Abstract
We report precise data on the equation of state near the Curie point of two uniaxial ferromagnets, LiTbF4 and Dy-ethylsulfate, obtained by a superconducting quantum-interference-device magnetometer. Analyzing the critical behavior of the coefficient of the third-order term in Landau's expansion of the magnetic field, the logarithmic divergence as expected from exact solutions of the renormalization-group equations is seen directly for the first time. The nonuniversal parameters describing this logarithmic singularity are compared to calculations based on the microscopic theory of Larkin and Khmel'nitzkii.