Classical-quantum crossover in magnetic stochastic resonance in uniaxial superparamagnets

It is shown that the signal-to-noise ratio of the magnetic moment fluctuations in the magnetic stochastic resonance of a quantum uniaxial paramagnet of arbitrary spin value S subjected to a weak probing ac field H(t) = H cos Omega t and a dc bias magnetic field H-0 displays a pronounced dependence on S. The dependence arises from the quantum dynamics of spins which differs markedly from the magnetization dynamics of classical superparamagnets. In the large spin limit, S -> 8, the quantum solutions reduce to those for a classical uniaxial superparamagnet.