BlockingTemperatur

BlockingTemperatur is a term used in magnetism to describe the temperature below which the magnetic moments of nanoscale particles become thermally blocked on the timescale of observation. Below this temperature, the moments remain fixed in a preferred orientation for practical timescales, whereas above it they can rapidly fluctuate due to thermal energy, a regime known as superparamagnetism.

The concept arises from the competition between magnetic anisotropy energy and thermal energy. For a single-domain particle with anisotropy energy KV (K is the anisotropy constant, V is the particle volume), the relaxation time for reorientation follows Neel-Arrhenius kinetics: τ = τ0 exp(KV/kB T). The blocking temperature TB is often defined by the condition τ ≈ tobs, leading to TB ≈ KV/(kB ln(tobs/τ0)). Here τ0 is a characteristic attempt time (typically 10^-9 to 10^-12 s) and tob s is the measurement timescale.

Experimentally, TB is commonly identified in zero-field-cooled and field-cooled magnetization measurements. The temperature at which the zero-field-cooled magnetization peak occurs is associated with TB for the given experimental timescale. TB is sensitive to particle size, shape, material, interparticle interactions, and measurement conditions, and it can vary within an ensemble.

In applications, TB influences data stability in magnetic recording, the behavior of magnetic nanoparticles in biomedical contexts, and the effectiveness of hyperthermia heating. Because TB is a cross-over property dependent on timescale rather than a true phase transition, it can shift with changing measurement or operating conditions.

See also: superparamagnetism, magnetic anisotropy, Neel relaxation.