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The results of magnetization and thermal expansion measurements on samples of the copper oxide superconductor La(sub 2-x)Sr(sub x)CuO(sub 4) are discussed. These characterize the oxygen-isotope effects on the carrier density and on the in-plane penetration depth. Results also showed that polaronic charge carriers exist and condense into Cooper pairs in high-temperature superconductors. It was also confirmed that a strong Jahn-Teller effect can result in the formation of these polarons.

Magnetic hysteresis loops were measured for ferrimagnetic iron chalcogenide Fe3Se4 nanoparticles in the whole temperature range below the Curie temperature TC (315 K). The coercivity of the material is huge, reaching about 40 kOe at 10 K. The magnetic anisotropy constant K was determined from the magnetic hysteresis loop using the law of approach to saturation. The deduced anisotropy constant at 10 K is 5.22×106 erg/cm3, which is over one order of magnitude larger than that of Fe3O4. We also demonstrated that the experimental magnetic hysteresis loop is in good agreement with the theoretical curve calculated by Stoner and Wohlfarth for a noninteracting randomly oriented uniaxial single-domain particle system. Moreover, we show that K is proportional to the cube of the saturation magnetization Ms, which confirms earlier theoretical models for uniaxial magnets. [ABSTRACT FROM AUTHOR]

High-temperature magnetic measurements were carried out on barium ferrite BaFe12 O19 nanoparticles coated with amorphous silica. We find that the Curie temperature of this material decreases with decreasing particle size, in agreement with the finite-size scaling theory. In contrast to what one expects, the observed particle-size dependence of the Curie temperature does not follow a finite-size scaling relation for a zero-dimensional magnetic system. Instead, the data follow a finite-size scaling relation for a two-dimensional magnetic system with the scaling exponent ν = 0.78±0.06. The validity of the two-dimensional scaling relation in this material is due to the fact that the nanoparticles have a platelet-like shape. [ABSTRACT FROM AUTHOR]

The magnetoresistance in the perovskite La(sub 0.8)Ca(sub 0.2)MnO(sub 3+y) is probably due to the large isotope shift of over 20 kelvin in the ferromagnetic transition temperature on replacing the oxygen atom, 16O, with 18O. The coupling of Jahn-Teller lattice defects with charge carriers produces the isotope shift. The ferromagnet SrRuO(sub 3) that has an insignificant Jahn-Teller effect shows no isotopic shift. This indicates that lattice distortions affect the magnetic and electric properties of perovskites.

We report magnetic hysteresis loops in a wide temperature range (4-700 K) for silica-coated barium ferrite BaFe12O19 nanoparticles. The saturation magnetization Ms and the first magnetic anisotropy constant K are determined simultaneously from the magnetic hysteresis loop using the law of approach to saturation. It is remarkable that K is linearly proportional to Ms and varies precisely with temperature as K(T) = K(0)[1 - (T/TC)1.58] in the whole temperature range below the Curie temperature TC (740 K). The unusual temperature dependence of the anisotropy constant and its linear relation with the saturation magnetization in BaFe12O19 are not predicted from the existing theoretical models. [ABSTRACT FROM AUTHOR]

Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized α-Fe2O3 nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1×10-5 Torr) during the magnetic measurement up to 1000 K leads to a complete reduction of α-Fe2O3 to Fe3O4. This precludes the determination of the Néel temperature for the α-Fe2O3 nanoparticles. In contrast, coating α-Fe2O3 nanoparticles with SiO2 stabilizes the α-Fe2O3 phase up to 930 K, which allows us to determine the Néel temperature of the α-Fe2O3 nanoparticles for the first time. The Néel temperature of the 60-nm α-Fe2O3 nanoparticles is found to be 945 K, about 15 K below the bulk value. The small reduction of the Néel temperature of the α-Fe2O3 nanoparticles is consistent with a finite-size scaling theory. [ABSTRACT FROM AUTHOR]

We report high-energy synchrotronX-ray diffraction spectrum andhigh-temperature magnetic data for multiwalled carbon nanotubes (MWCNTs)embedded with Fe and Fe3O4nanoparticles. Weunambiguously show that the saturation moments of the embedded Feand Fe3O4nanoparticles are enhanced by a factorof about 3.0 compared with what would be expected if they would beunembedded. More intriguingly the enhanced moments were completelylost when the sample was heated up to 1120 K, and the lost momentswere completely recovered through two more thermal cycles below 1020K. These novel results cannot be explained by the magnetism of theFe and Fe3O4impurity phases, the magnetic proximityeffect between magnetic nanoparticles and carbon, and the ballistictransport of MWCNTs. [ABSTRACT FROM AUTHOR]

Abstract: We present numerical calculations of the nuclear spin-lattice relaxation rate in the superconducting state of pure indium and slightly underdoped n-type cuprate Pr0.91LaCe0.09CuO4− y . By properly taking into account electron–phonon coupling, our calculated for the conventional s-wave superconductor, indium, is in quantitative agreement the experimental data with a clear Hebel–Slichter peak. In contrast, the absence of the Hebel–Slichter peak in the data of Pr0.91LaCe0.09CuO4− y can be explained by either d-wave or highly anisotropic s-wave gap symmetry. Thus, the absence of the Hebel–Slichter peak does not necessarily argue against an s-wave gap symmetry in this electron-doped cuprate. [Copyright &y& Elsevier]

Many anomalous physical properties have been observed in the high-T[sub c] cuprate superconductors for the last decade. These include: (1) the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the spin-glass freezing temperature and on the antiferromagnetic ordering temperature; (2) the exotic pairing symmetry shown by many bulk-sensitive experiments; (3) the magnetic resonance peak in the superconducting state revealed by inelastic neutron scattering; (4) the peak, dip and hump features seen in angle-resolved photoemission spectroscopy and in tunnelling spectra; (5) the strong-coupling features in optical and tunnelling data; (6) the pseudogap in the normal state of underdoped cuprates and (7) the unusually large supercarrier mass anisotropy and its novel doping dependence. We review these important experimental results, which can place crucial constraints on the physics of cuprates. The conclusion is that high-T[sub c] superconductivity in cuprates is a cooperative phenomenon between the strong electron-electron correlation and strong electron-phonon coupling. [ABSTRACT FROM AUTHOR]

Reports the results of magnetization and thermal expansion measurements on samples of the copper oxide superconductor La2-xSrxCuO4. The characterization of the oxygen-isotope effects on the carrier density and on the in-plane penetration depth; What the specific quantitative features show; The existence of polaronic charge carriers; The condensation into Cooper pairs; The implications for the Jahn-Teller effect.

Deals with a study that described a method for preparing single crystals of Ba[sub2]GdCu[sub3]O[sub7-y] of much larger size. Factors that were obtained from a superconducting crystal; Step taken to characterize the electrical quality of the annealed crystal; Cause of the large resistivity in the single crystal.

We report high-temperature magnetic measurements for SiO2-coated ultrafine Fe3O4 nanoparticles. The Curie temperatures of the ultrafine Fe3O4 nanoparticles are significantly reduced and follow a finite-size scaling law predicted from Monte Carlo simulations. Our current result provides the first quantitative confirmation of the finite-size scaling law for quasi-zero-dimensional magnetic systems. [ABSTRACT FROM AUTHOR]

Abstract: We have successfully synthesized single-domain barium ferrite particles with uniaxial anisotropy. We have coated them with amorphous silica to reduce interparticle interactions so that the assembly of these particles behaves like a noninteracting randomly oriented uniaxial single-domain particle system, a prototype for the Stoner–Wohlfarth model. From the magnetic hysteresis loops of the particle system in a wide temperature range ( ), we simultaneously determine the magnetic anisotropic field , the reduced remanence , and the coercive field in the whole temperature range below the Curie temperature. These complete sets of data allow us to quantitatively test the Stoner–Wohlfarth theory and the agreement between experiment and theory is good. [Copyright &y& Elsevier]

Abstract: Magnetic measurements have been performed on 40-nm sphere-like Fe3− δ O4 () nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe3− δ O4 nanoparticles with SiO2 effectively eliminates magnetic interparticle interactions so that the coercive field in the high-temperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be , which is very close to the bulk magnetocrystalline anisotropy constant of . Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the ensemble of our SiO2-coated sphere-like Fe3− δ O4 nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles. [Copyright &y& Elsevier]

Abstract: We report magnetic properties of silica-coated CoFe2O4 nanoparticles (about 32 nm) in a wide temperature range (4–700 K). Coating CoFe2O4 nanoparticles with amorphous silica yields an assembly of noninteracting single-domain particles with cubic magnetocrystalline anisotropy. We find that the reduced remanence is about 0.81–0.82 at low temperatures (), very close to the theoretical prediction (0.83) for cubic anisotropy. The room-temperature anisotropy constant inferred from the data is also in quantitative agreement with the bulk value (). The current work thus provides a quantitative test for a theory of noninteracting single-domain particles with cubic anisotropy. [Copyright &y& Elsevier]