Electrical conductivity and magnetic properties of La1 − x Ca x Mn1 − y Fe y O3 ceramic samples (x = 0.67, y = 0, 0.05)

Abstract

The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La1 − xCaxMnO3 with x = 0.67 (LCMO) and La1 − xCaxMn1 − yFeyO3 with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–105 G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures TCO ≈ 272 K for LCMO and TCO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers peff = 4.83 μB (LCMO) and 4.77 μB (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < TCO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and aund for LCMO is a/aund = 0.88.