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How does superparamagnetism occur in extremely small magnetic grains?

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Diagram showing a circular loop of wire with an electric current, surrounded by curved blue lines representing magnetic field lines that loop around the wire and pass through its center. — magnetism Some lines of the magnetic field B (blue) for an electric current I (gold) in a loop.

Superparamagnetism occurs in materials composed of extremely small magnetic grains, typically around 100 angstroms in size. In these tiny particles, the volume is so limited that the thermal energy present at a given temperature is sufficient to overcome the forces that would otherwise cause a cooperative alignment of magnetic dipole moments.1

Because thermal fluctuations are strong enough to disrupt the spontaneous ordering of these moments, the grains do not exhibit the stable, permanent magnetization characteristic of ferromagnetism or ferrimagnetism in the absence of an external field. Instead, the magnetic moments of the individual grains fluctuate randomly, resulting in a state that behaves similarly to paramagnetism, but with a much larger magnetic susceptibility due to the collective nature of the moments within each grain.1