Influence of Fe and Co additions on magnetization and magnetoresistivity of MnAu2 melt-spun alloys

Melt-spun MnAu2 ribbons modified by ternary magnetic additions (Fe and Co) show a large giant magnetoresistance (GMR) effect up to a magnetoresistance ratio MR ={[R(H=7T)-R(0)/R(0)]} of the order of -10% at room temperature. Pure MnAu2 samples show a transition from positive magnetoresistance at low temperatures (e.g., MR=+23% at 10 K and mu(0)H=5 T) to a GMR behavior with values of about -8% at 295 K and 5 T. At 135 K the resistivity is nearly i ndependent of the applied magnetic field. The strong increase of \MR\ at ro om temperature is only found with applied fields larger than a threshold fi eld, where a metamagnetic transition occurs. Alloying the compound with Co or Fe this threshold field can be reduced or suppressed. This effect is due to softening of the antiferromagnetic behavior of the (Mn; Co or Fe)Au-2 s amples. For an alloy with 5 at. % Fe as substitutes for Mn, no metamagnetic transition is observed in the temperature range between 10 and 300 K. For both 5 at. % Co or Fe additions a GMR behavior is observed in this temperat ure range with higher sensitivity at low fields, e.g., (Mn0.95Fe0.05)Au-2 h as values of MR=-12% and -1% in a field of 1 T at 10 and 295 K, respectivel y. At low temperatures, an anomalous increase of resistance at higher field s (above 1-2 T) is observed for these alloys. (C) 2000 American Institute o f Physics. [S0021- 8979(00)16308-X].