Magnetic aftereffect experiments at low temperature: Linear response and 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: S0 due to the response to an excitation field and S1 due to the response to an initial thermal perturbation. The thermal behavior of the magnetic viscosity is found to be S0K(T)/kBT in the first case, and S1K(T) in the second case, where K(T) is the thermal noise. Experimental results, interpreted as the manifestation of the magnetic viscosity S0, show some evidence of a noise K(T), which can be interpreted as a quantum form of the noise.