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The high-resolution Bitter technique for visualization (decoration) of the magnetic-flux structure in superconductors and magnets at low temperatures is presented. This method is based on the preparation of magnetic nanoparticles directly during a low-temperature experiment (in situ) by evaporating a magnetic material in the atmosphere of a buffer gas (helium) above the sample surface. An apparatus was constructed and a technique was proposed to stabilize the temperature of a sample during decoration with an accuracy of ±1 K within a wide temperature range. As an example, the results of observation of the magnetic-flux structure in single crystals of BSCCO(2212) high-temperature superconductor and EuFe2(As0.79P0.21)2 ferromagnetic superconductor at T ≤ 18 K are presented.

A spontaneous vortex structure in magnetic domains (vortex domains) has been visualized by the decoration technique near the ferromagnetic transition temperature (TC = 18 K) in a ferromagnetic superconductor EuFe2(As0.79P0.21)2 single crystal with the superconducting transition temperature (TSC = 22 K). Meissner domains previously observed by magnetic force microscopy have also been observed in a narrow temperature range (ΔT ∼ 1 K) near TC. A domain structure with the alternating direction of the magnetic flux has been observed upon cooling below TC. Experimental results are in reasonable agreement with a theoretical estimate of the periods of domain structures and the temperature interval below TC in which they are observed.

Magnetic flux structure on the surface of EuFe$_2$(As$\rm_{1-x}$P$\rm_x$)$_2$ single crystals with nearly optimal phosphorus doping levels $x=0.20$, and $x=0.21$ is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with $x=0.21$ in the temperature range between the critical temperatures $T_{\rm SC}=22$ K and $T_{\rm C}=17.7$ K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below $T_{\rm C}$. The nature of this structure is discussed., 6 pages, 3 figures

Transport, magnetic and optical investigations on EuRbFe$_4$As$_4$ single crystals evidence that the ferromagnetic ordering of the Eu$^{2+}$ magnetic moments at $T_N=15$ K, below the superconducting transition ($T_c=36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_N$, the zero resistance state is preserved and the superconductivity is affected by the in-plane ferromagnetism mainly at domain boundaries; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_c$, associated with the opening of a complete superconducting gap at $2\Delta = 10$ meV. A gap smaller than the weak coupling limit suggests the strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap., Comment: 4 pages, 5 figures

The vortex structure of Bi2Sr1.65La0.35CuO6+δ single crystals in tilted magnetic fields has been studied by the decoration method. From the observed pattern of vortex chains in the basal plane, the parameter of anisotropy in the superconducting state has been estimated as γS = 460 ± 40. The electric resistance of Bi2Sr1.65La0.35CuO6+δ single crystals has been studied in a broad range of temperatures (T c < T < 300 K) and magnetic fields (up to 16 T). The ratio of resistivities in the direction perpendicular to the basal plane and in plane near the critical temperature T c amounted to ρ⊥/ρ‖ = 3.2 × 105. A possible relationship between the anisotropy in the normal and superconducting states is discussed.

Single crystals of iron-containing Ba(FeAs)2 superconductors doped with phosphorus have been studied. The vortex structure has been studied by means of the decoration method. Large domains of the triangular vortex structures have been observed in the phosphorus-doped single crystals of iron-containing superconductors. The effect of doping on pinning in iron-containing superconductors has been discussed.

The vortex structure in the iron pnictide single crystals Ba 1– x K x Fe 2 As 2 and Sr 1– x K x Fe 2 As 2 of the 122 type and SmFeAsO 1– xFx of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.

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.

The magnetic structure of the doped perovskite manganites La0.7Sr0.3MnO3 and La0.8Ca0.2MnO3 at a temperature of 10 K has been investigated by using the decoration technique in a wide region of external magnetic fields (up to 1000 Oe). We observe the multidomain ferromagnetic state in La0.7Sr0.3MnO3 and the coexistence of the ferro- and nonmagnetic phases in La0.8Ca0.2MnO3. The dependence of the structure on an external magnetic field has been investigated in La0.8Ca0.2MnO3.

Vortex structure in FeTe0.66Se0.44 and FeTe0.6Se0.4 single crystals with Tc ∼ 11.7 and 14.5 K, respectively, has been studied using the decoration technique. It has been found that in single crystals with the simplest crystalline structure of 11-family iron-containing superconductors (without interlayers), no regular vortex lattice is observed, similar to the case of the previously studied 122 and 1111 families. Using transmission electron microscopy, the dislocation structure with a density of ∼109 cm−2 has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.

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.

With the aid of the decoration technique, a method is developed for observing the vortex structure forming at different temperatures. Behavior due to the effect of volume pinning and the geometric barrier in thin BSCCO (2212) single crystals are found.

The domain structure of TbNi2B2C was studied by the finest (⋍0 nm) magnetic-particle decoration below 13 K. The twin domain boundaries caused by magnetoelastic stresses were observed in the {110} planes.

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.

Since the discovery of superconductivity above liquid nitrogen temperature in copper oxide based compounds many experiments have been performed on the one hand to study their mixed state properties and on the other hand to check their technological applicability. Due to their well defined cylindrical shape latent particle tracks (also known as columnar defects) are a powerful tool to investigate the mixed state in high temperature superconductors (HTSC) as they provide optimized pinning centers. Therefore in this paper we will present experiments performed with HTSC samples (epitaxial thin films on SrTiO 3 substrates, single crystals and technical tapes) containing columnar defects. We will outline the influence of these linear defects on the resistive phase transition and on transport I–V curves of YBa 2 Cu 3 O 7−δ (123) thin films. All HTSC known up to now exhibit a pronounced structure-related anisotropy. Hence we present magnetic and transport measurements concerning the angular dependence of the critical current in various irradiated HTSC compounds. Furthermore experiments with technical tapes of (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10 (Bi2223) will be shown giving an overview of problems concerning technological applications. Finally in irradiated and nonirradiated Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) single crystals the flux line lattice is visualized via the high resolution Bitter decoration technique.

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.

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0,47}$Ni$_{0,53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the top of the films well below the Curie temperature T$_{Curie}$ ($\sim$ 60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm., 4 pages, 5 figures, will be published in JETP Lett

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.

We report the observation of superconducting vortices in pure and lightly Al doped ${\mathrm{MgB}}_{2}$ single crystals. Low field experiments allow for the estimation of the absolute value of the London penetration depth, $\ensuremath{\lambda}=1900\ifmmode\pm\else\textpm\fi{}600\mathrm{\AA{}}$ for $T\ensuremath{\sim}6\mathrm{K}.$ Experiments in higher fields (e.g., 200 Oe) clearly show a triangular vortex lattice in both real space $(13\ifmmode\times\else\texttimes\fi{}13\ensuremath{\mu}{m}^{2}$ Bitter decoration image of over 1000 vortices) and reciprocal space.

Evolution of the vortex lattice, the rhombus-square transition included, for ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ in the field along the c crystal axis, is tracked by the decoration technique pushed up to the record high (for this method) field of 1480 Oe. Decoration images are analyzed with the help of the Fourier transform, which indicates disordered structures in small fields of a few Oe. In fields $Hl200 \mathrm{Oe}$ the coexisting domains of different structures are observed. A technique based on the Fourier transform is employed to see the domains separately. The transition to the square lattice is recorded near 900 Oe. The results are in agreement with predictions of the nonlocal London theory.

The family of rare-earth metal nickel-borocarbides RNi2B2C (R=Er, Lu, Tm, Ho and so on) is an object of considerable attention due to the interplay between magnetic and superconducting properties [1]. Unconventional behavior of vortex lattices in these compounds and, in particular, the phase transition from triangular to square vortex lattice driven by the magnetic field, has been discovered. Various experimental methods of studying vortex lattices were employed: the small-angle neutron scattering (SANS) [2], the scanning tunneling microscopy (STM) [3], and the decoration by small magnetic particles [2]. One should note that whereas SANS and STM studies are effective at rather high magnetic fields (from a few kOe to T), the decoration technique is commonly used in the low field domain (about 100 Oe or less).

The magnetic flux structure in type-II superconductors was predicted in 1957 by Abrikosov [5.1]. According to that theoretical work the magnetic field penetrates into a superconductor in the form of individual magnetic lines (vortices) each carrying one magnetic flux quantum Ф0 = hc/2e = 2.07.10−7 G.cm2. The idea of a vortex state proved to be extremely valuable in explaining electrodynamic properties of type-II superconductors. The most convincing evidence of the validity of this theory was the direct observation of the Abrikosov vortices using the high-resolution technique of decoration with dispersed ferromagnetic particles. This technique was developed in 1966 by Traumble and Essmann in Stuttgart [5.2]. Their works marked the beginning of magnetic structure investigations in conventional low-Tc type-II superconductors; the results have been summarized in reviews and monographs [5.3,5].

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.

RNi2B2C single crystals, where R = Lu, Y, Er, Ho and Tb have been studied by high-resolution Bitter decoration technique, over the wide range temperatures and magnetic fields up to 2 T. Diverse vortex lattice structures were investigated: transition from triangular to square lattice for LuNi2B2C single crystals; peculiar vortex structures associated with antiferromagnetic (AFM) and weak-ferromagnetic (WFM) states below Tc for ErNi2B2C single crystals . In addition, Bitter decoration revealed structures associated with a long range magnetic order in the non-superconducting TbNi2B2C as well as ErNi2B2C in the normal but magnetically ordered state.

The mixed state of highly anisotropic high-temperature superconductors like BSCCO has a very complicated phase diagram. The vortex structure has been investigated in optimally doped BSCCO single crystals at helium temperatures and magnetic fields up to 600 Oe by high- resolution Bitter technique. The transition from regular triangular vortex lattices to disordered state was revealed within an interval of magnetic fields 230–400 Oe for different samples. The anisotropy parameter was measured from observation of chain vortices at tilted magnetic fields. The results are discussed in framework a model taking in to account the influence of anisotropy and defects on vortex phase diagram.

The magnetic structure of the high-TC superconductors has been investigated using the high-resolution technique of decorating the sample by small ferromagnetic particles. The flux line lattice was observed in single crystals Y1Ba2Cu3Ox,Ho1Ba 2Cu3Ox' Tl2Ba2Ca1Cu2O and Bi2Sr2Ca1Cu2Ox. It was found out that the flux quantum is ΦO = hc/2e = 2.07*10-7 G*cm2 in the investigated high-Tc superconductors, as in traditional superconductors. A possibility of estimating the penetration depth from observation of single vortices in low magnetic fields is discussed. An interaction between the flux line lattice and the crystal lattice is considered. A compression of the regular triangular lattice of vortices was observed in the a - b plane in a single domain area of the orthorhombic YBa2Cu3Ox single crystal. This compression is an evidence of vortex anisotropy in this plane. The ratio of superconducting carrier effective masses in the a and b directions was estimated as μa μb = 1.4 ± 0.2. Strong pinning of Abrikosov vortices has been found by twin boundaries in Y Ba2Cu3Ox and by unidentified plane defects along (100) or (010) in Bi2Sr2Ca Cu2Ox single crystals. A triangular FLL with a long-range order has been observed in the perfect Tl2Ba2Ca Cu2Ox single crystal. The elementary pinning force exerted on the FLL from twin boundaries has been estimated for YBa2Cu3Ox single crystals.

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.

A detailed investigation of the flux line lattice (FLL) interaction with nonuniformly distributed, mainly screw dislocations in deformed Nb-Mo single crystals has been made using the high resolution technique of decorating a sample by small Fe particles. The interaction has been considered from the point of view both of elementary pinning force summation and of the particular pinning mechanism. The FLL has been observed to split into short-range order regions at a low dislocation density and to be amorphous at densities exceeding 1010 cm−2. The dimensions of these regions determine the critical current value in agreement with the collective pinning theory. The observed distribution of the flux lines has been shown to result from the dielastic ΔE-interaction of the dislocations with the FLL.

Part of the phase diagram of the La 2 O 3- CuO system has been studied and single crystals of La 2 CuO 4, (LaSr) 2 CuO 4 (up to 20 × 8 × 6 mm 3), YBa 2 Cu 3 O 7−δ (up to 8 × 6 × 0.1 mm 3) and HoBa 2 Cu 3 O 7-(up to 3 × 2.5 × 1 mm 3) have been grown by the method of spontaneous crystallisation, using a rotational crucible. It has been established that thermal treatment of "123" single crystals under oxygen pressure to 10 bars improves their superconducting properties. Electrical, magnetic, optical properties, and the real structure of the single crystals produced have been studied.

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.

This paper reports observation of oscillation on current—voltage characteristics in deformed Nb single crystals in the vicinity of fields corresponding to the growth of the critical current density, near the upper critical field (peak effect).

The magnetic structure of the YBa2Cu3Ox single crystal has been investigated using the technique of decorating the sample with small ferromagnetic particles. The lattice of the Abrikosov vortices and the effects caused by pinning on the twin boundaries have been observed in both the electron and optical microscopes. The twin structure of the single crystals has been investigated using the polarized light in the optical microscope. The penetration depth for the magnetic field parallel to the c axis has been evaluated as ≲ 0.3 μm at 4.2 K for the YBa2Cu3Ox single crystal.

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.

The flux-line lattice (FLL) has been observed on the (001) face of high Tc Bi2.2Sr2CaCu2OX and Ti2Ba2CaCu2OX single cristals using the technique of decorating the sample with small ferromagnetic particles. Strong pinning of the Abrikosov vortices by plane defects along (100) and (010) planes has been found out on Bi2.2Sr2CaCu2OX. The triangular FLL with a long-range order has been observed on the perfect Ti2Ba2CaCu2OX single crystal. The penetration depth for a magnetic field parallel to (001) axis has been evaluated as ≤0.2μm at 4.2K for both materials.