Anomalous behavior of the magnetoresistance of NiMn alloys in the spin-glass and ferromagnetic states

Abstract

The longitudinal and the transverse magnetoresistances of disordered NiMn alloys with about 21, 26, 28, and 30 at. % Mn have been measured in magnetic fields up to ∼75 kG and in the temperature range 1.2–80 K. For the 21 at. % Mn alloy the magnetoresistance is anisotropic (Δ ρ∥>0, Δ ρ⊥<0) at the lowest fields and temperatures. No detectable anisotropy is seen in the more concentrated sepcimens. In the critical concentration region (∼25 at. % Mn) the competition between ferromagnetic and spin glass effects is very severe and the magnetoresistance is marked by strong and unusual hysteresis effects in the Δ ρ/ρ0 vs H curves. When the sample is prepared with a remanent magnetization both Δ ρ/ρ0 and its field derivative undergo a discontinuity at H∼−1 kG and T=1.2 K.