Magnetoresistance in Elemental Boron

Abstract

The magnetoresistance of crystalline, beta-rhombohedral boron and amorphous boron film was studied as a function of the magnetic field, the temperature, the specimen purity, and the crystal orientation. The monocrystals exhibited anomalous negative magnetoresistance for fields up to 5 kG, and a positive magnetoresistance for higher fields. The polycrystals showed positive magnetoresistance, with complete saturation for fields of 2 kG. The resistance did not exceed 3 • 10–³ at room temperature, and depends upon the type of boron, the temperature, and the magnetic field. In the range from 200 to 400°K, Δρ/ρ is proportional to T ^{- 5•6} for the polycrystalline samples, and to T ^-3 for the monocrystalline. The magnetoresistance decreased with increasing specimen purity for the polycrystalline samples. The negative magnetoresistance in the monocrystals, the positive effect in the polycrystals, and the negative effect in the film are isotropic with respect to the angle between the directions of the electric current and the magnetic field. The positive effect in the single crystals, however, is anisotropic and may be described by the empirical formula Δρ/ρ = a + b cos² θ’ where the coefficients depend on the specimen temperature, and θ’ is the angle between the magnetic field and the [111] rhombohedral axis.

A preliminary report of this work was presented at the Philadelphia Meeting of the American Physical Society, March 1964; Bull. Am. Phys. Soc. 9: 279 (1964).