Direct observation of the logarithmic correction to Landau's equation of state at marginal dimension

Abstract

We report precise data on the equation of state near the Curie point of two uniaxial ferromagnets, LiTb${\mathrm{F}}_{4}$ 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.