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The relaxation dynamics of a uniaxial paramagnet of arbitrary spin value S, interacting with a thermal bath, are treated using the spin-boson model for a particular spin Hamiltonian consisting of the simplest uniaxial potential in the high temperature, Ohmic damping, and weak spin-bath coupling limits. The signal-to-noise ratio of the magnetic moment fluctuations in the stochastic resonance of such a paramagnet displays a pro- nounced dependence on S for values of S20. The dependence arises from the quantum spin dynamics which differs markedly from the magnetization dynamics of classical superparamagnets with S10 3 -10 5 . A particularly interesting aspect of the motion of Brown- ian particles in a multiwell potential is the phenomenon of stochastic resonance SR, whereby a weak periodic forcing synchronized with the thermally activated hopping between the potential wells greatly enhances the rate of switching between them. The archetypal theoretical model of SR Ref. 1 is a Brownian particle in a bistable potential sub- jected to noise arising from the thermal bath. The particle is excited by a weak ac driving force of frequency close to the rate of transitions escape rates between the wells. How- ever, the amplitude of the force is in itself insufficient to induce the transitions. Consequently, switching may occur only by the combined effect of the deterministic ac force and the noise. The resulting signal-to-noise ratio SNR as a function of the noise intensity has a bell-like shape, i.e., it passes through a maximum thus exhibiting stochastic reso- nance due to the ability of noise to enhance the intensity of the interwell hoppings in the system. Stochastic resonance is now well known but still remarkable effect allowing one to control the behavior of periodic signals passing through noisy systems and is a universal manifestation of the inter- play between stochastic and regular motions. Comprehensive reviews of diverse aspects of SR are available in Refs. 1-3. The behavior of magnetic nanosystems such as super- paramagnetic particles, nanoclusters, and molecular mag- nets forced by a weak ac magnetic field is yet another im- portant manifestation of SR. Here the magnetic anisotropy provides the multistable states for the magnetization while the thermal fluctuations due to the bath are the source of the noise. These conditions give rise to magnetic stochastic reso- nance which may be defined as the enhancement of the SNR of the magnetic moment fluctuations due to noise. 4 The SNR of the magnetic moment fluctuations is of interest because nanoparticle magnetism is a rapidly expanding area of re- search with many applications. These arise both in the ap- plied area of information storage and in other fundamental aspects such as the crossover between classical and quantum behavior of the magnetization since single-domain particles exhibit essentially classical behavior while smaller entities such as nanoclusters made of many atoms and molecular magnets exhibit pronounced quantum behavior.

Spin-size effects in stochastic resonance in uniaxial superparamagnets