Superparamagnetism occurs for very small crystallites (1-50 nm) with a single magnetic domain.
Thermal energy can change the orientation of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero.
The material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field.
Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are superparamagnets (rather than ferromagnets).
Superparamagnetism sets a limit on the storage density of hard disk drives due to the minimum size of particles that can be used.