Order Parameter Criticality of the d=3 Random-Field Ising Antiferromagnet Fe(0.85)Zn(0.15)F2

The critical exponent beta =0.16 ± 0.02 for the random-field Ising model order parameter is determined using extinction-free magnetic x-ray scattering for Fe(0.85)Zn(0.15)F2 in magnetic fields of 10 and 11 T. The observed value is consistent with other experimental random-field critical exponents, but disagrees sharply with Monte Carlo and exact ground state calculations on finite-sized systems.