Variable-range hopping conductivity ofLa1 − xSrxMn1 − yFeyO3

Abstract

The temperature dependence of the resistivity,ρ, of ceramicLa1 − 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 temperaturesT ∼ 5 and 310 K inmagnetic fields B up to 8 T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behaviorof ρ(T) is more complicated, comprising of two extremes, divided byan interval of metallic behavior in #15, and two inflections ofρ(T) in #20 withinsimilar intervals ΔT 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 atT > TC,T closeto TC and T < TC. In #25, the Mott VRH conductivity is observed in two different intervals, above and belowTC, divided by an intermediate interval of the Shklovskii–Efros VRH conduction regime.Analysis of the VRH conductivity yielded the values of the localization radius,α, and thedependence of α and of the density of the localized states,g, near the Fermilevel, on B. AboveTC the localization radiusin all samples at B = 0 hassimilar values, α≈1.0–1.2 Å,which is enhanced to α≈3.3 Å (#20) and 2.0 Å (#25) below TC. The sensitivity of α and g toB dependon y andT. Thecomplicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopicparameters α and g, is attributable to different electronic and magnetic phases of LSMFO varying withtemperature and Fe doping.