D. C. Cronemeyer

New mechanisms for irreversibility in high‐Tc superconductors (invited)

We discuss the zero‐field cooled (ZFC), field‐cooled (FC), and remanent (rem) magnetization of YBaCuO crystals in the context of an extended Bean critical‐state model. We introduce a novel way to include the reversible magnetization in this theory and show how this can explain the concave‐downwards shape of the ZFC magnetization near Tc. We explain the persistence of remanence in a temperature range where FC and ZFC magnetizations merge in terms of a residual irreversibility arising from a step in the reversible B(H) curve. We also introduce the concept of a flux‐trapping depth which helps explain the field dependence of the Meissner effect.

LOW-FIELD MEISSNER FRACTION OF YBaCuO IN A FLUX PINNING MODEL

Abstract We report a large increase in Meissner fraction (flux expulsion) at low fields in dense ceramic and crystal YBaCuO. The effect can be interpreted in terms of the irreversability line 1 − t∝ H 2/3 observed in these materials, and is at least qualitatively consistent with a flux pinning picture.

The temperature dependence of the uniaxial anisotropy of Gd1−x−yCoxMoy amorphous alloy films on glass substrates

The temperature dependence of the uniaxial anisotropy of Gd1−x−yCoxMoy films is described in this study as measured by ferromagnetic resonance. The magnetic moment of the films is obtained from a Faraday balance measurements. For this ferrimagnet, the sub‐network moments and g factors are related by the Wangsness equation which gives the net g‐factor in terms of these sub‐network moments and g‐factors. Having these sub‐network moments, the anisotropy energy can be described in terms of them, and it is found that the temperature dependence of the anisotropy constant can be fitted to a dipolar equation of the form: Ku=i,j=1Σ2CijMiMj

Magnetic anisotropy in Pb‐substituted Eu3 Fe5 O12 films

A large uniaxial magnetic anisotropy has been observed in (100) and (111) Pb‐substituted Eu3 Fe5 O12 garnet films grown by the LPE technique from PbO:B2O3 fluxes. For a Pb content of about 5.0 wt% the uniaxial anisotropy was 1.9×105 erg/cm3 in films of (100) orientation and 0.5 × 105 erg/cm3 for (111) film orientations. This uniaxial anisotropy is shown to be induced by the Pb and is similar to the growth‐induced anisotropy observed in mixed iron garnets.

Demagnetization factors for general ellipsoids

Convenient and inclusive tables for the demagnetization factors of general ellipsoids are calculated by an APL program using the mathematical framework of Osborn [Phys. Rev. 67, 351 (1945)] along with elliptical integral function tables of Spenceley and Spenceley [Smithsonian Elliptic Function Tables, 109, 3863 (1947)]. The new demagnetization tables have immediate application to magnetic and superconductor studies of present day prominence. For an ellipsoid of major axes a≥ (R18)b≥ (R18)c the range of variables is from 1≥ (R18)(c/a)≥ (R18)0.0001, and 1≥ (R18)(b/a)≥ (R18)0.01; appropriate approximations are indicated for smaller values of these ratios. In addition details of the mathematical calculation are included so that those interested may duplicate the procedure. Methods of applying this ‘‘ellipsoidal’’ approximation to other shapes such as parallelopipeds are also indicated.

Optical probing technique for inhomogeneous superconducting films

We report a nondestructive optical probing technique for superconducting films, by which a cross‐sectional gradient of the local transition temperature Tc and a two‐dimensional map of the local critical current Ic of an Al film were obtained. The two‐dimensional map clearly shows a variety of defects of the Al film. Some of them can be correlated to visible pinholes.

Magnetocrystalline anisotropy of Eu:YIG LPE films grown on (111) substrates

As previously reported in part by us, epitaxial films of Eu:YIG exhibit a strong dependence of the uniaxial anisotropy Ku upon Pb‐content. In the present work, magnetocrystalline and uniaxial anisotropies have been derived simultaneously from an analysis of the FMR rotation spectra of (111) films about an inplane 〈110〉 axis. The (111) EuIG films are grown on NdGaG, Eu2Y1IG films on SmGaG, and Eu1Y2IG and YIG films on GdGaG substrates by the LPE tipping process. The Pb contents range from 0.5 to 6 wt.% for growth temperatures ranging from 895 to 740°C. Bulk values are assumed or extrapolated for the magnetization. For EuIG, the room temperature value of K1 is −34,000 ergs/cm3 fro zero Pb; this is decreased in magnitude by about 30% by a 6 wt.% Pb addition. Similar dramatic effects occur in the other members of the Eu:YIG system. In the case of YIG, the magnitude of K1 is almost doubled by a similar addition of Pb. These changes in K1 cannot be explained on the basis of a simple dilution model in which Pb d...

EUROPIUM‐YTTRIUM IRON‐GALLIUM GARNET FILMS GROWN BY LIQUID PHASE EPITAXY ON GADOLINIUM GALLIUM GARNET

(Eu,Y)3(Fe,Ga)5012 garnets have desirable magnetic properties for bubble domain applications and can be grown as epitaxial thin films on simple Gd3Ga5012 substrate crystals. These films have the general formula EuRY3−RFe5−xGax012 and are in the composition range of the prototype Eu.5Y2.5Fe4Ga012. Room temperature magnetization can be estimated from the expression 4πM = 1170 − 120.R − 790.x. Uniaxial anisotropy in these films results from biaxial compressive stress induced by a lattice mismatch with the GGG substrates (ao = 12.384 A) . Lattice parameters for films grown from Pb0‐B203 fluxed melts by the LPE method are ao = 12.376 + 0.041·R − 0.015·x + 0.013·p, where p = wt.% Pb incorporated into the films. The effects of growth parameters on Ga content and Pb incorporation in these films are discussed. The Ga‐ and Pb‐contents of films are critical factors in determining magnetization and anisotropy, respectively.

Lattice mismatch between Pb‐substituted EuIG LPE films and NdGG substrates

The lattice mismatch Δa as a function of Pb content of the film, for LPE grown EuIG films on (100) and (111) NdGG substrates, deviates from linearity at Pb contents greater than 1 wt % Pb. At these Pb contents the films are compressively strained. It is suggested that the deviation is caused by a change in valence state of Pb and Eu. Heat treating films of high Pb content above 900°C resulted in stress‐relief.

Residual irreversibility in high-Tc superconductors

Magnetization measurements on many single crystals of YBaCuO as well as BiSrCaCuO show a non-vanishing remanent magnetization in a region where the field-cooled and zero-field-cooled magnetizations coincide. We call this kind of remanence “residual irreversibility”. We argue that this behavior is not compatible with over-simplified critical-state models or surface barriers. We show how it can arise from weak “residual” pinning in a regime where vortices are further apart than the penetration depth and the reversible B(H) curve shows a sharp initial rise just above Hcl. The “residual irreversibility” can be enhanced in crystals with an inhomogeneous or non-superconducting core, as seen in some of our thicker YBa/CuO crystals and also in BiSrCaCuO. We also show how the field-cooled Meissner fraction can measure the superconducting volume fraction in the low-field limit.