arXiv subjects

C. A. Cardoso

Publications and source records attributed to C. A. Cardoso.

15 recordsLinked to original sources

arXiv2010

Pinning of spiral fluxons by giant screw dislocations in YBa_2Cu_3O_7 single crystals: Josephson analog of the fishtail effect

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in YBa_2Cu_3O_7 single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting a=3 spiral turnings, the pitch b=18.7 microns and the step height c=1.2nm (the last parameter is responsible for creation of extended weak-link structure around the giant defects). The magnetic field applied parallel to the surface enters winding around the weak-link regions of the screw in the form of the so-called spiral Josephson fluxons characterized by the temperature dependent pitch b_f(T). For a given temperature, a stabilization of the fluxon structure occurs when b_f(T) matches b (meaning an optimal pinning by the screw dislocations) and manifests itself as a pronounced low-field peak in the dependence of the susceptibility on magnetic field (applied normally to the surface) in the form resembling the high-field (Abrikosov) fishtail effect.

cond-mat.supr-con↗

arXiv2009

Manifestation of finite temperature size effects in nanogranular magnetic graphite

In addition to the double phase transition (with the Curie temperatures T_C=300K and T_{Ct}=144K), a low-temperature anomaly in the dependence of the magnetization is observed in the bulk magnetic graphite (with an average granular size of L=10nm), which is attributed to manifestation of the size effects below the quantum temperature. The best fits of the high-temperature data (using the mean-field Curie-Weiss and Bloch expressions) produced reasonable estimates for the model parameters, such as defects mediated effective spin exchange energy J=12meV (which defines the intragranular Curie temperature T_C) and proximity mediated interactions between neighboring grains (through potential barriers created by thin layers of non-magnetic graphite) with energy J_t=exp(-d/s)J=5.8meV (which defines the intergranular Curie temperature T_{Ct}) with d=1.5nm and s=2nm being the intergranular distance and characteristic length, respectively.

cond-mat.mtrl-sci↗

arXiv2009

Synthesis, characterization and magnetic properties of room-temperature nanofluid ferromagnetic graphite

We report the chemical synthesis route, structural characterization, and physical properties of nanofluid magnetic graphite (NFMG) obtained from the previously synthesized bulk organic magnetic graphite (MG) by stabilizing the aqueous ferrofluid suspension with an addition of active cationic surfactant. The measured magnetization-field hysteresis curves along with the temperature dependence of magnetization confirmed room-temperature ferromagnetism in both MG and NFMG samples.

cond-mat.mtrl-sci↗

arXiv2009

Temperature oscillations of magnetization observed in nanofluid ferromagnetic graphite

We report on unusual magnetic properties observed in the nanofluid room-temperature ferromagnetic graphite (with an average particle size of l=10nm). More precisely, the measured magnetization exhibits a low-temperature anomaly (attributed to manifestation of finite size effects below the quantum temperature) as well as pronounced temperature oscillations above T=50K (attributed to manifestation of the hard-sphere type pair correlations between ferromagnetic particles in the nanofluid).

cond-mat.mtrl-sci↗

arXiv2009

Unusually strong coherent response from grain-boundary Josephson network in polycrystalline Pr_{x}Y_{1-x}Ba_2Cu_3O_7

By applying a highly sensitive homemade AC susceptibility technique to Pr_{x}Y_{1-x}Ba_2Cu_3O_7 polycrystals (with x=0.0, 0.1 and 0.3), we observed very sharp Fraunhofer type low-field periodic oscillations of the real part of the AC susceptibility which were attributed to Josephson vortices penetrating intergranular regions of grain-boundary Josephson network in our samples. Assuming the Lorentz type distribution of single-junction contact areas, we were able to successfully fit the experimental data.

cond-mat.supr-con↗

arXiv2005

Elastic properties of polycrystalline YBa_2Cu_3O_7: Evidence for granularity induced martensitic behavior

In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa_2Cu_3O_7 prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensitic-like phase above the superconducting critical temperature T_C. We argue that this peculiar behavior can be attributed to the granularity present in the samples.

cond-mat.supr-con↗

arXiv2005

Magnetic and superconducting properties of RuSr2 Gd1.5Ce0.5Cu2O10-x samples: dependence on the oxygen content and ageing effects

The magnetic and superconducting properties of RuSr2Gd1.5Ce0.5Cu2O10-x polycrystalline samples with different oxygen doping level are presented. A strong suppression of the superconducting temperature (Tc), as well as a reduction in the superconducting fraction, occurs as the oxygen content is reduced by annealing the samples in oxygen deprived atmospheres. Drastic changes in the electrical resistivity are observed above Tc, possibly associated with oxygen removal mainly from grain boundaries. However, the magnetic ordering is relatively less affected by the changes in oxygen content of the samples. The spin glass transition is enhanced and shifted to higher temperatures with the reduction in oxygen content. This could be correlated with an increase in the spins disorder and frustration for the oxygen depleted samples. Also, the same oxygen vacancy induced disorder could explain the reduction in the fraction of the sample showing antiferromagnetic order. We also report significant changes in the measured properties of the samples as a function of time.

cond-mat.supr-con↗

arXiv2004

Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system

Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is \~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.

cond-mat.str-el↗

arXiv2004

Stable room temperature magnetic graphite

Carbon materials are attracting increasing attention due to the novelty of the associated physical properties and the potential applications in high-tech devices. The possibility to achieve outstanding properties in macroscopic carbon materials opens up a profusion of new striking applications. Magnetic properties induced by defects on graphite structures, such as pores, edges of the planes and topological defects, have been theoretically predicted. The possible coexistence of sp3 and sp2 bonds have been also invoked to predict this behavior (for a review, see ref. 1). Some reports have proved the existence of weak ferromagnetic-like magnetization loops in highly-oriented pyrolytic graphite (HOPG) (ref. 2-3). Very recently two reports showed that the existence of ferromagnetism in pure carbon is unambiguously possible (ref. 4-5). Here we report on a novel and inexpensive chemical route consistent in a controlled etching on the graphite structure to obtain macroscopic amounts of magnetic pure graphite. This material has a strong magnetic response even at room temperature where it can be attracted by a commercial magnet and would be the experimental confirmation for the defect induced magnetism previously predicted.

cond-mat.mtrl-sci↗

arXiv2004

Comparative study of specific heat measurements in LaMnO3, La1.35Sr1.65Mn2O7, La1.5Sr0.5NiO4 and La1.5Sr0.5CoO4

We present the temperature dependence of the specific heat, without external magnetic field and with H= 9 T, for LaMnO3, La1.35Sr1.65Mn2O7, La1.5Sr0.5NiO4 and La1.5Sr0.5CoO4 single crystals. We found that spin-wave excitations in the ferromagnetic and bilayer-structure La1.35Sr1.65Mn2O7 were suppressed by the 9 T magnetic field. On the other hand, the external magnetic field had no effect in the specific heat of the other three antiferromagnetic samples. Also, the electronic part of the interactions were removed at very low temperatures in the La1.5Sr0.5NiO4 single crystal, even with a zero applied magnetic field. Below 4 K, we found that the specific heat data for La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 crystals could be fitted to an exponential decay law. Detailed magnetization measurements in this low temperature interval showed the existence of a peak close to 2 K. Both results, magnetizations and specific heat suggest the existence of an anisotropy gap in the energy spectrum of La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 compounds.

cond-mat.mtrl-sci↗

arXiv2003

Physical properties of single-crystalline fibers of the colossal-magnetoresistance manganite La0.7Ca0.3MnO3

We have grown high-quality single crystals of the colossal-magnetoresistance (CMR) material La0.7Ca0.3MnO3 by using the laser heated pedestal growth (LHPG) method. Samples were grown as fibers of different diameters, and with lengths of the order of centimeters. Their composition and structure were verified through X-ray diffraction, scanning electron microcopy with EDX (Energy Dispersive X-ray Analysis) and by Rietveld analysis. The quality of the crystalline fibers was confirmed by Laue and EBSD (Electron Backscatter Diffraction) patterns. Rocking curves performed along the fiber axis revealed a half-height width of 0.073 degrees. The CMR behavior was confirmed by electrical resistivity and magnetization measurements as a function of temperature.

cond-mat.mtrl-sci↗

arXiv2003

Spin Glass Behavior in the Ru-1222 and Ru-1212 rutheno-cuprate families: a comparative study

Strong evidences of a spin glass phase on polycrystalline RuSr2Ln1.5Ce0.5Cu2O10-x (LnRu-1222, Ln = Y, Ho and Dy) is provided by ac susceptibility and dc magnetization measurements. The fingerprint of a spin glass transition, the frequency-dependent cusp in ac susceptibility vs. T measurements at low magnetic field, is observed for all LnRu-1222 samples. The strong supression of the cusp in the ac susceptibility measurement in the presence of magnetic fields as low as 500 Oe, and the results of thermoremanent magnetization (TRM) and isothermal remanent magnetization (IRM) measurements are also evidences of a spin glass behavior. By comparing these results with measurements on a YRu-1212 sample we conclude that the glassy behavior is characteristic of the Ru-1222 phase, not being observed for the studied YRu-1212 sample. We suggest that the spin glass transition occur only in the Ru-1222 phase possibly due to the larger number of oxygen vacancies present in these compounds.

cond-mat.supr-con↗

arXiv2002

Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10

The dynamics of the magnetic properties of polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K. The exact nature of Ru spins magnetic ordering is still debated and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac susceptibility vs. T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with magnetically interacting clusters. Such interpretation is supported by themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations are well described by a stretched exponential relation, and present significant ageing effects.

cond-mat.supr-con↗

arXiv2001

Angular dependence of the bulk nucleation field Hc2 of aligned MgB2 crystallites

Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.

cond-mat.supr-con↗

arXiv2001

Anisotropic superconducting properties of aligned MgB2 crystallites

Samples of aligned MgB2 crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy Hc2^{ab}/Hc2^{c}= xi_{ab}/xi_{c} ~ 1.73; with xi_{o,ab} ~ 70 A, xi_{o,c} ~ 40 A, and a mass anisotropy ratio m_{ab}/m_{c} ~ 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.

cond-mat.supr-con↗