Nearest neighbour hopping conductors with negative magnetoresistance

• Published in: Physica A 1994-11, Vol.211 (4), p.393-398
• Main Authors: Pakhomov, A.B., McLachlan, D.S., Oblakova, I.I., Virnik, A.M.
• Format: Article
• Language: English
• ISSN: 0378-4371; 1873-2119
• DOI: 10.1016/0378-4371(94)00132-4

DC resistance and magnetoresistance measurements are performed on insulating AlAl 2O 3 samples which are prepared from conducting metal-insulator granular composite samples by subjecting them to high density current pulses. This treatment drastically reduces the room-temperature conductivity of the mixture, due to the explosion of a fraction of the metallic grains in percolation channels. Providing the pulse current density is high enough, the samples show the hopping conductivity. Two distinctly different nearest-neighbour hopping (NNH) types of samples may be obtained. In the first (M) type samples, conduction is dominated by hopping between adjacent metallic grains imbedded in a dielectric host medium. Negative orbital magnetoresistance (NMR), with a quasilinear field dependence, is observed at both 77 and 300 K. In the samples of the second (H) type (those which have been subjected to higher current pulses or to several sequential pulses of the same amplitude), conduction is found to be dominated by NNH between single impurity atoms. Positive magnetoresistance is now observed.