Non-universal low-field magnetic scaling and variable-range hopping conductivity as a consequence of disorder in La1−xCaxMn1−yFeyO3 Original Research Article

Low-field dc magnetic susceptibility, χ, investigated in hole-doped La1−xCaxMn1−yFeyO3 (LCMFO) with x=0.3 and y=0.05−0.10, exhibits non-uniform critical behavior near the paramagnetic (PM)-to-ferromagnetic (FM) transition temperature, TC. We observe the scaling law, χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ, with γ≈1.4 corresponding to a 3D Heisenberg spin system below τcr∼0.1–0.2, and with γ≈1.7 characterizing a 3D percolation system, above τcr. The non-universal scaling results from strongly inhomogeneous distribution of holes leading to formation of percolation clusters from the hole-rich nanoscale FM particles, embedded in the host PM matrix. The resistivity, ρ, of LCMFO exhibits between a temperature and TC the Shklovskii–Efros-type variable-range hopping conductivity law, ρ(T)=ρ0(T)exp[(T0/T)1/2], governed by generation of a complex gap in the density of states. The prefactor follows the law ρ0(T)∼Tm, where m changes from 9/2 at y=0 to 5/2 at y=0.03, 0.07 and 0.09, reflecting an additional fluctuating short-range potential induced by doping with Fe.