Depression of the critical exponent of the susceptibility by low-concentration impurities in uniaxial ferromagnets

Zero-field susceptibility data for GdCl₃ and LiTbF₄ containing up to 12 at.% impurities (Nd³⁺, La³⁺, Y³⁺) reveal a distinct depression of the effective critical exponent γeff at small concentrations ({}7%). This new effect is explained quantitatively by renormalization-group calculations, using the measured shift of the Curie temperature. For larger impurity concentrations γeff tends to rise as expected for an increasingly unstable structure.