A model of superconductivity in high-temperature superconducting layered cuprates is proposed, based on the extended saddle point singularities in the electron spectrum, weak screening of the Coulomb interaction and phonon-mediated interaction between electrons plus a small short-range repulsion of Hund's, or spin-fluctuation origin. This permits to explain the large values of T_c, features of the isotope effect on oxygen and copper, the existence of two types of the order parameter, the peak in the inelastic neutron scattering, the positive curvature of the upper critical field, as function of temperature, etc. The resonant tunneling mechanism for the c-axis transport is proposed. Real physical properties are calculated and compared with experimental data. These included the temperture dependence of the static c-axis conductivity in the normal state, frequency dependence of the optical conductivity and stationary supercurrent along the c-axis. It is demonstrated that for the latter the coherence of resonant tunneling through different centers is of primary importance. The resonant tunneling idea is used for description of the origin and some properties of the "pseudogap phase". The superconducting critical temperature in this picture is defined at low doping by establishment of a 3-dimensional phase correlation between the layers, and at high doping by destruction of a d-wave superconductivity by disorder. The result is a nonmonotinic dependence of T_c on doping. The pseudogap phase is described on the basis of the Franz-Millis model of superconducting fluctuations, consisting of small superconducting domains with uncorrelated supercurrents. The calculated characteristics, namely, the spectral function, the inelastic neutron scattering cross section, and the spin susceptibility agree wtih experimental data.

It has been known for a long time that many systems, including disordered metals and metallic oxides, undergo a metal/insulator transition (MIT). We have found that the superconducting transition temperature, T_c, of such materials is enhanced in the vicinity of the MIT. We have constructed superconductivity phase diagrams (T_c vs σ, the conductivity) for many materials whose only common feature is their proximity to the MIT and found that they are remarkably similar. These results suggest that there is a common mechanism for the enhancement of superconductivity near the MIT. Following this clue, we developed a simple, heuristic model based on scaling theory near the MIT that accounts for the observed features.

The ab-plane infrared and visible (3 meV - 3 eV) response of Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) thin films (prepared by r.f. sputtering on SrTiO₃) has been measured between 300 K and 10 K for different doping levels. In the superconducting state, dramatic differences appear between the underdoped and overdoped regimes regarding the electrodynamics of the formation of the superfluid condensate. In the over-doped regime, the superfluid grows up by removing states from energies below 60 meV. This energy is of the order of a few times the superconducting gap. In this respect, overdoped Bi-2212 exhibits a conventional behavior. In the underdoped regime, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of electronic kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.

It has recently been shown that x-ray diffraction from the doped holes in cuprates can be enhanced by 3-4 orders of magnitude by exploiting resonance effects in the oxygen K shell. This new type of anomalous scattering is direct way of probing ground state inhomogeneity in the mobile carrier liquid of high temperature superconductors. Here we describe a model which quantifies the relationship between experimental count rates and the structure factor for doped holes in this technique. We describe first efforts to detect inhomogeneity in thin films of La₂CuO_4+δ and report some peculiar observations. We attempt to offer some explanation.

DC voltage versus current measurements of superconductors in a magnetic field are widely interpreted to imply that a phase transition occurs into a state of zero resistance. We show that the widely-used scaling function approach has a problem: Good data collapse occurs for a wide range of critical exponents and temperatures. This strongly suggests that agreement with scaling alone does not prove the existence of the phase transition. We discuss a criterion to determine if the scaling analysis is valid, and find that all of the data in the literature that we have analyzed fail to meet this criterion. Our data on YBCO films, and other data that we have analyzed, are more consistent with the occurrence of small but non-zero resistance at low temperature.

The concept of ferroelasticity and ferroelastic transition is perovskite oxides is reviewed. The presence of ferroelasticity in perovskite cuprates has been revealed by the stress-strain ferroelastic hysteresis, abrupt changes in the temperature dependence of atomic displacements and by formation of ferroelastic domain walls and nanodomains in the copper-oxygen planes. Ferroelastic properties could be responsible for many macroscopic physical properties of HTSC cuprates, including electric transport and magnetic properties, tunneling spectra and phase diagrams.

Orientational dependent x-ray absorption fine structure (XAFS) measurements on the local structure of the La₂CuO₄ based high T_c superconductors La_2-xSr_xCuO₄, La_2-xBa_xCuO₄, La_1.6-xSr_xNd_0.4CuO₄, La_1.85Sr_0.15Cu_1-yNi_yO₄, find both displacement and orientational disorder induced by doping (Sr,Ba) and alloying (Nd,Ni) atoms. The displacement disorder is in the first few neighboring atoms to the perturbing atom while the orientational disorder is of nanometer-sized regions which have disordering of the tilt angles of their CuO₆ octahedra with respect to neighboring regions. This disorder affects both the normal and superconducting properties of the materials. It is found that disorder induced in the Cu-O₂ planes has the greatest detrimental effect on T_c and conductivity.

We use a pulsed laser deposition (PLD) setup to grow ultra-thin films of high temperature superconductors (HTSC) and transfer them in-situ into a photoemission chamber. Photoemission measurements on such films allow us to study non-cleavable materials, but can also give insights into aspects never measured before, like the influence of strain on the low energy electronic structure. Systematic studies of many different materials grown as films showed that Bi₂Sr₂CaCu₂O_8+x, Bi₂Sr₂Cu₁O_6+x, Bi₂Sr₂Ca₂Cu₃O_10+x and La_2-xSr_xCuO₄ films exhibit a conductor-like Fermi edge, but materials containing chains (such as YBa₂Cu₃O_7-x) are prone to very rapid surface degradation, possibly related to critical oxygen loss at the surface. Among HTSC materials, La_2-xSr_xCuO₄ is extremely interesting because of its rather simple structure and the fact that its critical temperature T_c can be enhanced by epitaxial strain. Here we present our first high resolution angular resolved photoemission spectroscopy (ARPES) results on 8 unit-cell thin La_2-xSr_xCuO₄ films on SrLaAlO₄ [001] substrates. Due to the lattice mismatch, such films are compressed in the copper oxygen planes and expanded in the c-axis direction. Results show a surprisingly modified Fermi surface compared to the one of non-strained samples.

The physical properties of the perovskite-type oxide RuSr₂GdCu₂O₈ have been recently discussed in the view of a simultaneous occurrence of superconductivity and ferromagnetism. In order to explore some peculiarities of these compounds we have prepared superlattices of oxides that are known to be either ferromagnetic [La_.67Ca_.33MnO₃ ] or superconducting [YBa₂Cu₃O₇]. Superlattices of different periodicity serve as model systems for the understanding of the features of the RuSr₂GdCu₂O₈ system and are used to compare their properties with those of single phase epitaxially grown RuSr₂GdCu₂O₈ thin films. The YBCO/LCMO superlattices have been grown by pulsed laser deposition with individual layer thickness ranging from 4 to 200 unit cells for the YBa₂Cu₃O₇ and 10 to 500 unit cells for the La_.67Ca_.33MnO₃ . Measuring dc-transport and magnetic properties some novel effects have been found due to a coupling between the layers observed in the superlattices. Superlattices with individual thickness of the constituent materials of 4 nm e.g. show a reduced Curie temperature of 120K and a superconducting transition temperature of 52K. Lowering the temperature a reentrant normal state occurs at T = 25K. Switching off the electronic interlayer coupling by the introduction of insulating SrTiO₃ spacer layers leads to the intrinsic critical temperatures. For the explanation of the results several novel concepts have to be developed based on a long range ferromagnetic interlayer coupling and a novel long range superconducting proximity effect.

(BaCuO_2+δ)_m/(CaCuO₂)₂ superconducting artificial structures were synthesized by layer by layer pulsed laser deposition. The interlayer coupling between adjacent superconducting CaCuO₂ blocks was investigated. The thickness of the BaCuO_2+δ charge reservoir (CR) layer was gradually increased up to m=10. It was found that interlayer coupling is not essential for high T_c superconductivity: giant CR blocks, thicker than 20 angstrom, (m ≈ 5), depress only moderately T_c while increase the 2D character of superconductivity. Furthermore hetero-epitaxial (BaCuO_2+δ)_m/(CaCuO₂)₂/(BaCuO_2+δ)_m tri-layers made of a single superconducting block sandwiched between two CR blocks were deposited. Electrical transport measurements showed that such ultra thin structures are superconducting with T_c up to about 60 K and critical current densities (at 4 K) as high as 10⁸ A cm^-2. Such finding confirmed the occurrence of purely intralayer superconductivity in these artificial structures.

In high temperature superconductors (HTS) the coherence length along non-c axis directions is longer. This feature can be useful wh en designing electronics devices based on HTS. Therefore growth and characterization of non-c axis oriented thin HTS films is of great interest. In this paper we present a short review of our data regarding (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO₃ and (110) SrTiO₃. The emphasis is made on improvement and control of the quality of the films by the "two-temperature" technological approach and/or use of the vicinal substrates. Phase and morphology evolution for different processing conditions, substrate's type and off-angle are presented. The highest critical temperatures of T_c0=67.2 K and T_c0=74 K for the "single" and "two-"temperature routes were obtained on vicinal SrTiO₃ with the off-angle of 20°. A higher off-angle promoted the formation of a specific step-like morphology with lower roughness. For the films grown on flat substrates the morphology was of mountain-range shape. Surface morphology as a result of two types of growth mechanisms (two-dimensional (2D), assisted by a so-called "twin"-growth and step-flow growth) for the (119)Bi-2223 filmes are discussed.

The lanthanide (Ln) copper oxides of the general chemical formula Ln₂CuO₄ take two different crystal structures: K₂NiF₄(T) and Nd₂CuO (T'). La₂CuO₄ takes the T structure by high-temperature bulk processes. The "thermal expansion mismatch" between the La-O and Cu-O bonds predicts that the T' phase of La₂CuO₄ can be stabilized at synthesis temperatures below 425^oC. Such low synthesis temperatures are difficult to access by bulk processes, but easy by thin-film processes. We have surveyed growth parameters in molecular beam epitaxy, and succeeded in the selective stabilization of T- and T'-La₂CuO₄. From our observations, it turns out that the growth temperature as well as the substrate play a crucial role in the selective stabilization: the T' structure is stabilized at low growth temperatures (< 600^oC) and with substates of a_s < 3.70 angstrom or a_s > 3.90 angstrom, while the T structure is stabilized at high growth temperatures (>650^oC) or with substrates of a_s ~ 3.70 - 3.85 angstrom. We have also been attempting hole (Ca, Sr, and Ba) and electron (Ce) doping into both of T- and T'-La₂CuO₄. In T-La₂CuO₄, hole doping produces the well-known LSCO and LBCO. Surprisingly, contrary to the empirical law, electron doping is also possible up to x~ 0.06 - 0.08, although the films do not show superconductivity. In T'-La₂CuO₄, electron doping produces superconducting T'-(La,Ce)₂CuO₄ with T_c~ 30 K, although hole doping has as yet been unsuccessful.

Epitaxial strain in La_2-xSr_xCuO_4+δ thin films (0 ⩽ x ⩽ 0.30) is controlled by using SrLaAlO₄ buffer layers of different thicknesses on SrTiO₃ substrates. We found that compressive epitaxial strain results in higher T_c for all the Sr concentrations. Better oxygenation by cooling the films in ozone/molecular oxygen mixture also leads to higher T_c. In undoped and lightly-doped ultrathin films, the samples are insulating under tensile strain, but superconducting when the strain is sufficiently compressive. We suggest that the epitaxial strain affects the insertion of interstitial oxygen, which is responsible for the observed effects. Hall measurements confirm the change in carrier density in films of different strain. The Hall angle also changes with epitaxial strain. The epitaxial strain dependence of the slope in the T² dependence of the cotangent of the Hall angle is different for underdoped and optimally-doped samples.

In this report we review recent experimental results on photoexcited carrier relaxation dynamics on high temperature superconductors (HTSC) probed by a femtosecond time-resolved optical spectroscopy, and compare the results with the data obtained on quasi two dimensional charge density waves. In these experiments, a femtosecond laser pump pulse excites electron-hole pairs via an inter-band transition in the material. These hot carriers rapidly release their energy via electron-electron and electron-phonon collisions reaching states near the Fermi energy within ~100 fs. If an energy gap is present in the low-energy density of states (DOS), it inhibits the final relaxation step and photoexcited carriers accumulate above the gap causing a transient change in reflectivity arising from excited state absorption. The relaxation and recombination processes of photoexcited quasiparticles, governed by the magnitude, anisotropy and the T-dependence of the low energy gap, are monitored by measuring the resulting photoinduced absorption as a function of time after the photoexcitation. This way, the studies of carrier relaxation dynamics give us direct information of the T-dependent changes in the low energy DOS. The technique is particularly useful to probe the systems with spatial inhomogeneities, where different local environments give rise to different relaxation rates. The data on series of HTSC-s show evidence for the coexistence of two distinct relaxation processes, whose T-dependences seem to be governed by two different energy scales: a T-independent pseudogap and a mean-field-like T-dependent gap that opens at T_c. The data suggest the origin of the two-gap behavior is in the intrinsic microscopic spatial inhomogeneity of these materials.

Femtosecond optical reflectivity measurements of La_2-xSr_xCuO₄, La₂CuO_4+y, Bi₂Sr₂CuO_6+z and Bi₂Sr₂CaCu₂O_8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.

We report scanning tunneling spectroscopic studies of the effects of quantum impurities on cuprate superconductors. The samples include p-type YBa₂Cu₃O_7-δ single crystals with spinless impurities of Zn²⁺ and Mg²⁺ ((Zn,Mg)-YBCO) and n-type infinite-layer system Sr_0.9La_0.1CuO₂ with 1% magnetic Ni²⁺- or 1% non-magnetic Zn²⁺-impurities that substitute the Cu²⁺ in the CuO₂ plane. The local effects of spinless impurities on the quasiparticles spectra of (Zn,Mg)-YBCO are analogous to those of Zn-substituted Bi₂Sr₂CaCu₂O_8+x, and the global effect is manifested by the suppression of the pairing potential Δd and of the spin excitation energy. In contrast, spectroscopic studies of Sr_0.9La_0.1CuO₂ reveal momentum-independent spectra and superconducting gap Δ , with (2Δ/k_BT_c) ~ 7 for T_c = 43 K and no pseudogap above T_c. The global response of Sr_0.9La_0.1CuO₂ to quantum impurities is similar to that of s-wave superconductors, being insensitive to small concentrations of spinless impurities (Zn) while showing rapid degradation in T_c with increasing magnetic impurities (Ni). Moreover, the spectra of the Ni-substituted Sr_0.9La_0.1CuO₂ reveal strong electron-hole asymmetry and long-range impurity effects, in contrast to the localized impurity effects in the p-type cuprates, and the introduction of Zn yield no reduction in either Δ or T_c. The physical implications of these findings are discussed.

We report, on the basis of our recent tunneling experiments over wide temperatures (T) and hole-doping (p) ranges in Bi2212/vaccum/Bi2212 junctions fabricated using STM, that in the electronic excitation spectrum, there exists two kinds of pseudogaps (LPG and SPG) with different characteristic energies. The LPG, which is 3 to 4 times larger than the superconducting (SC) gap magnitude 2Δ₀ develops below ~T_max, where the magnetic susceptibility starts to decrease because of the gradual development of antiferromagnetic spin fluctuations. On the other hand, the SPG, whose magnitude is comparable to 2Δ₀, develops progressively, in addition to the LPG, below the mean-field characteristic temperature T_co for d-wave superconductors, and then evolves into the SC-gap below T_c,suggesting that it will be some kind of precursor of superconditivity. Furthermore, in accordance with the SC transition, the high-energy feature of quasiparticle spectrum outside the SC-gap, changes from a broad hump to a clear dip and hump accompanied by a shift of the hump position toward lower energies, in addition to the rapid growth of SC-gap from SPG. We also report that T_c nearly scales with the product of Δ₀ and p, k_BT_c~pΔ₀' the effective SC gap is ~pΔ₀ in high T_c cuprates, instead of Δ₀ in conventional BCS superconductors.

Reproducibility of phonon structures in the tunneling conductance is studied by using mechanical Bi₂Sr₂CaCu₂O₈/Au contacts. The structures are much weaker than a previous clear phonon structure for a BiSr₂CaCu₂O₈/GaAs junction, and a smearing of the structures is discussed. The step-like increase in a critical temperature T_c with a number of CuO₂ layer is explained as a natural consequence of the phonon mechanism in a layered lattice. For explaining an extraordinarily large relative gap, 2Δ(0)/k_B T_c, and a T_c dependent isotope effect, an Eliashberg-magnetic pair break scenario is proposed for underdoped oxides.

We present a microscopic theory of coherent quantum transport through a superconducting film between two ferromagnetic electrodes. The scattering problem is solved for the general case of ferromagnet/superconductor/ferromagnet (FSF) double-barrier junction, including the interface transparency from metallic to tunnel limit, and the Fermi velocity mismatch. Charge and spin conductance spectra of FSF junctions are calculated for parallel (P) and antiparallel (AP) alignment of the electrode magnetization. Limiting cases of nonmagnetic normal-metal electrodes (NSN) and of incoherent transport are also presented. We focus on two characteristic features of finite size and coherency: subgap tunneling of electrons, and oscillations of the differential conductance. Periodic vanishing of the Andreev reflection at the energies of geometrical resonances above the superconducting gap is a striking consequence of the quasiparticle interference. Also, the non-trivial spin-polarization of the current is found for FSF junctions in AP alignment. This is in contrast with the incoherent transport, where the unpolarized current is accompanied by excess spin accumulation and destruction of superconductivity. Application to spectroscopic measurements of the superconducting gap and the Fermi velocity is also discussed.

We present various concepts and experimental procedures to fabricate biepitaxial YBa₂Cu₃O_7-x grain boundary Josephson junctions. Different types of structures can be realized including "π-loops" in which one of the junctions is intrinsically π-phase shifted. Due to its versatility the biepitaxial technique offers the possibility to investigate basic aspects of the physics of grain boundary Josephson junctions, and also the influence of intrinsic and extrinsic d-wave induced effects in transport propertis. Further, we discuss the possibility to produce circuits in which "0-" and "π- loops" are controllably located on the same chip, and intrinsic d-wave effects can be exploited.

We repoert on ferroelectric field effect experiments in correlated oxide films, demonstrating that a reversible, nonvolatile change in the electronic properties can be obtained upon reversing the ferroelectric polarization in epitaxial heterostructures consisting of ferroelectric Pb(Zr_0.2Ti_0.8) O₃ and metallic or superconducting oxide layers. In particular, we show that a T_c modulation of 7 K can be obtained in very thin (~20 Å) films of high T_c superconductors. We also discuss conventional field effect experiments, where we have used a SrTiO₃ gate insulator to modulate the electronic properties of a thin NdBa₂Cu₃O_7-Δ film. In this device, the dielectric constant of SrTiO₃ reaches a value of 2800 at 18 K. The polarization obtained is a few μC/cm², which induces a modulation of the resistivity of the NdBa₂Cu₃O_7-δ layer of ~9%.

The ability to analyze the chemistry, atomic and electronic structure of interfaces with atomic spatial resolution is afforded by modern scanning transmission electron microscopy techniques. By combining atomic resolution imaging with spectroscopy, structure-property relationships of functional oxide thin films can be established. In this paper, we describe two specific examples where we have applied high-spatial resolution electron energy-loss spectroscopy to dielectric thin films.

New compounds with spinel structure CuCr_1.5+xSb_0.5-xS₄(0≤x≤0.3) were obtained and studied in detail. All the compounds are non-degenerate semiconductores. The compounds (0≤x≤0.1) were found to have the magnetic properties characteristic for antiferromagnets. Compounds (0.2≤x≤0.3) have a spontaneous magnetization, with the Curie point of the compound with x=0.3, T_c = 334 K, being higher than room temperature. The re-entrant spin glass transition is observed in the compounds with x=0.17;0.2)

In this paper we discuss how atomic force microscopy can be used to locally study polarization phenomena, elucidating the fundamental properties of ferroelectrics. By combining local probe switching, time dependent electric field microscopy and piezoelectric microscopy, it is possible to address the question of ferroelectricity in ultra thin films. Using these techniques e demonstrate that Pb(Zr_0.2Ti_0.8)O₃ films are ferroelectric down to 40Å. We also discuss the use of the ferroelectric field effect to study switching in thin samples. Furthermore, by examining with a nanometer resolution the writing and reading of ferroelectric regions, the electric field dependence of the domain wall velocity can be quantified, demonstrating that ferroelectric domain wall motion is a creep process in thin films.

Thermoelectric power(TEP) mesurement has been carried out on the series of Pr_0.5Sr_0.5Mn_1-xRu_xO₃ (0.0 ≤ x ≤ 0.1) under zero and 6 tesla magnetic fields. In Pr_0.5Sr_0.5MnO₃, a large negative peak of TEP was observed below the ferromagnetic(FM) to antiferromagnetic(AFM) transition temperature, T_N ~ 165K. Under H = 6 tesla, the magnitude of the negative TEP peak is slightly reduced with decrease of T_N down to 105K. For Mn-site doped samples with Ru, however, the negative TEP peak is drasically suppressed by only 2% of Ru doping and completely disappears with further Ru doping. This indicates that the FM metallic state is induced more strongly by Ru substitution than by the magnetic field. In the paramagnetic(PM) regime above the Curie temperature, T_C, it was found that TEP as well as resistivity for Pr_0.5Sr_0.5Mn_1-xRu_xO₃ can be described by the polaronic transport mechanism. The systematic changes of TEP in the PM regime with variation of Ru concentration is discussed in relation to the effects of Ru doping at Mn sites which extends the FM phase at low temperatures and increases T_C similarly to the application of magnetic field.

We present fabrication and optical time-resolved photoresponse characterization of MgB₂ superconducting thin films. The films were prepared on crystalline and flexible plastic substrates by vacuum co-deposition of B and Mg precursors and high-temperature annealing in an Ar or vacuum atmosphere. The post-annealed films exhibited very smooth surfaces and amorphous structures with nanocrystal inclusions. The best films exhibited the critical temperature Tc of up to 38 K, the transition width of 1 K, and the current density j_c at 4.2 K of about 10⁶ A/cm². In our pump-probe photoresponse experiments, we used 100-fs-wide optical pulses generated by a Ti:Sapphire laser. The pump and the probe beams had 800-nm wavelength and the measurements were performed in the temperature range from 3.5 K to room temperature. The transient reflectivity change (ΔR/R) signals exhibited around 300-fs (10%-90%) risetime. At room temperature and far above Tc, (ΔR/R) the transient reflectivity change was characterized by a ~160-fs, single-exponential decay, interpreted as the electron-Debye-phonon interaction time. Below 60 K and in the superconducting state, the ΔR/R photoresponse was biexponential, with the initial femtosecond decay followed by a much slower, several-ps-long relaxation. We associate the latter slow relaxation with the electron-phonon interaction related to the Cooper pair recombination dynamics. The existence of this signal above the nominal T_c of our films, we tentatively interpret as the presence of superconducting fluctuations in our MgB₂ films. Our work gives the first insight into the carrier dynamics in MgB₂ by time-resolved experimental studies of the Cooper pair breaking and thermalization mechanisms for the films perturbed by femtosecond optical excitations.

The magnetic-field dependence of the irreversible magnetization of the binary superconductor MgB₂ was measured. For most of the temperature region below T_c, bulk pinning dominated surface pinning, thus, the magnetization curves, M(H), were well described by the critical-state model. Moreover, the M(H) curves at various temperatures scaled when the field and the magnetization were properly normalized. The universal scaling of the magnetization indicated that the pinning was uniquely determined at temperatures below T=T_c.

Eu_2-xCe_xRuSr₂Cu₂O_10-δ(Ru-2122) is the first Cu-O based system in which superconductivity (SC) in the CuO₂ planes and weak-ferromagnetism (W-FM) in the Ru sublattice coexist. The hole doping in the CuO₂ planes, is controlled by appropriate variation of the Ce concentration and/or increasing the oxygen concentration. SC occurs for Ce contents of 0.4-0.8, with the highest T_C=35 K for Ce=0.6. Due to the granular nature of the materials the magneto-resistance R(H) below T_C is positive and unexpected hysteresis loops are observed. The R(H) curve on decreasing the applied field (H_ext) is much smaller than the R(H) curve for increasing (H_ext). The width of the loops depends strongly on the weak-link properties. The as-prepared non-SC EuCeRuSr₂Cu₂O₁₀ (x=1) sample exhibits magnetic irreversibility below T_irr=125 K and orders anti-ferromagnetically (AFM) at T_M =165 K. Systematic magnetic studies on Eu_2-xCe_xRuSr₂Cu2O_10-δ show that T_M, T_irr and M_sat decrease with x, and the full Ce dependent magnetic-SC phase diagram is presented. A simple model for the SC state is proposed. The interpretation of the magnetic behavior is: (i) the system becomes antiferromagnetically (AFM) ordered at T_M; (b) at T_irr < T_M, W-FM is induced by the canting of the Ru moments, and (c), at lower temperatures the appropriate samples become SC at T_C. The magnetic features are not affected by the SC state, and the two states coexist.