Precise Measurements of the Low-Field Hall Coefficient and Magnetoresistance in Metals

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In the present investigation, a superconducting chopper amplifier has been improved. First, the external input for the reference signal of a lock-in amplifier was connected through a photo-fibre to a pulse generator which operates the superconductiong chopper. The influence of inductive noises was reduced to the extent that the amplifier could operate with a sensitivity of 5 <p^V> under noisy conditions. Next, a digital multimeter of 4(1/2) digits was replaced by that of 5(1/2) digits as a recording instrument for a specimen current and a reference current. In this way, a full sensitivity of 5 <p^V> became available in various measurements.Precise measurements of the low-field Hall and magnetoresistance coefficients were performed at 4.2 K on a monocrystalline specimen of high-purity aluminium with quenched-in vacancies of a few at.ppm, and on polycrystalline specimens of dilute aluminium alloys with 3d metal elements. As solute elements, Sc, Ti and Cu of 10-1000 at.ppm were used. Hitherto, it has been reported that the low-field Hall coefficients of such alloys are parabolic with respect to the atomic number of solute elements. The centre of the parabola is at Cr. But, in the present investigation, it has been found that the Hall coefficients increase monotonically from Sc to Cu. Moreover, anomalies of the Hall and magnetoresistance coefficients in the low-field limit have been found in Al-100 at.ppm Ti and Al-1000 at.ppm Ti alloys. The Hall coefficient became more negative than the free-electron value of -3.41x <10^(-11)> <m^3> /C, and the magnetoresistance coefficient became much larger than that in other alloy systems. More detailed investigations will be desired both on experiments and theories.