B. Loegel

Epitaxial magnetic Fe layers grown on Si(001) by means of a template method

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.

The influence of thermal treatments, fabrication methods and shapes on the magnetic transition of high temperature superconducting compounds

Abstract 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.

Flux pinning and dynamical critical fields in some YBaCuO compounds

Abstract 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.

Magnetization reversal mechanisms in epitaxial Fe/Si(001) layers with twofold and fourfold magnetic anisotropies

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.

Irreversibility line and anisotropy of magnetic melt textured Y1Ba2Cu3O7−δ studied by AC susceptibility

Abstract Magnetically melt textured YBa 2 Cu 3 O 7−δ samples have been studied by AC initial susceptibility measurements in the low field limit. The critical dynamic fields h ∗ c and the relative position of the irreversibility line and of the critical line show that magnetic melt texturing leads to the creation of strong pinning sites. The anisotropy factor A is in agreement with the value obtained from magnetization under high DC fields. The A values are lower than in single cystals, but in agreement with the important orientation effects introduced by magnetic field texturing. These values also are typical of fabrication techniques involving a melting at some stage.

Homogeneity of high and low temperature superconducting compounds in relation with magnetic characterization

Abstract 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.

Surface effects in bulk high temperature superconducting ceramics

Abstract 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

Magnetic characterization of superconductors in the low a.c. field limit

Abstract 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”.

Irreversibility line and the appearance of energy dissipations in inhomogeneous superconductors

Abstract 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.

Superconducting properties of epitaxial laser ablated thin films

Abstract We present experimental results obtained for high quality epitaxial thin films (film thicknesses around 5000 A; rocking curve FWHM down to 0.1°). These films are obtained by laser ablation of REBa 2 Cu 3 O 7−δ (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 ≤ 300 Oe) and show sharp magnetic transition width and high critical currents (10 7 A cm −2 j c (77K) 8 A cm −2 ). The loss peak is only weakly depressed by an increasing AC field 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.