ALTERNATING TECHNIQUES TO MEASURE MAGNETORESISTANCE EFFECTS WITH A SENSITIVITY OF 2 P-OMEGA UNDER 3 T DOWN TO 60 F-OMEGA IN ZERO-FIELD

Two methods have been developed to measure variations with magnetic fi eld of electrical resistances in the range 0.1 n Omega-1 mu Omega, 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 sensit ivity is 3 x 10(-14) V/root Hz with a short circuit at input. It becom es on a 1 mu Omega sample: 6 x 10(-14) V/root Hz in zero field, and 2 x 10(-12) V/root Hz under 3 T. Averaging the signal over 100 s, the se nsitivity is 6 x 10(-15) V in zero field and 2 x 10(-13) V under 3 T. In terms of resistance, using a sensing current of 100 mA, it correspo nds to a sensitivity of 6 x 10(-14) Omega in zero field and 2 x 10(-12 ) Omega in the maximum field. The reproducibility of this method is 0. 3%. The second method is a sinusoidal field modulation technique givin g the field derivative of the resistance: dR/dH. It has the same absol ute sensitivity as the R(H) method and it achieves ultrahigh resolutio n. It is sensitive to variations of the resistance as small as 6 x 10( -14) Omega in zero field and 2 x 10(-12) Omega in 3 T independent of t he absolute resistance of the sample, thus the resulting resolution De lta R/R is, for example, 6 x 10(-8) in zero field and 2 x 10(-6) in th e maximum field for a 1 mu Omega sample. To demonstrate the high sensi tivity of the setups, we measured the magnetoresistance of a magnetic multilayer with the current applied perpendicular to the multilayer pl ane. (C) 1996 American Institute of Physics.