What is the coercivity limit for superparamagnetic behavior in M-H graph?
Coercivity in the superparamagnetically blocked state is a pure kinetic effect. In equilibrium meaning in the limit of arbitrary low sweep rate of the applied magnetic field there is no coercivity. The kinetics of the system is simple very slow in the blocked state, but still there is relaxation. That means that increasing the sweep rate of the magnetic field will increase the "coercive field". I don't think there is a well defined limit if you could only arbitrarily fast ramp the applied magnetic field. This is very similar to the fact that the blocking temperature is also not a system property but depends on the characteristic time scale on which you measure. Moessbauer and SQUID magnetometry provide for example extremely different blocking temperatures.
For a Ni-Zn ferrite superparamagnetic nanoparticles you can observe 50-200 Oe coercive field at RT. In addition to Dr. Binek, particle size and morphology affect the magnetic ordering too.
By definition a superparamagnetic state is a magnetic monodomain whose coercivity field is then zero in a M-H hysteresis loop. But below the blocking temperature and in the ZFC it is possible to achieve one equivalent field equivalent because the superparamagnetic moments are frozen in every nanoparticle or nanomagnetic domain with a Neel relaxation time to destroy such field in a given time. That is to say it is not an stable state. Thus the coercivity limit could be understood as states with temperatures higher than the blocking one coming from the zero field cooling process.
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