Paramagnetic Resonance as a Probe for Critical Phenomena

Critical phenomena have been greatly in evidence in recent years, especially last year in view of the Nobel Prize being awarded to K. G. Wilson for his Renormalization-Group (R.G.) theory (1). With this theory, one can quantitatively calculate the response of cooperative systems undergoing phase transitions, be they of the continuous, second-order, or discontinuous, first-order, variety. Especially for the former, experimentalists had observed appreciable deviations from so-called mean-field theory. The properties observed in early experiments were, for example, specific heat c , compressibility K, and the so-called order parameter n. The latter quantity measures the degree, of order below a transition Tc in a ferromagnetic system, the magnetization m, in an antiferromagnet, the staggered magnetization ms, etc. For the above-mentioned quantities, c (T), K(T) or ri(T) power-law dependences were observed as