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Superparamagnetism and Spin Glass Dynamics of Interacting Magnetic Nanoparticle Systems

The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a nontrivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependence is interpreted in terms of the nonaxially symmetric potential created by the transverse component of the dipolar field. Strongly interacting nanoparticle systems are investigated experimentally in terms of spin glass behavior. Disorder and frustration arise in samples consisting of frozen ferrofluids from the randomness in particle position and anisotropy axis orientation. A strongly interacting FeC system is shown to exhibit critical dynamics characteristic of a spin glass phase transition. Aging, memory and rejuvenation phenomena similar to those of conventional spin glasses are observed, albeit with much weaker rejuvenation effects than in both a Ag(11 at% Mn) Heisenberg and an Fe_{0.5}Mn_{0.5}TiO_3 Ising spin glass. Differences in the nonequilibrium dynamics of the strongly interacting nanoparticle system and the two spin glass samples are discussed in terms of anisotropy and different timescales, due to the much longer microscopic flip time of a magnetic moment than of an atomic spin.