Alternating techniques to measure magnetoresistance effects with a sensitivity of 2 pΩ under 3 T down to 60 fΩ in zero field

Two methods have been developed to measure variations with magnetic field of electrical resistances in the range 0.1 nΩ–1 μΩ, for applied magnetic fields up to 3 T, and for temperatures less than 9 K. The first method provides direct access to R(H). Its absolute sensitivity is 3×10−14 V/√Hz with a short circuit at input. It becomes on a 1 μΩ sample: 6×10−14 V/√Hz in zero field, and 2×10−12 V/√Hz under 3 T. Averaging the signal over 100 s, the sensitivity is 6×10−15 V in zero field and 2×10−13 V under 3 T. In terms of resistance, using a sensing current of 100 mA, it corresponds to a sensitivity of 6×10−14 Ω in zero field and 2×10−12 Ω in the maximum field. The reproducibility of this method is 0.3%. The second method is a sinusoidal field modulation technique giving the field derivative of the resistance: dR/dH. It has the same absolute sensitivity as the R(H) method and it achieves ultrahigh resolution. It is sensitive to variations of the resistance as small as 6×10−14 Ω in zero field and 2×10−12 Ω in 3...