Conductivity and magnetoresistance of La1-xSrxMnO3 and La1-xSrxMn1-yMgyO3 single crystals

We present conductivity and MR measurements on single crystals of the mixed perovskite manganites La1-xSrxMnO3 (0.1 < x < 0.25) as a function of temperature and in magnetic fields up to 20 T. The samples exhibit a semiconducting behavior up to x = 0.17 and a metallic one at higher Sr concentrations. The MR is maximum near T-c, which is strongly concentration dependent. In the sample with x = 0.15, the MR is not saturated even at 20 T. For the same composition, an important anisotropy of the electrical conductivity with respect to the [001] direction appears at T-c and increases at lower temperatures. In the compound La0.825Sr0.175Mn0.94Mg0.06O3, the substitution of Mg for Mn lowers T-c, in contrast to what is observed by substituting Sr for La. The crystal exhibits a well defined first order transition from orthorhombic to rhombohedral structure at a temperature T-s which can be shifted by a magnetic field.