Low-field magnetic properties as indication of disorder, frustration and cluster formation effects in La 1− xCa xMn 1− yFe yO 3

Abstract

Low-field ( B=2–80 G) DC magnetic susceptibility, χ, is investigated in ceramic La 1− x Ca x Mn 1− y Fe y O 3 (LCMFO) samples with x=0.3. When y is increased from 0 to 0.1 the ferromagnetic (FM) Curie temperature, T C, 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=10 5 s) relaxation of thermoremanent magnetization. Critical behavior of χ( T) obeying the scaling law χ −1( T)− χ −1( T C)∼( T/ T C−1) γ ≡ τ γ is observed below τ cr∼0.05–0.2 with γ= γ 1≈1.4 corresponding to a three-dimensional (3D) Heisenberg spin system. Above τ cr, χ( T) can be fitted with the same scaling law as for τ< τ cr, but with another value of γ= γ 2≈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 T C. The magnetic moments, radii, and concentration of these particles are determined.