superparamagnetic

Superparamagnetism is a form of magnetism that occurs in small ferromagnetic or ferrimagnetic particles, typically single-domain, where thermal fluctuations can spontaneously flip the direction of the particle’s magnetic moment. In zero external field, the net magnetization averages to zero over time; in an applied field, the moments align with the field, producing a large magnetic susceptibility but little residual magnetization when the field is removed.

The effect arises when the magnetic anisotropy energy KV of a particle (K is the anisotropy constant, V is the particle volume) is comparable to or smaller than the thermal energy kBT. The magnetization can flip with a characteristic Neel relaxation time tau = tau0 exp(KV/kT). If tau is shorter than the measurement timescale, the particle behaves superparamagnetically; if tau is longer, the particle becomes “blocked” and may display hysteresis. The temperature at which tau equals the measurement timescale is called the blocking temperature TB. Above TB, particles are superparamagnetic; below TB, they exhibit stationary magnetization with magnetic memory.

Properties and implications include negligible remanence and coercivity in the absence of a field, rapid magnetic response, and high effective susceptibility. Interparticle interactions and a broad size distribution can modify TB and lead to deviations from ideal behavior.

Materials and applications commonly involve iron oxide nanoparticles such as magnetite (Fe3O4) or maghemite (gamma-Fe2O3). They are used as MRI contrast agents (SPIONs), in magnetic hyperthermia, and for targeted drug delivery or magnetic separation, where controllable magnetization dynamics and biocompatibility are important.