Superparamagnetism: Nanoparticle Size Threshold

A grid of single-domain ferromagnetic nanoparticles, each with its own magnetic-moment arrow. Shrink the particle size below the critical threshold and the anisotropy energy barrier that normally locks each moment in place collapses toward the ambient thermal energy, so moments start flipping spontaneously and the time-averaged magnetization falls toward zero even though each particle stays fully magnetized instant to instant. Toggle the applied field to see both size regimes align while it is on — but only the large, stable particles keep that alignment once it is switched off.