Observation of isotropic dipolar to uniaxial dipolar crossover in gadolinium

Extensive high-resolution magnetic-susceptibility ({χ}) measurements have been performed along the c axis (easy direction of magnetization) of a high-purity gadolinium single crystal in the reduced temperature range 5.1×10^-5<~ε=(T-TC)/TC<~1.2×10^-1. The results of an elaborate data analysis permit the first accurate determination of asymptotic critical exponent for {χ} and unambiguously demonstrate that the asymptotic critical behavior of gadolinium is that of a uniaxial dipolar (UD) ferromagnet. In addition, present results using existing data on critical spin dynamics provide conclusive evidence for the occurrence of U→Disotropic dipolar (ID){→}Gaussian crossovers. While the U→DID crossover is sharply defined and occurs at εCO^U→DID=2.05(10)×10^-3, the I→DGaussian crossover proceeds without the (theoretically predicted) intervening isotropic short-range Heisenberg regime and takes place over a wide temperature range.