Electric and magnetic behaviour in double doped La_2/3+4x/3Sr_1/3-4x/3Mn_1-xMg_xO_3

The double-doped La_2/3+4x/3Sr_1/3-4x/3Mn_1-xMg_xO_3 samples with fixed Mn³⁺/Mn⁴⁺ ratio equal to 2/1 are investigated by means of magnetism and transport measurements. Phase separation is observed at temperature higher than T_c^onset for x=0.10 and 0.15. For x=0.10, rather strong phase separation induces drastic magnetic random potential and results in the localization of carriers. Thus, the variable-range hopping process dominates. For other samples, there is no or only weak phase separation above T_c^onset. Thus, thermal activation mechanism is responsible for the high temperature transport behaviour. For x=0.20 and 0.25, unexpected AFM behaviour is observed at low temperature. All these results are well understood by considering the special role of the ``double-doping".