Critical phenomena studied via nuclear techniques

Since pioneering NMR studies of magnetic critical phenomena in MnF₂ by Heller, a range of hyperfine techniques, particularly Mössbauer effect and perturbed angular correlations, have been successfully applied to the problem. When optimally designed, these methods provide experimental approaches that are in some ways more successful than magnetic neutron scattering. Although the number of hyperfine results now number in the hundreds, only a few are sufficiently asymptotic to provide clearcut tests of theory. In this paper we outline the theoretical framework used to describe the modern theory of critical phenomena, discuss the strengths and weakness of various experimental methods, and provide a selection of the best experiments that test the theory. Whereas we emphasize hyperfine interaction studies, we also describe the principles and results of other methods where appropriate. Particular topics include: (1) static critical phenomena in chemically ordered systems studied as a function ofn andd, the spin and lattice dimensionality; (2) static critical phenomena in magnetically disordered systems studied as a function of the type of disorder; and (3) dynamic critical phenomena in chemically ordered systems.