Low-field magnetic properties as indication of disorder, frustration and cluster formation effects in La1−xCaxMn1−yFeyO3

Low-field (B=2–80 G) DC magnetic susceptibility, χ, is investigated in ceramic La1−xCaxMn1−yFeyO3 (LCMFO) samples with x=0.3. When y is increased from 0 to 0.1 the ferromagnetic (FM) Curie temperature, TC, decreases from 259 to 107 K, indicating growth of disorder by iron doping. A transformation of LCMFO from a weakly frustrated FM phase between y≈0–0.05 to a strongly frustrated mixed, FM and glassy, phase between y≈0.07–0.10 is revealed by measurements of magnetic irreversibility and long-time (up to t=105 s) relaxation of thermoremanent magnetization. Critical behavior of χ(T) obeying the scaling law χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ is observed below τcr∼0.05–0.2 with γ=γ1≈1.4 corresponding to a three-dimensional (3D) Heisenberg spin system. Above τcr, χ(T) can be fitted with the same scaling law as for τ<τcr, but with another value of γ=γ2≈1.7, which characterizes a 3D percolation system. The change of γ upon lowering the temperature can be explained by the formation of percolation clusters from an assembly of hole-rich nanoscale particles of the second FM phase embedded into the host LCMFO matrix at T well above TC. The magnetic moments, radii, and concentration of these particles are determined.