Magnetization behavior of ferromagnetic and superparamagnetic NPs under an external magnetic field. (a) Under an external magnetic field, domains of a ferromagnetic NP align with the applied field. The magnetic moment of single domain superparamagnetic NPs aligns with the applied field. In the absence of an external field, ferromagnetic NPs will maintain a net magnetization, whereas superparamagnetic NPs will exhibit no net magnetization due to rapid reversal of the magnetic moment. (b) Relationship between NP size and the magnetic domain structures. Ds and Dc are the 'superparamagnetism' and 'critical' size thresholds.
Akbarzadeh et al. Nanoscale Research Letters 2012 7:144 doi:10.1186/1556-276X-7-144