We discuss the magnetic properties of lead doped Bi-2223 bulk samples obtained through combined magnetic melt texturing and hot pressing (MMTHP). The ac complex susceptibility measurements are achieved over a broad ac field range (1 Oe < hac < 100 Oe) and show highly anisotropic properties. The intergranular coupling is improved in the direction perpendicular to the applied stress and magnetic field direction, and an intragranular loss peak is observed for the first time. A comparison is made with other bismuth-based compounds and it is shown that the MMTHP process shifts the ac irreversibility line (ac IL) toward higher fields. It is also shown that all the ac IL’s for quasi 2D bismuth-based compounds show a nearly quadratic temperature dependence and deviate therefore strongly from the linear behavior observed in quasi 3D compounds and expected from a critical state model.

Good quality body-centered cubic iron layers can be grown on Si(001) at room temperature using thin FeSi2 or CoSi2 silicide buffer layers. The in-plane and out-of-plane strains are measured by x-ray diffraction (XRD). Due to the difference in Fe and Si parameters of +5.6%, the thin Fe layers undergo a strong tetragonal distortion. For the samples deposited on FeSi2, the measured biaxial in-plane compressive strain ranges from values as large as -3.8% in the thinnest films (<25 ML) to -1% in the thicker layers (>40 ML). In the samples deposited on CoSi2, a part of the iron layer clearly observable up to 40 ML, apparently grows in pseudomorphy with the silicon, whereas another dominant part above 15 ML partly relaxes and behaves as on FeSi2 templates but with systematically larger strains. The evolution of the in-plane cubic effective magnetic anisotropy constant versus Fe film thickness observed in previous work, can be explained by means of fourth-order (in spin) magnetoelastic coupling in the iron lattice with the strain components determined by XRD. Finally, the observed ratio between the perpendicular and in-plane strain components differs substantially from linear elasticity theory predictions and indicates the importance of anharmonic effects and/or defects in the description of the elastic properties of such films.

Structural information on ultrathin films of FexCo1−x alloys grown epitaxially by coevaporation on Si(001) 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 (001)[100]FexCo1−x//(001)[100]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.

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 Néel 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.

Epitaxially grown iron thin films (thickness: 8 Angstrom < d < 120 Angstrom) on Si(001) with body centred cubic structure have been structurally and magnetically studied at room temperature. All films 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. (C) 2001 Elsevier Science B.V. All rights reserved.

Binary Fe(Si1-xFex) iron and ternary Fe3-yCoySi iron cobalt silicide thin films (thickness: 200 Angstrom) 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(Si1-xFex) films have ferromagnetic properties at room temperature in the range 0.09 less than or equal to x < 1 and the cobalt-substituted Fe3-yCoySi films an ferromagnetic as well in the whole range 0 < y < 3 at room temperature. The off-normal evaporation geometry leads to strong in-plane uniaxial magnetic anisotropy (including the non-epitaxial pure cobalt silicide Co3Si) 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(Si1-xFex) films increases the coercive field but does not strongly affect the uniaxial anisotropy constant. In Fe3-yCoySi 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. (C) 2000 Elsevier Science B.V. All rights reserved.

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 [greater than or equal to 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. (C) 2000 Elsevier Science B.V. All rights reserved.

Iron silicide thin films (200Å Fe(Si1−xFex) with 0⩽x⩽1 and local cubic CsCl structure) have been grown by coevaporation at room temperature (RT) on Si(111). 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 (Fe1.35Si). The magnetization is parallel to the film surface, and the magnetic anisotropy is uniaxial, with the easy axis lying along a [1̄01]Si crystallographic direction and the hard axis along a [12̄1]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.

Texturing of high-temperature superconducting ceramics leads to an improved intergranular coupling. This is demonstrated on the basis of ac susceptibility measurements obtained when texturing Bi-2223 polycrystals through combined magnetic melt texturing and hot pressing (MMTHP) and Y123 bulk samples obtained through combined magnetic melt texturing and zone melting (MMTZM).

La caracterisation de materiaux composites par des techniques simples telles que la resistivite electrique pose probleme des lors que l'on est place en dessous de la limite de percolation et seules des techniques de caracterisation de type magnetique sont capables de deceler la presence de diverses phases supraconductrices. Nous discutons ici de resultats obtenus sur des composites YBa 2 Cu 3 O 7-x par une etude en susceptibilite complexe qui permet, outre l'effet Meissner, de determiner les dissipations energetiques et la ligne d'irreversibilite (ac-LI). Cette derniere est fortement deplacee vers les champs forts par rapport a toute ac-LI de type intergranulaire et est proche des ac-LI obtenues par exemple pour les contributions intragranulaires dans des echantillons massifs ou pour des couches minces. L'emploi d'un modele de type etat critique qui permet egalement d'estimer le courant cntique intragranulaire (j c (77K) = 1,7 a 2 10 5 A/cm 2 ).

We present experimental results obtained forRBa2Cu3O7−x(R=Y,Er) expitaxial thin films obtained through pulsed laser deposition (PLD) and grown on yttria stabilized zirconia (YSZ) and SrTiO3(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.

We investigated the coupling of specific ferroelectric-superconducting composites with variable volume fraction, particularly because they have presented a superconducting transition at 85 K. The intra- and intergranular coupling of these composites has been considered from the resistivity and susceptibility measurements. Our results show that the transport properties of this kind of composite are governed by different mechanisms, depending on the volume fraction of the ferroelectric material, . Up to intragranular coupling exists between the YBCO superconducting grains, but when the electrical transport is greatly related to intergranular coupling. In this case, two distinct and superconducting phases are evidenced by the susceptibility measurements; they maintain the superconducting character of the composites. Moreover, at the superconducting percolation threshold, the electrical response exhibits some hysteresis giving evidence that internal polarization is important and can play a role in the composite properties.

Superconducting ReBa2Cu3O7 (R = Y, Er) epitaxial thin films were deposited by reactive laser ablation on YSZ (Yitria Stabilized Zirconia) and SrTiO3 substrates under various conditions. The structural quality of the films were characterized by complete XRD measurements. The superconducting properties have been investigated by AC susceptibility in an extended AC fields range (hAC≤400 Oe). We focused our attention on the imaginary part χ″(T) (loss peak) for fields applied parallel and perpendicular to the film surface. The use of “high” parallel AC fields leads to a precise determination of the irreversibility lines (IL) in the (hAC, T) plane. The IL are weakly frequency dependent and exhibit a nearly linear behavior. The observation of fine structures of the loss peak are correlated to the presence of different phases and/or grain orientations.

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.

We present experimental results obtained for high quality epitaxial thin films (film thicknesses around 5000 Angstrom rocking curve FWHM down to 0.1 degrees). These films are obtained by laser ablation of REBa(2)Cu(3)O(7-delta) (RE = Y, Er) deposited on SrTio(3) and YSZ substrates. The superconducting properties have been studied by complex susceptibility in an extended AC field range (h(ac) less than or equal to 300 Oe) and show sharp magnetic transition width and high critical currents (10(7) A cm(-2) < j(c)(77 K) < 10(8) A cm(-2)). The loss peak is only weakly depressed by an increasing AC held and the observed shifts are up to an order of magnitude lower than those observed for intragranular contributions in bulk samples. The agreement with the behaviour expected from a critical state model is also discussed.

RE1Ba2Cu3O7−δ (RE=Er or Y) superconducting films were fabricated by reactive laser deposition, in situ, on YSZ(100), SrTiO3(100) and Si(001), under various conditions. A complete XRD investigation showed that the films grown with high substrate temperatures (Ts740°C) combined with low fluences (1.5J/cm2 < 2 J/cm2) and laser repetition rate (4 Hz) have the best structural characteristics, very good electrical and magnetic properties.

High quality ErBa2Cu3O7−x thin films were grown by reactive laser deposition on yttria-stabilized zirconia substrate. The structural and electrical properties of the samples were investigated by x-ray double-crystal diffractometry, texture analysis and reciprocal space mapping, and resistivity measurements, respectively. The best films were obtained by using high substrate temperature combined with a low fluence and low laser repetition rate. In these conditions the deposition rate results were very low, namely <0.25 Å/s. We found that in this case we can grow films with a very high degree of crystalline perfection. Preliminary results show that this process promotes the fabrication of very thick films (thickness ≳500 nm) which are highly c-axis oriented.

Complex a.c. susceptibility measurements in a broad field range () have been made on Bi - Pb - Sr - Ca - Cu - O (Bi-2223 phase) bulk samples obtained by combined magnetic melt texturing and hot pressing. Highly anisotropic properties are observed. Improved intergranular coupling is obtained in the direction perpendicular to the applied stress and the magnetic field direction. An intragranular peak is also observed for the same direction. This is the first time that an intragranular loss peak has been clearly seen in a bismuth-based compound, which indicates that the intragranular contribution is strongly increased.

Low field a.c. susceptibilities of HgBa2Ca2Cu3O8+δ (Hg-1223) and Bi2Sr2CaCu2O8+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 Ea ≈ 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.