Concentration scaling of the specific heat in the d=3 random exchange Ising model system FexZn1−xF2

Abstract

Linear magnetic birefringence Δnm was used to study the concentration dependence of both the total magnetic specific heat Cm and critical amplitudes in the random exchange Ising model (REIM) system FexZn1−xF2. We find the proportionality between Δnm and the magnetic energy Um is x independent for 0.24≤x≤1.0, since Δnm scales with x2 as does Um. However, the critical part of Cm is found to decrease with dilution very much faster than x2. To explain this, a scaling form is derived which describes the x-dependent crossover from pure Ising to REIM with a crossover exponent φ=α, the pure Ising specific heat exponent. The critical amplitudes A± of Cm = A±‖t − tn‖−α̃ are shown to scale as A±(x)A±(x)∼ (1 − x)(α̃−α)/(φ)∼ (1 − x)−1.9(3), using the experimental REIM value of α̃=−0.09(3), and theoretical value of α=0.105(7). Good agreement between theory and experiment is found.