Anisotropy of hopping magnetoresistance in antiferromagnetic La2CuO4+d single crystals

The results on anisotropy of hopping conductivity (5-295 K) and magnetoresistance (MR) (5-55 K) of La2CuO4+d antiferromagnetic single crystals with TN approx 188 K are reported. The resistance was measured by the Montgomery technique for different combinations of transport current and magnetic field directions with respect to the crystallographic axes. In the case where the field and transport current were parallel to the CuO2 layers, a transition (at T approx 20 K) from negative MR to positive MR was detected with a rise of temperature. In the fields perpendicular to the CuO2 layers, only negative MR was observed. The nature of the positive MR is discussed. It is shown that the effect is more likely attributable to the orbital motion of charge carriers rather than to the interaction of their spins with the magnetic surroundings. The correlation is found between the value, magnetic-field and temperature behavior of the positive MR and the known Shklovsky-Efros model based on the allowance for compression of impurity wave functions of charge carriers in magnetic field