Akio Ono

Anomalous giant magnetoresistance in Heusler-type alloys Fe2+xV1-xGa

Electrical resistance, magnetoresistance and magnetization were measured for Heusler-type alloys, Fe 2+ x V 1- x Ga. An anomalous giant magnetoresistance (GMR) near 50% was observed for Fe 2 VGa which was a weak and inhomogeneous ferromagnet. For alloys with x >0, which show clear ferromagnetism, the electrical resistance showed an anomaly below the Curie temperature. For alloys with x <0, the temperature dependence of the resistance indicated behavior like that of a semiconductor. The energy gap was tentatively estimated to be 0.03 eV for x =-0.10. The origin of the present GMR is discussed in terms of the metal-insulator transition arising from ordering of the magnetic moment.

Variable-range hopping conduction and negative magnetoresistance of disordered carbons at low temperature

Abstract Temperature dependence of the electrical conductivity in the range from room temperature down to 1.31 K was studied for three kinds of cokes heat treated at 900°C, bamboo char heat treated at 900°C and glassy carbon heat treated at 1000°C. Localized states as evidenced by variable-range hopping conduction (conductivity proportional to exp T −1/4 ) was observed for these carbons at temperatures below 4 K. Room temperature Hall coefficient was studied for these carbons. Magnetoresistance was measured for the samples in the temperature range between 1.31 and 4.2 K. Positive magnetoresistance related to variable-range hopping conduction was observed for a bamboo char to increase with increasing magnetic field and with decreasing temperature. The mechanism of conduction can be interpreted by use of a model of density of states suggested by Davis and Mott for amorphous semiconductor. For cokes and glassy carbon negative magnetoresistance superposes with the positive magnetoresistance. Dependence of conductivity and of magnetoresistance on heat-treatment was investigated in the temperature range from 1.31 to 4.2 K for cokes heat treated at 1100, 1300 and 1500°C. Conductivity was found to be constant against temperature for these samples. With increasing heat-treatment temperature the contribution of negative magnetoresistance was observed for samples heat treated at 1500°C.

Galvanomagnetic Properties of Well-Oriented Graphite in Relation to the Structural Imperfections

The Hall coefficient and magnetoresistance in well oriented graphite are measured at 77 K with the field oriented parallel to the c-axis. Various degrees of structural perfections are observed in the samples investigated. The magnetic field is continuously changed from 0 to 10 kG for each run. The concentrations and mobilities of conduction carriers in the samples are estimated by an analysis based on coventional multi-carrier models. Special care is taken in accurate determination of the Hall coefficient and magnetoresistance at low field (H<3 kG), which leads to a conclusion that highly mobile electrons do exist as minority carriers in each sample. With increasing imperfection, the concentrations of majority electrons and holes are found to increase, and mobilities to decrease. The same trend is also observed for minority electrons. These results imply that the introduction of structural imperfections into graphite causes considerable perturbation on the band structure.

Field Dependence of Magnetoresistance in No-binder Soft Carbons at 77°K

The dependence of magnetoresistance on magnetic field is investigated at 77°K for soft carbons prepared from Texas petroleum coke (no-binder) by heat treatments in the range 1400°–3000°C. For specimens heat treated at temperatures between 1400° and 2200°C, the magnetoresistance is negative; its absolute value increases monotonically with increasing heat treatment temperature (HTT) and with increasing magnetic field. For specimens heat treated between 2400° and 2450°C, the magnetoresistance is negative and tends to saturate at higher fields. The absolute value becomes larger for lower HTT. Specimens heat treated between 2400° and 2450°C show a magnetoresistance which is negative at low fields, passes through minimum and becomes greater for lower HTT. For specimens with HTT≥2570°C, no negative magnetoresistance is observed even at low fields. Some discussion of the effects found is included.

Magnetic-field dependence of low-temperature anomalies in the thermoelectric power of highly oriented graphite

Abstract Highly oriented graphites show three distinct anomalies in the temperature dependence of the thermoelectric power (TEP). The first anomaly is a pronounced negative dip observed at 30–40 K and has been ascribed to an ordinary phonon-drag effect of in-plane vibrations. The second anomaly is a peak around 10–15 K attributed to the two-stage drag effect of in-plane vibrations. The third anomaly is a negative dip observed around 4 K associated with the phonon-drag effect of out-of-plane vibrations. The second and third anomalies depend strongly on the degree of structural perfection. In the present work the dependence of the second and third anomalies on magnetic field was studied experimentally in kish graphite at magnetic field below 0·1 T. At low fields the magnitude of the TEP at the peak of the second anomaly increases with increasing magnetic field. The TEP value at the dip of the third anomaly initially increases, reaches a maximum and then decreases, as the field is increased. At temperatures ...

Magnetoresistance of two-phase carbons

Abstract The transverse magnetoresistance was studied at liquid nitrogen temperature for carbons with two phases, the turbostratic and the graphitic. Samples investigated were carbons prepared by heat treatment at temperatures between 1320 and 1780°C under pressure of 5 kbar. The starting materials were a coke obtained from polyvinyl chloride carbonized up to 680°C, and an equi-weight mixture of the same coke with a thermal black. Samples obtained from the coke alone by the heat treatment above 1470°C consist of two phases. When the fractionof the graphitic phase ranges 0.5–6%, two new types of field dependence of the transverse magnetoresistance, types 1 and 2, are observed. The field dependence of the magnetoresistance were reproduced by using a model composed of a 2900°C-treated pyrolytic graphite and of a 1900°C-treated extruded petroleum coke carbon, when the resistance ratio between the two components was properly adjusted. This seems to show that the new types of field dependence found in this work are due to the presence of a mixture of graphitic and turbostratic grains in our samples.

Low Temperature Anomalies in the Thermoelectric Power of Highly Oriented Graphite

The magnetic field dependence of the thermomagnetic tensor components β x x and β y x for a high quality kish graphite specimen is studied in the temperature range 1.5-15 K with application of the field up to 0.1 T. β is a more fundamental quantity than thermoelectric power. β x x shows a negative dip at 3.8 K and a sharp peak at 11 K in zero magnetic field. With application of magnetic field the temperatures for the dip and peak change in complicated ways. The peak disappears when a filed of 0.1 T is applied. β y x is negative and decreases with increasing temperature. At temperatures below 4 K β x x shows a positive peak at a field around 0.01 T in the field dependence. The previous theory for the anomaly of β x x around 4 K is extended to the case with magnetic field. The observed behavior for β x x in weak field region is ascribed to the trigonal band warping effect.

Diamagnetic Susceptibility and Magnetoresistance of Sintered Polyvinyl Chloride Char in Relation to preferred Orientation

The diamagnetic susceptibility of sintered polyvinyl chloride char, prepared by heat treatment at various temperatures between 1300°C and 1900°C under the pressure of 5 kbar, has been measured at room temperature. The magnetoresistance of the specimens has also been determined at liquid nitrogen temperature. The anisotropy of the susceptibility is compared with that of the transverse magnetoresistance, and some differences between them are discussed.

Automatic Measurement of The Hall Voltage and Magnetoresistance in Carbon Materials as Functions of Magnetic Field

An electronic device for the use of the continuous and automatic measurement of the Hall voltage and magnetoresistance as functions of the magnetic field has been assembled, with several commercial instruments, and has yielded a great success for carbon materials. In practice, one has to take best care of the perfect shield of various leads as well as the one point earthing. Especially, when the specimen is of high resistance and of low magnetoresistance, every effort should be made for suppressing the current fluctuation as small as possible.