and Magneto-Optical Kerr effect as a function of their growth temperature. The surface roughness is a competition between the roughness of the bare substrate and the roughness resulting from the oxide layer growth. Cr oxide is present at some places on the surface at low growth temperature as shown by SKPFM. The observed decrease of surface potential with the oxide layer thickness indicates an effective protection against corrosion. Magnetic measurements demonstrate that the outer layer contains a magnetite phase (in-plane magnetization).

The influence of Si capping layers on the magnetic properties of thin Fe films grown on Si(1 1 1) has been studied by means of shape anisotropy calculations. Fe surface morphology simulations are realized using experimental STM data. Surface modifications induced by the interaction between the Si overlayer and the Fe surface are performed in agreement with the model proposed in a previous work by Stephan et al. [J. Magn. Mater. 293 (2005) 746]. Calculations of the uniaxial anisotropy energy constant K u are then performed on the modified Fe surface morphology for different Si deposition geometries as proposed in the model. The relevant data deduced by this method such as anisotropy constants and their related easy axis direction, are directly compared to the experimental ones obtained by ex situ magneto-optical Kerr effect (MOKE) measurements at room temperature using the transverse bias initial inverse susceptibility and torque (TBIIST) method. We show that a very good agreement between those results leads to a confirmation of the proposed model.

This article presents the results obtained from the analysis of the I–V electrical characteristics of photovoltaic arrays that were tested in a region of the Sahara. Experiments were carried out at Adrar in the southern part of Algeria. The study includes the determination of the most representative parameters of the arrays by using numerical methods for the one exponential model. After having determined these parameters, they were numerically used in order to carry out the plotting of the theoretical I–V characteristics of modules for the considered environmental variables such as ambient temperature and solar irradiance. Other model (Rauschenbach's model) has also been considered in this article to make a comparison with the previous one. The study has been completed by carrying out the translation of the experimental characteristics for the standard conditions. This translation allowed the comparison to be made between the performances of tested modules. In the last part of the paper, some elements related to the degradation of arrays performances have been presented.

The magnetic and structural properties of epitaxial Fe films grown on Si(111) are investigated by polarized neutron re. ectometry (PNR) at room temperature. The influence of different types of interfaces, Fe/Si, Fe/FeSi2 and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(111) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi2 template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface. (C) 2008 Elsevier B.V. All rights reserved.

We report on the investigation of structural and magnetic properties of the ternary Heusler alloy Co 2 MnSi grown on Si(0 0 1) by molecular-beam epitaxy. Low-energy electron diffraction (LEED), inelastic medium-energy electron diffraction (IMEED) and X-ray photoelectron diffraction (XPD) measurements clearly show the growth of crystalline Co 2 MnSi . The best crystallographic and magnetic quality of the Co 2 MnSi films have been achieved after codeposition of the three Co, Mn and Si elements on the Si(0 0 1) substrate held at 587 K. Quantitative determinations of magnetic anisotropies were performed using transverse bias initial inverse susceptibility and torque measurements (TBIIST). Co 2 MnSi reveals to have an in-plane fourfold magnetocrystalline anisotropy with easy axis along 〈 0 1 0 〉 directions for evaporation fluxes perpendicular to the substrate surface. On the other hand, grazing-incidence fluxes invariably generate a dominant uniaxial in-plane magnetic anisotropy contribution with easy axis perpendicular to the incidence plane.

The strain state of Fe films grown on Si(1 1 1) has been investigated by X-ray diffraction (XRD) in the thickness range between 11 and 304 monolayers. Fe grows tetragonally distorted with the orientation relationship Fe(1 1 1) 〈 1 2 ¯ 1 〉 // Si(1 1 1) 〈 1 ¯ 2 1 ¯ 〉 . At low coverage, the films grow pseudomorphic. Above 15 monolayers the films are characterized by the coexistence of a pseudomorphic phase with another one which relaxes with the Fe thickness. This relaxation proceeds rapidly in the earlier stages then slowly with the film thickness. The XRD characterization allows one to obtain quantitative information on the in-plane and out-of-plane strains.

We present a sensitive and precise determination of an arbitrary thin film magnetic anisotropy energy E a (or E a / M S if the saturation magnetization M S is not known) versus in-plane polar angle θ by means of initial inverse susceptibility and torque measurements under constant transverse bias field H B . In this method coined TBIISTtwo magnetic fields H L and H B are applied in the plane of the film along directions defined by θ and θ - ( π / 2 ) , respectively, and m L the magnetization component in units of M S is measured versus H L around m L = 0 . This readily provides two quantities χ - 1 = ( ∂ H L / ∂ m L ) ( m L = 0 ) the initial inverse susceptibility and Δ H = H L ( m L = 0 ) the field offset or torque at m L = 0 , shown to be respectively related to the second ( 1 / M S ) ( ∂ 2 E a / ∂ θ 2 ) ( θ - ( π / 2 ) ) and first ( 1 / M S ) ( ∂ E a / ∂ θ ) ( θ - π / 2 ) derivatives of E a . This, in turn, yields two determinations of E a ( θ ) from χ - 1 ( θ ) and Δ H ( θ ) , respectively. Fourier analysis then easily resolves various contributions of different symmetries to the magnetic anisotropy. The magnetization m L ( H L ) is measured with a conventional magneto-optical Kerr experiment in longitudinal geometry with respect to H L and higher-order (nonlinear in m L ) contributions as well as polar or other contributions to the Kerr signal are carefully determined and corrected in order to obtain the desirable accuracy or even simply meaningful results. Typical acquisition times including all corrections are about a few minutes per angle θ . The power of the method is demonstrated on typical examples of epitaxial Fe layers on Si(0 0 1) and Si(1 1 1) substrates. For instance, it is shown that extremely small contributions up to the 12th order in-spin to the cubic magnetocrystalline anisotropy of iron are readily detected.

We demonstrate that an in-plane uniaxial magnetic anisotropy can be induced in Fe films grown on Si(1 1 1) by means of an appropriate Si capping layer. It is shown that grazing-incidence Si deposition reduces the three-fold symmetry of the body-centered-cubic (BCC) Fe(1 1 1) surface to two-fold, thus generating a surface-type uniaxial in-plane magnetic anisotropy contribution. The relevant magnetic easy axis direction depends in a critical way on the azimuth of the Si evaporation flux with respect to the Fe crystal surface directions. A simple qualitative model based on the change of the Fe surface by inhomogeneous chemical reaction with Si is proposed to interpret the magnetic anisotropy evolution.

Combined transverse biased initial inverse susceptibility and torque measurements were performed on thin Fe(1 1 1) films epitaxially grown on Si(1 1 1), using a conventional magneto-optical Kerr effect magnetometer. This new method, called TBIIST, is a well suitable and sensitive technique for the study of magnetic anisotropies and in particular provides an accurate value of the anisotropy field strength in the sample and a precise determination of the anisotropy axes. For this purpose a 160 monolayer Fe film grown on Si(1 1 1) at normal incidence was investigated. A very small sixth-order contribution to the cubic anisotropy is readily detected, as expected for a Fe(1 1 1) film, corresponding to anisotropy fields as low as a few tenths of an Oersted.

Structural information on ultrathin films of Fe x Co 1− x alloys grown epitaxially by coevaporation on Si(0 0 1) at room temperature is obtained by X-ray photoelectron diffraction, low-energy electron diffraction, inelastic medium-energy electron diffraction and transmission electron microscopy. The body-centered-cubic structure is found to be stabilized for alloy thin films up to very high concentration of Co beyond the thermodynamically stable bcc range (0.25⩽ x ⩽1). For alloy films with thickness up to 10 monolayers, the bcc structure with (0 0 1)[1 0 0]Fe x Co 1− x //(0 0 1)[1 0 0]Si is actually observed over the entire range of alloy concentrations that includes elemental bcc Co. All films are ferromagnetic with in-plane magnetization. They show clear in-plane fourfold magnetic anisotropy for all alloy concentrations and the relevant cubic anisotropy constant shows a strong composition and thickness dependence.

Epitaxially grown iron thin films (thickness: 8 A < d < 120 A) on Si(0 0 1) with body centred cubic structure have been structurally and magnetically studied at room temperature. All lilms are characterized by ferromagnetic properties as soon as the thickness reaches 4 ML. In-plane uniaxial and cubic magnetic anisotropy constants are measured on samples obtained by modifying the incidence angle of evaporating iron flux. The magnetization reversal mechanisms are analyzed by combining longitudinal magneto-optical Kerr effect of both parallel and transverse magnetization components. These mechanisms are strongly influenced by the nature of the magnetic anisotropies.

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe3Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K4eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Neel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.

We demonstrate the possibility of growing good-quality epitaxial Fe films on a Si(001) substrate, opening up new prospects to prepare ferromagnetic superlattices on this substrate. A template technique prevents the formation of disordered interfacial iron silicides. Transmission electron microscopy reveals that the Fe layers are fairly uniform in thickness with abrupt interfaces and in majority epitaxial relationship Fe(001)[100]//Si(001)[100]. Both the diffraction data and the observation of Moire fringes indicate that the Fe lattice is almost relaxed towards its bulk bcc phase in thick layers [≥40 monolayers (ML)] but is still strained by the substrate in the thinnest films. Magneto-optical Kerr effect measurements show that the films evaporated at normal incidence with a thickness above 4 ML are ferromagnetic at room temperature and exhibit in-plane biaxial anisotropy.

Binary Fe(Si 1− x Fe x ) iron and ternary Fe 3− y Co y Si iron cobalt silicide thin films (thickness : 200 A) with local CsCl structure epitaxially grown at room temperature on Si(1 1 1) have been studied through X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE) vector magnetometry. The binary Fe(Si 1− x Fe x ) films have ferromagnetic properties at room temperature in the range 0.09⩽ x 3− y Co y Si films are ferromagnetic as well in the whole range 0 y 3 Si) favored by the absence of orientation dependence of the fourth-order cubic term of the magnetocrystalline anisotropy energy in (1 1 1) crystallographic plane. The easy axis corresponds invariably to the direction normal to the incidence plane of the atomic flux during evaporation. The substitution of iron by silicon in Fe(Si 1− x Fe x ) films increases the coercive field but does not strongly affect the uniaxial anisotropy constant. In Fe 3− y Co y Si alloys, a similar trend is observed and the increase of coercive force may be related to defects generated by alloying. The change of the magnetic moments of Fe and Co versus y , as obtained by XMCD, can be understood in terms of simple models.

Iron silicide thin films (200 A Fe(Si 1− x Fe x ) with 0⩽ x ⩽1 and local cubic CsCl structure) have been grown by coevaporation at room temperature (RT) on Si(1 1 1). X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE) measurements indicate that the films are ferromagnetic at RT for x ranging from 1 (pure Fe) to 0.15 (Fe 1.35 Si). The magnetization is parallel to the film surface, and the magnetic anisotropy is uniaxial, with the easy axis lying along a [1 0 1] Si crystallographic direction and the hard axis along a [1 2 1] Si direction of the substrate. MOKE measurements show that the magnitude of the saturation field increases with increasing Si concentration, while XMCD data indicate that the average local magnetic moment carried by the Fe atoms decreases with decreasing Fe concentration. Models which involve the diminution of the number of Fe nearest neighbors are proposed for the description of the behavior of the Fe moments.

We present experimental results obtained for R Ba 2 Cu 3 O 7 − x ( R = Y,Er) expitaxial thin films obtained through pulsed laser deposition (PLD) and grown on yttria stabilized zirconia (YSZ) and SrTiO 3 (STO) substrates. The films have been deposited by using low deposition rates ( f = 4 Hz) and with control of the film surface temperature rather than that of the sample holder leading to a high quality of the epitaxy.

RE 1 Ba 2 Cu 3 O 7-δ (RE = Er or Y) superconducting films were fabricated by reactive laser deposition, in situ, on YSZ(100), SrTiO 3 (100) and Si(001), under various conditions. A complete XRD investigation showed that the films grown with high substrate temperatures (T s > 740°C) combined with low fluences (1.5 J / cm 2 < 2 J / cm 2 ) and laser repetition rate (4 Hz) have the best structural characteristics, very good electrical and magnetic properties.

Initial AC susceptibility measurements on REBa2Cu3O7−δ (RE = Y, Er), Bi2Sr2CaCu2O8+δ and HgBa2Ca2Cu3O8+δ polycrystals are used in order to obtain the irreversibility lines (IL) in the (hac, T) plane. The deviation of IL from linearity below a critical field value h c ∗ is discussed and related to the dimensionality of the compound. The effects of the fabrication techniques are also discussed.

Low field a.c. susceptibilities of HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) have been studied. The loss peak of χ″, relevant to intergranular pinning, shifts to higher temperatures for increasing frequencies. The results can be fitted with an Arrhenius law, leading to activation energies E a ≈ 1eV, but the hopping frequencies 0 are not realistic. More realistic values can be obtained by introducing a Fulcher law or a power law which is also characteristic of strongly interacting systems.

The complex AC susceptibility of HgBa2Ca2Cu3O8+δ(Hg-1223) has been studied for DC magnetic fields 0 Oe≤HDC≤50 Oe and AC magnetic fields 0.1 Oe≤hAC≤70 Oe allowing the determination to be made of the irreversibility line both in the (HDC, T) plane and in the (hAC, T) plane. The critical dynamic field h∗c has furthermore been obtained in the low AC field limit. All these results are consistent with a strong increase of the intergranular pinning strength which is one order of magnitude higher than in similar as-sintered 123 compounds, suggesting possibilities for the critical currents.

Characterization of a superconducting transition is commonly achieved by a.c. susceptibility measurements. The technique involved is rather simple, but interpretation of experimental results in inhomogeneous compounds such as superconducting ceramics is difficult because of the great sensitivity to numerous experimental parameters which also govern sample fabrication techniques (geometry, stoichiometry and thermal treatment) and measuring conditions (frequency, magnitude and orientation of the a.c. and/or d.c. magnetic fields). We present an analysis of both the real part x′ and the imaginary part x″ of the complex susceptibility x = x′ + i x″ in the low field limit which allows an effective comparison between various superconducting ceramics. A review of experimental results includes comparison of flux pinning strength (given by the critical dynamic field h c ∗ ) and phase diagram in the ( h a.c. , T ) plane. The critical line obtained in such a way differs markedly from the so-called “irreversibility line”.

We discuss the problem of characterizing a superconducting transition and the question of comparing samples originating from different fabrication processes and/or obtained under various shapes or morphologies (bulk, wires, thin films, single crystals, …). We present two different methods for the analyses of the AC susceptibility (for both the real and the imaginary part) in the low field limit. Strength of flux pinning and intergrain/intragrain contributions are discussed. Both analyses are applied to different inhomogeneous superconductors like high-Tc compounds (YBCO, LCO, LSCO, LBCO, B(P)SCCO, NCCO), Chevrel phases, etc. The dynamic critical field given by such an analysis allows an estimation of the quality and/or homogeneity of the superconducting sample and is a measure of the first intergrain (or Josephson) critical field.

We discuss the problem of characterizing a superconducting transition and the question of comparing samples originating from different fabrication processes and/or obtained under various shapes or morphologies (bulk, wires, thin films, single crystals, …). We present different methods for the analysis of the very low field a.c. susceptibility (for both the real and the imaginary part). The dynamic critical field obtained from our analysis (which can be applied to different inhomogeneous superconductors like high T c compounds, Chevrel phases, etc.…) is a measure of the quality and/or homogeneity of the superconducting sample.

The controversial shapes of the transition in HTSC (e.g. width and ac field amplitude dependences) are resolved by introducing the dynamic critical field deduced from our susceptibility measurements. The dynamic critical field criterion (which is also a quality factor) is studied for various compounds (YBaCuO, BiSrCaCuO) and morphologies (bulk- and single crystals, thin films, wires) by studying the ac field dependences of both the real and imaginary parts of the initial susceptibility over nearly five orders of magnitude and down to very low ac fields (0.02 mOe h ac

When discussing critical properties of inhomogeneous high temperature superconducting compounds it is preferable to compare experimental data collected from the greatest possible number of published results. However, such comparison is made difficult because of the wide variety of elaborate techniques, thermal treatments, characterization methods and sample morphologies involved in the experimental work reported in the literature. Here a phenomenological model, based on Clem's formulation and utilizing very low a.c. susceptibility measurements, is presented which demonstrates the existence of a critical field, h c ∗ , corresponding to the appearance of flux pinning. The analysis allows the classification of a large variety of high temperature superconductors including bulk samples, thin films and single crystals; the strength of the critical dynamic field (varying over three orders of magnitude: 50 mOe c ∗ Oe ) is a measure of the quality (and also homogeneity) of the sample. The analysis applies to YBaCuO compounds as well as to LaBaCuO and BiSrCaCuO compounds or Chevrel phases. The best quality appears to be for YBaCuO single crystals, while sintered BiSrCaCuO exhibits peculiar behaviour with very low h c ∗ or no h c ∗ at all.

ac initial susceptibility of sintered, field textured, zone melted polycrystals and single crystals has been studied in the low field limit. Critical dynamic fields (h c ∗ ) and relative position of irreversibility line (I.L) and critical line (C.L) are consistent with the creation of strong pinning sites. High anisotropy is observed in the case of field textured samples.

The irreversibility line (I.L.) is often deduced from initial susceptibility measurements and taken at temperature TM corresponding to the maximum of the imaginary part χ″ of the susceptibility. It is expected that such a line plays a part analogous to that of the Tg (spin freezing temperature) in spin glasses, i.e. leads to a phase diagram in the H, T plane. But TM corresponds to the strongest dissipations and low field initial susceptibility measurements have shown that dissipations already occur well below the χ″ susceptibility maximum, and even in zero d.c. fields. We discuss an alternative way to define a critical line separating reversible and irreversible regimes; such a line is obtained (in a 3D extended phase diagram) by analyzing the χ′ real part of the initial susceptibility in the low a.c. field limit. It also represents the limit between dissipative and non dissipative regimes.

We compare the irreversibility lines deduced from ac susceptibility measurements on Y-123, Bi- and Hg compounds obtained by various techniques (sintering, zone melting, magnetic field texturing). Field and frequency dependences are studied. Including results from the literature, our analysis indicate that fabrication methods containing a melting process favour the creation of pinning sites with high activation energies. Although weaker than in single crystals, an important anisotropy is induced by texturing processes.

We have studied the AC initial complex susceptibility of sintered samples of high temperature superconducting YBaCuO compounds and the importance of using very low external magnetic AC fields (h AC h ∗ c which is much lower than the first critical (static) field H c1 . The characterization and the quality of the true transition (which is field independent) can only be obtained with measuring AC fields whose values are lower than the critical AC field. The shape of the magnetic transition is otherwise strongly affected by the amplitude of the AC field and makes difficult a comparison between samples of different origins, fabricated by different processes and characterized under different conditions.

Field, temperature and frequency dependences of the a.c. initial complex susceptibility of sintered samples of high-temperature superconducting YBaCuO compounds have been studied. We point out the importance of using very low external magnetic a.c. field (hac < 1 mOe) to prevent dynamic saturation (flux-pinning) on reaching the critical a.c. field hc∗, which is much lower than the first critical (static) field Hc1. Our results are also consistent with a rather large spread of critical temperatures through the volume: the highest (Tc = 90 K) is found at the surface and the lowest (50 K < Tc < 80 K) in the inner part of the volume. Aging effects and restoration by appropriate thermal treatment (15 days at 450 °C) are also controlled by studying the imaginary part of the susceptibility.

We present results on complex susceptibility measurements for the high temperature superconducting compound YBa 2 Cu 3 O x . It is found that the imaginary part of the susceptibility shows strong dependence both on frequency and amplitude of the external a.c. magnetic field, whereas the real part of the susceptibility shows weak dependence. These results are compared with the behaviour of other inhomogeneous type II superconducting systems. Meissner effects, power losses and current wave forms are consistent with the existence of weakly connected loops and/or flux pinning.

The a.c. initial susceptibility of bulk sintered samples of high temperature superconducting YBaCuO compounds has been studied in detail. By analyzing the field, temperature and frequency dependences of both real part (diamagnetic shielding) and imaginary part (power losses) we point out the compositional differences in oxygen content between the surface and the volume of a bulk sample. Our results are also consistent with a rather large spread of critical temperatures over the volume: the highest (Tc = 90K) at the surface and the lowest (50K < Tc < 80K) in the inner part of the volume. Results on aged and restored samples are also consistent with modifying of oxygen gradients.