Editing Superparamagnetism (section)

==Measurements==
A superparamagnetic system can be measured with [[magnetic susceptibility|AC susceptibility]] measurements, where an applied magnetic field varies in time, and the magnetic response of the system is measured. A superparamagnetic system will show a characteristic frequency dependence: When the frequency is much higher than 1/τ<sub>N</sub>, there will be a different magnetic response than when the frequency is much lower than 1/τ<sub>N</sub>, since in the latter case, but not the former, the ferromagnetic clusters will have time to respond to the field by flipping their magnetization.<ref>{{cite web |url=http://www.qdusa.com/resources/pdf/1078-201.pdf |last1=Martien |first1=Dinesh |title=Introduction to: AC susceptibility |publisher=Quantum Design | access-date= 15 Apr 2017}}</ref> The precise dependence can be calculated from the Néel–Arrhenius equation, assuming that the neighboring clusters behave independently of one another (if clusters interact, their behavior becomes more complicated). It is also possible to perform magneto-optical AC susceptibility measurements with magneto-optically active superparamagnetic materials such as iron oxide nanoparticles in the visible wavelength range.<ref>{{cite journal |doi=10.1063/1.4801837|last1=Vandendriessche |first1=Stefaan|title=Magneto-optical harmonic susceptometry of superparamagnetic materials|journal=Applied Physics Letters|volume=102|number=16|pages=161903–5|bibcode = 2013ApPhL.102p1903V |display-authors=etal|year=2013 |url=https://lirias.kuleuven.be/handle/123456789/402755 }}</ref>