Magneto-optical Analysis Research Articles

Quantitative magneto-optical analysis of twinnedYBa2Cu3O7−δ/La1−xSrxMnO3 bilayers

Magneto-optical analysis was performed onYBa2Cu3O7−δ/La1−xSrxMnO3 bilayers grown ontwinned LaAlO3 substrates. Thetwin boundaries in the LaAlO3 substrate induced splitting of the manganite magnetic domains with either in-plane or out-of-planemagnetization, depending on the strontium doping level affecting the magnetic pattern. In theYBa2Cu3O7−δ film, the spontaneous vortex phase, as induced by the out-of-plane magnetic moments inthe manganite films, was directly visualized. For vortices nucleated by an externalmagnetic field, we distinguished different vortex dynamics due to the out-of-planemagnetic moments located either in correlated defects (twin boundaries) or in ahomogeneous part of the heterostructure. Vortex guidance over the magnetic domains,dependent only on the relative orientation of the vortex driving force with respectto the local out-of-plane magnetization in the manganite layer, was observed.By means of the inversion of the Biot–Savart law, the local modulations of thesupercurrent by the spontaneous vortex phase were imaged. Moreover, a localenhanced/depressed critical current was found across domains of opposite magneticpolarization.

Anisotropic temperature-dependent current densities in vicinalYBa2Cu3O7−δ

Thin epitaxial films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grown on vicinal cut substrates exhibit a substantial anisotropy of the critical current density ${j}_{c}$ in the film plane. By means of a quantitative magneto-optical analysis it is possible to investigate the current densities along different directions independently from each other. We performed a detailed analysis of the temperature dependence of the critical currents in the range $T=5--90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and found a completely different behavior of ${j}_{c}(T)$ along different directions. Describing the data by a step-wise power-law ansatz allows us to distinguish between different current limiting mechanisms.

Performances under ion irradiation of weak pinning superconductors MgB 2 and Bi 2 Sr 2 Ca 2 Cu 3 O x

A very significant figure of merit of a superconductor, for what concerns perspective applications, is related to its radiation hardness. In this paper we focus on the effect of high energy ions (4.2 GeV Au-ions) and low energy proton (3.5 MeV) irradiation on the superconducting properties of MgB2 tapes and films, respectively. Ions and energies are chosen as representative of the main radiation exposure in hard ambient. For sake of comparison, the behaviour of conventional high-Tc Superconductor with weak pinning, as Ag-sheated/Bi2Sr2Ca2Cu3Ox tapes, irradiated with 0.25 GeV Au-ions, is also reported. The main analysis methods consists in ac/dc magnetic and magneto-optical analysis. The obtained results indicate an high radiation hardness of MgB2 as a valuable figure of merit with respect to the application of this material in hard ambient, as well as, a radiation-induced significant enhancement of the critical current density and a large shift of the irreversibility line towards high temperatures in Ag-sheated/Bi-2223 tapes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magneto‐optical analysis of the local magnetic field distribution across superconducting/magnetic bilayers

The paper reports on an investigation of the superconducting and magnetic properties of a YBa2Cu3O7/La0.67Sr0.33MnO3 bilayer grown on LaAlO3. Beside structural, transport and optical characterisation the magneto-optical analysis was the main tool to study the local properties of the bilayer. The optical analysis shows the twinned structure exhibited by the substrate and the magneto-optical analysis allows visualizing the manganite magnetic domain structure with in plane-field magnetization, divided by domain walls where magnetic moments tilt out of the film plane. After cooling below Tc the domain-wall stray field induces vortex decoration patterns. Under an external magnetic field flux penetration is strongly modulated by the manganite domain walls. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Non-uniform columnar defect implantation in YBCO coplanar resonators for the control ofvortex-induced microwave dissipation and nonlinearity

This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed atcontrolling vortex-induced microwave dissipation and nonlinearity by means of asuitable distribution of columnar defects. Two procedures were used: (i) uniformirradiation at a given dose and (ii) non-uniform irradiation, obtained by means of amoving-target technique, producing a tailored profile of columnar defect density. The lastconfiguration is needed in order to attain an optimal compromise between defect-inducedenhancement of vortex pinning and increase of dissipative scattering. The resonatorswere characterized before and after irradiation in order to evaluate penetrationdepth and surface impedance, by means of a data analysis also suitable to accountfor the substrate properties. It turns out that in the case of uniform irradiationthe induced damage, attributed to the addition of dissipative regions, alwaysprevails over the improvement due to vortex pinning. In contrast, in the case of thetailored dose profile, significant damage is absent, while the onset of non-linearity isshifted towards higher circulating power. The need to control vortex-inducedeffects is confirmed by magneto-optical analysis that puts in evidence that vortexpenetration occurs even at low values of transverse dc magnetic field and temperature.

Characterization of silicon–YBCO buffered multilayers grown by sputtering

In recent years, the scientific community has considered with interest the possibility to integrate YBCO-based devices with silicon-based electronics. In fact, the proved YBCO radiation hardness makes this integration appealing from the point of view of space and telecommunication applications. In this paper we report on the influence of buffered substrate properties on the superconducting performances of YBCO films. In this framework we here consider the Si/CeO2/YBCO multilayer. The non-satisfying quality of the YBCO film in this multilayer is attributed to an unavoidable interlayer of SiO2 between Si and CeO2. On the other hand, we prove, by means of quantitative magneto-optical analysis, the excellent properties of the bi-layer CeO2/YBCO on YSZ substrate. Thus, these measurements indicate YSZ as the best candidate to be deposited between Si and CeO2 for optimal YBCO performances on silicon.

Magneto-optical analysis of the critical current density dependence on temperature inproton irradiated YBCO films

A quantitative magneto-optical analysis of YBCO thin films, as grown and irradiated with3.5 MeV protons, is presented. Induction field maps are drawn in the temperature range 5–70 Kand, by means of the Biot–Savart law inversion, the local current density distributions areevaluated. The paper is focused on the temperature dependence of the critical current densities(Jcs) beforeand after irradiation at increasing fluence. Temperature-modulated damage thresholds are reached. To accountfor the Jc dependence on temperature for all the considered defect densities and in the wholeinvestigated temperature range, a model based on a Josephson junction formalism isproposed. Namely, the vortex distribution across the whole sample results in a currentdensity that mirrors the current through an average short Josephson junction (JJ) row. TheJJ length is modulated by the defect distribution. By applying this model, asatisfying agreement between the experimental data and the model is found in thewhole temperature range, both before and after irradiation. A suitably definedtemperature-dependent magnetic thickness of the junctions is the chief quantity that allowsapplying this JJ formalism to the vortex distribution across the defected matrix.

Temperature dependence of the critical current density in proton irradiated YBCO films by magneto-optical analysis

In this paper we present a magneto-optical analysis of local current densities in YBCO films, before and after 3.5 MeV proton irradiation. The main issue consists into measuring and interpreting the temperature dependence of the critical current density (J c ) in samples with different, increasing defect density. Proton irradiation adds more point defects into the as-grown films. The new defect density as well as the related strain-induced modifications of the order parameter are pushed in our experiment up to temperature-modulated damage thresholds. First of all model-independent J c data were analysed in the framework of different pinning models, all of them based on mechanisms related to the temperature induced change of the effective pinning centre distribution as well as to the shape of single pinning wells. It turns out that in such a framework the fit parameters are, generally speaking, not suitable to interpret the changes of the pinning landscape across the whole investigated temperature range. Then a model based on a vortex distribution across the whole sample, resulting in a current density that mirrors the current through a defect-modulated average short Josephson junction (JJ) row, is successfully tried. The J c dependence in the whole temperature range and for all the considered defect densities is accounted for by means of a coherent set of fit parameters. It turns out that the chief quantity that allows applying the JJ formalism to a vortex distribution across the defected matrix is a suitably defined temperature-dependent magnetic thickness of the junctions, which substitutes the usual magnetic penetration in JJs.

Current density distribution in superconducting YBCO films with correlated defects

The quantitative magneto-optical analysis (QMO) is a powerful tool to observe in a parallel way the local electrodynamics properties of superconducting films of whatever geometry. If the analysis directly provides the correlation between the geometric constraints and the superconducting properties, at the micron scale, we can find out signatures of anisotropic pinning due to vortex interaction with correlated defects. In this paper we present a set of QMO measurements on YBCO films, as grown and irradiated with heavy ions, in order to produce extrinsic nanometric-size columnar defects. It will be shown (1) how the nanodefect arrangement should be controlled to obtain the wanted pinning characteristics in thin samples; (2) which are the most effective pinning centers among intrinsic defects due to film texturing and extrinsic columns; (3) what is the useful temperature range of actual critical current in each system.

Near-field magneto-optical analysis in reflection mode SNOM

Observation of magnetic domains with in-plane magnetisations is demonstrated by scanning near-field optical microscopy (SNOM) in reflection mode. The longitudinal and transverse magneto-optical Kerr effects are employed as the contrast mechanisms; these are observed as either a change in the polarisation of the reflected light or reflectance, depending on magnetisation direction. SNOM images of Co and Ni thin films show magneto-optical contrast depending on polarisation of the incident and detected light. For the smooth cobalt thin films, the orientation for magnetic domains is estimated, based on the correlation between the contrasts in SNOM images obtained in different polarisation configurations and the directions of the magnetic vectors of the incident and reflected light. For the nickel films with pronounced topographic structures, the resulting near-field polarisation dependencies are more complicated, suggesting that the magneto-optical contrast in SNOM images are affected by the topographic cross-talk due to the depolarisation effects on surface topographic features.

Thickness dependence of the current density distribution in superconducting films

By means of a quantitative magneto-optical analysis, we show how the current density distribution is affected by sample thickness, due to geometrical effects and changes in microstructure. We found a maximum into the critical current density for film thickness twice the London penetration depth. For thinner films, the magnetic field screening is poorer and the actual critical current decreases. For thicker specimens, the vortices are strongly tilted, as the magnetic field lines are, and the critical current decreases again, because the most effective pinning sites are correlated defects. Here, we present low temperature measurements on high temperature YBCO superconducting films, carefully chosen in order to compare films with similar microstructure, i.e. with the almost same pinning sites density and distribution. Finally, we put in evidence the interplay between sample geometry and microstructure as revealed by the current density distribution.

Effect of oxygen adsorption on the structure and spin-reorientation transition of Fe films on Cu(1 1 25)

The morphology, structure, and magnetism of Fe films on an oxygen-precovered, stepped Cu(1 1 25) surface are investigated by scanning tunneling microscopy, low-energy electron diffraction, and magneto-optical Kerr effect analysis. After exposure of Cu(1 1 25) to about 600 L of oxygen at 493 K, a well-ordered (2√×√)R45° superstructure is formed. The O-induced structure has a zig-zag morphology, showing a high density of facets and is very stable during subsequent Fe deposition. Fcc Fe can be stabilized up to 20 monolayer (ML) thickness, while the magnetization of the films reorients from perpendicular to in-plane at 15 ML. From 24 ML, the linear extrapolation line of the thickness dependence of the Kerr intensity of the transformed bcc Fe films does not go along with the pure fully magnetized bcc Fe films. We conclude that oxygen adsorption strongly affects the structure and spin-reorientation transition of Fe films.

Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni–Mn–Ga alloys

Abstract Direct observations of the austenite and martensite twin and magnetic domain structures are combined with thermomagnetic analysis (low-field AC susceptibility temperature measurements) of phase transitions in ferromagnetic shape memory alloys Ni 2+ x Mn 1− x Ga. Thermal hysteresis and magnetization jumps are clearly resolved by both techniques. Large Barkhausen-like jumps associated with discontinuous phase boundary and martensite twin motion are observed during the austenite–martensite transformation in contrast to the smooth ferromagnetic–paramagnetic transition near the Curie temperature.

SECOND PEAK FEATURES IN BSCCO SINGLE CRYSTALS OBSERVED BY THE MAGNETO-OPTICAL ANALYSIS

We performed magneto-optical analysis on slightly overdoped Bi 2 Sr 2 CaCu 2 O 8+x single crystals, looking for the second peak effect. On the basis of a method suitably developed to extract from the magneto-optical images the main electrodynamic quantities (magnetic induction field, B z , and current density, J), the local magnetisation, M s and the local J distribution maps were obtained, respectively. For the first time (at our best knowledge) patterns clearly due to a second peak in the M s vs. B z and J vs. B z curves were put in evidence and correlated each other by a magneto-optical analysis. The second peak slightly shifts with the temperature and with the position inside the sample, while its sharpness and depth do increase when the considered area is lowered. All the experimental evidences point towards a straightforward correlation between the second peak effect and the local disorder at micrometric scale.

Blocking temperature distribution in implanted Co–Ni nanoparticles obtained by magneto-optical measurements

Three samples of Co–Ni alloy nanoparticles with different compositions were prepared by sequential ion implantation in silica slides. Transmission electron microscopy (TEM) showed the presence of spherical nanoparticles dispersed in the matrix. Magneto-optical Kerr effect analysis identified two magnetic components attributed to superparamagnetic particles in unblocked and blocked states, respectively. Magnetic field loops were measured as a function of temperature. Blocking temperature distributions were obtained; and their comparison with the size distributions derived from TEM provided the average magnetic anisotropy of the particles.

Quantitative magneto-optical analysis of macroscopic supercurrent flow in MgB2

Magnetic flux penetration and trapped flux patterns formed into a MgB2 film during the increase and the subsequent decrease of the applied external magnetic field were studied using magneto-optical imaging. The film was grown by electron-beam evaporation and exhibits a granular structure. The non-homogeneous fan-like shaped penetration, already pointed out in the literature, was observed. To investigate the origin of this kind of penetration, a quantitative approach was chosen. The induction magnetic field map and the corresponding contour map of a framed zone show many isolated loops originating from the granular nature of the sample. Inside such loops, an estimation of the local current density was made through the inversion of the Biot–Savart law. All the results point towards the conclusions that the fan-like shaped patterns do mirror the percolation of dissipation paths. Along these paths, some interfaces, distributed in a hierarchical order, play the role of either pinning barriers or easy-flow channels.

An improved method for quantitative magneto-optical analysis of superconductors

We report on the analysis method to extract quantitative local electrodynamics in superconductors by means of the magneto-optical technique. First of all, we discuss the calibration procedure to convert the local light intensity values into magnetic induction field distribution and start focusing on the role played by the generally disregarded magnetic induction components parallel to the indicator film plane (in-plane field effect). To account for the reliability of the whole technique, the method used to reconstruct the electrical current density distribution is reported, together with a numerical test example. The methodology is applied to measure local magnetic field and current distributions on a typical YBa2Cu3O7−x good quality film. We show how the in-plane field influences the MO measurements, after which we present an algorithm to account for the in-plane field components. The meaningful impact of the correction on the experimental results is shown. Afterwards, we discuss some aspects about the electrodynamics of the superconducting sample.

MO analysis of macroscopic supercurrent flow in MgB 2

We present results from an extended magneto-optical (MO) analysis of two samples cut from high-density pellets of MgB2. The first sample was studied in order to show that no matter how large the sample is and despite the bulk granularity, the material enters into a critical state in a crystal-like fashion. The second sample was chosen for the quantitative analysis. A numerical approach based on an inverted 2D Biot-Savart model was used to calculate the current paths across the homogeneous polycrystalline bulk, as well as in the vicinity and across some morphological defects. Local current densities in the homogeneous part were estimated as a function of the applied magnetic field at different temperatures, in three regimes: below full penetration, at full penetration and above full penetration, respectively. A hypothesis of interpretation of the apparent absence of magnetic granularity inside the polycrystalline microstructure is presented. It is related to a critical state likely reached by a network of strongly coupled Josephson junctions.

Variation of magnetic properties along the film thickness in perpendicular thin film media investigated by optical method

Local variation of magnetic properties along the film thickness for CoNiCrTa perpendicular thin film media is discussed based on the magneto-optical analysis. It is found that the thickness of the bottom layer, which has low coercivity, is roughly estimated to be about 10 nm. Furthermore, the hysteresis loop of the bottom layer can be calculated by using magneto-optical multilayer model.

Effects induced by crossing columnar defects and blind holes on Bi-2212 single crystals

Blind holes (columnar defects created only across a part of the samples) and crossing columnar defects are compared as tools to enhance the superconducting properties of Bi-2212 single crystals. These two different kinds of defects are created, into two twin samples by means of Au ion irradiation, respectively. Extrinsic defects are competing with particular intrinsic defects (shaped as “lamellae”) which control the flux entering in such crystals. Complementary measurements are performed by means of susceptometric and magneto-optical analysis. It is shown that blind holes are at least as effective as crossing columnar defects in competing with these intrinsic defects in different vortex phases.