Negative magnetoresistance in GaAs containing Mn or Ni acceptors

Abstract

Transverse magnetoresistance was measured, from 20 to 30 K and for fields up to 1 T, in GaAs crystals doped with enough Mn or Ni acceptors to result in activationless impurity conduction. Anomalies in the weak-field magnetoresistance were seen in the impurity conduction range. Nickel-doped samples showed consistently zero or positive Δρ/ρo, though the near-zero magnetoresistance below 0.5 T for the temperature range 40–100 K suggested cancellation of conventional monotonically increasing positive magnetoresistance by a negative component that saturated for large fields. With manganese-doped crystals, a small negative total magnetoresistance (−Δρ/ρo<4×10−5) was seen for fields up to 0.6 T, but only within the limited temperature range 40–75 K. The field dependence again suggested that total magnetoresistance should be regarded as the algebraic sum of an increasing positive term and a saturating-type negative component. The magnitude and field dependence of the negative magnetoresistance of GaAs : Mn was compared with models such as those based on the magnetic ordering of localized spins, and yielded a localized impurity magnetic moment of 2.2μB when compared with the magnetic ordering model of Pöder.