MAGNETIC AFTEREFFECT EXPERIMENTS AT LOW-TEMPERATURE - LINEAR-RESPONSEAND QUANTUM-NOISE

An alternative approach to slow magnetic relaxation at low temperature is proposed in terms of a linear response theory involving collective variables characteristic of local metastable states. The model leads to the definition of two types of magnetic viscosity: S-0 due to the r esponse to an excitation field and S-1 due to the response to an initi al thermal perturbation. The thermal behavior of the magnetic viscosit y is found to be S-0 proportional to K(T)/k(B)T in the first case, and S-1 proportional to K(T) in the second case, where K(T) is the therma l noise. Experimental results, interpreted as the manifestation of the magnetic viscosity S-0, show some evidence of a noise K(T), which can be interpreted as a quantum form of the noise.