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 functio n of temperature and in magnetic fields up to 20 T. The samples exhibi t a semiconducting behavior up to x = 0.17 and a metallic one at highe r Sr concentrations. The MR is maximum near T-c, which is strongly con centration dependent. In the sample with x = 0.15, the MR is not satur ated even at 20 T. For the same composition, an important anisotropy o f the electrical conductivity with respect to the [001] direction appe ars at T-c and increases at lower temperatures. In the compound La0.82 5Sr0.175Mn0.94Mg0.06O3, the substitution of Mg for Mn lowers T-c, in c ontrast to what is observed by substituting Sr for La. The crystal exh ibits a well defined first order transition from orthorhombic to rhomb ohedral structure at a temperature T-s which can be shifted by a magne tic field.