Diamagnetism of Bulk Graphite Revised

Recently published structural analysis and galvanomagnetic studies of a large number of different bulk and mesoscopic graphite samples of high quality and purity reveal that the common picture assuming graphite samples as a semimetal with a homogeneous carrier density of conduction electrons is misleading. These new studies indicate that the main electrical conduction path occurs within 2D interfaces embedded in semiconducting Bernal and/or rhombohedral stacking regions. This new knowledge incites us to revise experimentally and theoretically the diamagnetism of graphite samples. We found that the c-axis susceptibility of highly pure oriented graphite samples is not really constant, but can vary several tens of percent for bulk samples with thickness t ≳ 30 μ m, whereas by a much larger factor for samples with a smaller thickness. The observed decrease of the susceptibility with sample thickness qualitatively resembles the one reported for the electrical conductivity and indicates that the main part of the c-axis diamagnetic signal is not intrinsic to the ideal graphite structure, but it is due to the highly conducting 2D interfaces. The interpretation of the main diamagnetic signal of graphite agrees with the reported description of its galvanomagnetic properties and provides a hint to understand some magnetic peculiarities of thin graphite samples.

Diamagnetism of Bulk Graphite Revised

Recently published structural analysis and galvanomagnetic studies of a large number of different bulk and mesoscopic graphite samples of high quality and purity reveal that the common picture assuming graphite samples as a semimetal with a homogeneous carrier density of conduction electrons is misleading. These new studies indicate that the main electrical conduction path occurs within 2D interfaces embedded in semiconducting Bernal and/or rhombohedral stacking regions. This new knowledge incites us to revise experimentally and theoretically the diamagnetism of graphite samples. We found that the $c$-axis susceptibility of highly pure oriented graphite samples is not really constant but can vary several tens of percent for bulk samples with thickness $t \gtrsim 30~\mu$m, whereas by a much larger factor for samples with smaller thickness. The observed decrease of the susceptibility with sample thickness resembles qualitatively the one reported for the electrical conductivity and indicates that the main part of the $c-$axis diamagnetic signal is not intrinsic of the ideal graphite structure but it is due to the highly conducting 2D interfaces. The interpretation of the main diamagnetic signal of graphite agrees with the reported description of its galvanomagnetic properties and provides a hint to understand some magnetic peculiarities of thin graphite samples.

Crystal Structures and Peculiar Magnetic Properties of α- and γ-Al2O3 Powders

The crystal structure and magnetic properties of the α- Al2O 3 and γ- Al2O 3 samples were investigated. The α- Al2O 3 and γ- Al2O 3 powders have the trigonal and cubic symmetries, respectively. Moreover, the α- Al2O 3 powders have better crystallinity than γ- Al2O 3 material. A peculiar magnetic behavior in these two materials was found. The weak diamagnetic signal appeared at the temperature range between room temperature and 100 K. Moreover, both materials have antiferromagnetic-like transitions at the temperature range between 30 K and 100 K. The materials changed to paramagnetic-like behavior at the temperature lower than 30 K. Based on our studies, we did not find any superconducting diamagnetic signal at the temperature range from 300 K to 5 K.

Magnetic Anomaly in the YBa2Cu3O7 System at Low External Magnetic Fields

ABSTRACTMagnetization curves for low external magnetic fields of field-cooled superconducting YBa2Cu3O7 samples show an increase in the diamagnetic signal when the sample temperature is near the transition to the normal state. The Increase of the diamagnetic signal depends on the external magnetic field (H ≤ 13.5 Oe). The effect is observed when the samples are field-cooled, but not when they are zero-field-cooled, that means that it is related with the behavior of the vortex lattice.

TRANSPORT AND LOW FIELD MAGNETIC PROPERTIES OF HIGH TcBi2CaSr2Cu2O8−δ AND Tl0.8Ca2Ba2Cu3O8+δ COMPOUNDS

Electrical resistivity, low field diamagnetic susceptibility (constant field-cooled in various magnetic fields) and low field magnetization measurements on new high T c superconducting copper oxides Bi 2 CaSr 2 Cu 2 O 8−δ (2122) (T c =89 K ) and thallium-deficient Tl 0.8 Ca 2 Ba 2 Cu 3 O 8+δ (0.8,2,2,3) (T c =117 K ) were reported. Meissner diamagnetic signal decreases with increasing magnetic field of these new superconducting copper oxides.

THE 110 K SUPERCONDUCTING TRANSITION IN Bi-Ca-Sr-Cu-O SYSTEM

Large resistive transitions with strong diamagnetic signals have been achieved in the Bi-Ca-Sr-Cu-O system. The fractional drop in resistivity at 110K is found not to correlate with the strength of the diamagnetic signal. For samples with nominal composition BiCaSrCu 2 O y containing two superconducting phases, greater than 50% of the susceptive transition can be associated with the 110K phase.