Asymptotic critical behaviour of quenched random-exchange Heisenberg ferromagnets

Asymptotic and leading 'correction-to-scaling' critical exponents and critical amplitudes for zero-field specific heat, spontaneous magnetization and initial susceptibility have been accurately determined for quenched random-exchange Heisenberg ferromagnets through an elaborate analysis of highly precise electrical resistivity, bulk magnetization and AC susceptibility data taken on amorphous Fe_xNi_80-x(B,Si)₂₀ (x=10, 13, 16 and 20) alloys in the critical region. The values of critical exponents and universal critical amplitude ratios so obtained do not depend on composition and conform very well with the corresponding estimates given by the renormalization-group calculations for quenched random site- and bond-diluted Heisenberg ferromagnets. The amplitude ratios a_c1⁺/a_c1^- and a_c1⁺/a_{chi 1}⁺, which are characteristic of ferromagnets with quenched random disorder and for which no theoretical estimates are presently available, seem to possess universal character for they too are composition-independent. The experimental results are consistent with the concept of scaling in that the exponent equalities alpha ⁺= alpha ^-, beta + gamma = beta delta and alpha + gamma =2(1- beta ) are obeyed to a high degree of accuracy and the magnetization data satisfy the scaling equation of state characteristic of second-order phase transition. The effect of the isotropic long-range dipolar interactions on the asymptotic critical behaviour is mainly felt through the enhanced value of the A⁺/A^$ratio. Only a small fraction of moments actually participates in the ferromagnetic(FM)-paramagnetic(PM) phase transition for the alloys with Fe concentration x not very far from, but above, the critical concentration x_c and this fraction reduces further at a rapid rate as x_c is approached along the FM-PM phase transition line in the magnetic phase diagram.