Detailed magnetization study of quenched random ferromagnets. II. Site dilution and percolation exponents

Abstract

We report the experimental determination of the crossover exponent (φ) and the percolation critical exponents for magnetization (β_p) and spin-wave stiffness (θ) for quench-disordered (amorphous) three-dimensional (d=3) dilute Heisenberg ferromagnets. The values of φ, θ, and β_p, so obtained, as well as those of the percolation correlation-length critical exponent V_p and the conductivity exponent σ , deduced from the exponent equalities V_p=φ-(θ/2) and σ=(d-2)V_p+φ, conform very well with the most accurate theoretical estimates published recently. A comparison of the presently determined exponent values with those theoretically predicted for site or bond percolation on a d=3 crystalline lattice asserts that the critical behavior of percolation on a regular d=3 lattice does not get altered in the presence of quenched randomness if the specific-heat exponent of the regular system is negative. Consistent with the Alexander-Orbach conjecture (Golden inquality), the fracton dimensionality d¯ of the percolating cluster at threshold (the conductivity exponent s) turns out to be d¯ ≈4/3 (σ≤ 2). The present results vindicate the universality hypothesis.