= Solution
Thermally activated moment reversal, or <Néel relaxation>, often has an activated timescale
$$
\tau_N\simeq\tau_0e^{E_a/(k_BT)}.
$$
Lowering <temperature> increases $\tau_N$. When it becomes comparable to the observation time $t_m$, the ensemble crosses an operational <magnetic blocking temperature>,
$$
T_B\simeq\frac{E_a}{k_B\ln(t_m/\tau_0)}.
$$
Above this blocking crossover, an equilibrated ensemble remains reversible; its low-field susceptibility can increase as <temperature> falls. Below it, moments cannot fully relax during the field sweep, so \b[<magnetic coercivity> and <remanence> can emerge and the loop opens]. A distribution of particle <volumes>/anisotropies broadens the crossover.
This is a timescale-dependent blocking effect, not necessarily a thermodynamic phase transition. Rotation of whole particles in a fluid can supply an additional relaxation channel, and the particle orientation distribution affects the observed loop. Compare sweep rates and <temperatures> before interpreting one curve as an intrinsic static material law.
Back to article page
