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We review superconducting vortex imaging in Co-doped BaFe 2 As 2 by Bitter decoration and small-angle neutron scattering (SANS). At all measured fields a highly disordered vortex configuration is observed, which is attributed to strong pinning. Further support of this conclusion comes from the absence of a Meissner rim in decoration images obtained close to the sample edge. The evolution of the SANS scattering vector with increasing applied field indicates vortex lattice domains of (distorted) hexagonal symmetry. This is consistent with the decoration images which show small, six fold coordinated ordered vortex domains. The SANS scattered intensity is found to decrease rapidly with increasing field, exceeding the rate expected from estimates of the upper critical field. This is consistent with the large degree of vortex “lattice” disorder.

With using the decoration technique the vortex distribution near steps on the surface of BSCCO (2212) single crystals has been investigated. The vortex structure visualization was carried out by optical and scanning electronic microscopy (SEM) as well as by atomic force microscopy (AFM). The Meissner rim was revealed on the top side of the steps which height h was more than 0.1 a 0 —vortex lattice parameter. The width of the band is proportional to the height of the step h and does not depend on the magnetic field. The vortex lattice behavior in the vicinity of the sample steps is discussed.

The Bitter decoration technique has been used to observe vortices in the superconducting state of single crystal MgB2. Experiments in low applied magnetic field (B≈4.4 G) allow for the estimation of the London penetration depth, λ, to be approximately 1900 A for T≈6 K. Experiments in higher applied fields (B≈200 G) unambiguously show a triangular vortex lattice in both real space (a larger than 10 μm ×10 μm image of over 1000 vortices) as well as in reciprocal space.

The high resolution Bitter technique was used for observation of the vortex structure in twinned YBaCuO single crystal in the magnetic field tilted with respect both to the c-axis and to the twin planes. The dominating effect in vortex distribution was strong pinning by twin planes which was seen as an alignment of vortices along twins with approximately two times density higher density than in the bulk. This strong pinning remained unchange for a surprisingly broad interval of tilt angles 0

We present small-angle neutron scattering (SANS) and Bitter decoration studies of the superconducting vortices in $\text{Ba}{({\text{Fe}}_{0.93}{\text{Co}}_{0.07})}_{2}{\text{As}}_{2}$. A highly disordered vortex configuration is observed at all measured fields and is attributed to strong pinning. This conclusion is supported by the absence of a Meissner rim in decoration images obtained close to the sample edge. The field dependence of the magnitude of the SANS scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry, consistent with the decoration images which show primarily sixfold coordinated vortex domains. An analysis of the scattered intensity shows that this decreases much more rapidly than expected from estimates of the upper critical field, consistent with the large degree of disorder.

The magnetic flux structure in the basal plane, (001), of single crystals of superconducting (R = Er) and non-superconducting (R = Tb) RNi$_2$B$_2$C was studied by high resolution Bitter decoration at temperatures below $T_c$ (superconducting transition) and/or $T_N$ (antiferromagnetic transition). For both materials two sets of domain boundaries, in \{110\} and \{100\} planes, were observed. The temperature ranges in which the \{100\} domain boundaries were observed in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C coincide with the weak ferromagnetic (WFM) ordering in these materials. On the other hand, the \{110\} twin boundaries - the antiferromagnetic domain boundaries - were observed in both compounds below $T_N$. The possibility of interpretation of \{100\} boundaries as Bloch domain walls in the weakly ferromagnetic phase, for $T < T_{WFM} < T_N$ (TbNi$_2$B$_2$C) or $T < T_{WFM} < T_N < T_c$ (ErNi$_2$B$_2$C) is discussed.

Vortex lattices in LuNi2B2C have been investigated by the decoration method at fields 20–1000 Oe at liquid helium temperatures and in a field cooling regime. At small fields of 20–200 Oe we have observed vortex lattice domains with different lattices orientations. Domains size increased with magnetic field increase. At low fields vortex lattices closepacked directions in small domains do not correlate with crystal directions and the correlations become stronger at greater fields. One of the closepacked vortex arrays directions within a domain becomes parallel to 〈1 1 0〉 or 〈1 0 0〉 directions. The vortex lattices themselves evolve at larger fields. The vortex lattice is aligned with the 〈1 0 0〉 direction and its rhombic cell apex angle β increases from 64° at 180 Oe to 75–90° at 1000 Oe.

Using a decoration technique, the influence of artificial columnar defects on the vortex lattice in BSCCO single crystals was investigated. A transition from a fully disordered to a “polycrystalline” lattice was observed as the ratio of columnar defects to magnetic induction decreased.

Pinning at the grain boundaries in PbIn alloys is investigated. It is shown that the interaction of a flux-line lattice with the grain boundaries is determined by anisotropies of the superconducting properties of the material under investigation. Such a conclusion confirms the basic assumption of the existing theoretical models. It is shown that the field and temperature dependence of the critical current due to the grain boundaries is in good agreement with the conclusions of Larkin and Ovchinnikov's microscopic theory on weak inhomogeneity of an electron—electron interaction constant. Es wird Versetzungsverankerung an den Korngrenzen in PbIn-Legierungen untersucht. Es wird gezeigt, das die Wechselwirkung eines Flusliniengitters mit den Korngrenzen durch Anisotropien der supraleitenden Eigenschaften des untersuchten Materials bestimmt wird. Dieser Schlus bestatigt die Grundannahmen der existierenden theoretischen Modelle. Es wird gezeigt, das die Feld- und Temperaturabhangigkeit des kritischen Stromes infolge der Korngrenzen mit den Schlusfolgerungen der mikroskopischen Theorie von Larkin und Ovchinnikov uber schwache Inhomogenitat einer Elektron—Elektron-Wechselwirkungskonstante gut ubereinstimmt.

In the best crystals of YBa 2 Cu 3 O 7−x the temperature behaviour of the c-axis resistivity is metallic. The structure of Abrikosov's vortexes in ab-plane does not reveal the anisotropy of the penetration depth, possibly, due to the presence of a dense net of twins.