Superparamagnets

Superparamagnetism describes the behavior of very small ferromagnetic or ferrimagnetic particles that form a single magnetic domain. In zero field, thermal fluctuations cause the particle’s magnetic moment to randomly flip, so the time-averaged magnetization is essentially zero. When a field is applied, the moments align quickly and reversibly.

The effect depends on size, material anisotropy, and temperature. Below a blocking temperature TB the magnetization is frozen on experimental timescales; above TB the particle behaves as a true superparamagnet with negligible remanence. Relaxation follows an Arrhenius law, with the energy barrier KV/kBT governing the flipping time.

In practice, superparamagnets show minimal coercivity and remanence but high susceptibility and a fast, non-hysteretic response to applied fields. In suspensions, flipping can occur via Néel rotation or Brownian rotation; the measured behavior reflects the faster mechanism.

Common room-temperature materials are iron oxide nanoparticles, such as magnetite Fe3O4 and maghemite γ-Fe2O3, and ferrites. They are typically 5 to 20 nm in diameter, with coatings to improve stability and biocompatibility for biomedical use.

Applications include magnetic resonance imaging contrast enhancement, magnetic hyperthermia, and targeted drug delivery. The absence of remanence reduces aggregation after field removal, a desirable property in ferrofluids and biomedical contexts. Superparamagnetism also challenges data storage at very small bit sizes due to thermal instability.