Aging in a Magnetic Particle System

The influence of dipolar interaction in a frozen ferrofluid has been experimentally studied. The ferrofluid consisted of particles of $\gamma$- ${\mathrm{Fe}}_2$${\mathrm{O}}_3$ with mean diameter 70 Å. Four samples with volume concentration of magnetic particles ranging from 0.03% to 17% have been investigated. The magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field—the system ages. The most diluted sample shows isolated particle dynamics and no aging.