Variable-range hopping conductivity of La1 −xSrxMn1 −yFeyO3

The temperature dependence of the resistivity, ?, of ceramic La1 ? xSrxMn1 ? yFeyO3 (LSMFO) samples with x = 0.3 and y = 0.03, 0.15, 0.20 and 0.25 (or simply #03, #15, #20 and #25, respectively) is investigated between temperatures T ~ 5 and 310?K in magnetic fields B up to 8?T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behavior of ?(T) is more complicated, comprising of two extremes, divided by an interval of metallic behavior in #15, and two inflections of ?(T) in #20 within similar intervals ?T below ~ 100?K. Mott variable-range hopping (VRH) conductivity is observed in #15 above the ferromagnetic Curie temperature, TC. In #20 the Mott VRH conductivity takes place in three different temperature intervals at T > TC, T close to TC and T < TC. In #25, the Mott VRH conductivity is observed in two different intervals, above and below TC, divided by an intermediate interval of the Shklovskii?Efros VRH conduction regime. Analysis of the VRH conductivity yielded the values of the localization radius, ?, and the dependence of ? and of the density of the localized states, g, near the Fermi level, on B. Above TC the localization radius in all samples at B = 0 has similar values, ??1.0?1.2??, which is enhanced to ??3.3?? (#20) and 2.0?? (#25) below TC. The sensitivity of ? and g to B depend on y and T. The complicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopic parameters ? and g, is attributable to different electronic and magnetic phases of LSMFO varying with temperature and Fe doping.