We present ultrasonic techniques for magnetic measurements. Acoustically modulated magnetization is investigated with sensitive rf detection by narrowband loop antennas. Magnetization on the surface of ferromagnetic metals is temporally modulated with the rf frequency of the irradiated ultrasonic waves, and the near-field components emitted from the focal point of the ultrasonic beam are detected. Based on the principle of the acoustically stimulated electromagnetic (ASEM) response, magnetic sensing and tomography are demonstrated by ultrasonic scanning. We show that ASEM imaging combines good acoustic resolution with magnetic contrast. The sensitivity of this method is estimated to be about 6 G/Hz(1∕2) in our current setup.
Carrier doping effects of water vapor and an adsorbed water layer on single-crystal graphene were evaluated. After annealing at 300 °C in nitrogen ambient, the sheet resistance of epitaxial graphene on a SiC substrate had a minimum value of 800 Ω/sq and the carrier density was estimated to be 1.2 × 10 13 cm-2 for an n-type dopant. The adsorbed water layer, which acted as a p-type dopant with a carrier density of-7.4 × 10 12 cm-2 , was formed by deionized (DI) water treatment. The sheet resistances of graphene samples increased with humidity, owing to the counter doping effect. The estimated p-type doping amounts of saturated water vapor were-2.5 × 10 12 cm-2 for DI-water-treated graphene and-3.5 × 10 12 cm-2 for annealed graphene.
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