There is a old paper of Hansen&MOrup about how to estimate the blocking temperatures from ZFC/FC curves. You can find this paper Journal of Magnetism and Magnetic Materials 203 (1999) 214.
Dear Amir,
The bifurcation point* on ZFC/FC dependence can be considered as a blocking temperature but measured under the finite magnetic field (often called as a measuring field). Non-zero measuring field decreases the energetic barrier to overcome for the magnetization to achieve its equilibrium, thus the ensemble will be unblocked at the lower temperatures. Therefore, one should make a series of ZFC/FC measurements to obtain a dependence of the bifurcation point versus the measuring field. The next step is to choose an appropriate model for the ensemble being meausred (what kind of magnetic anisotropy exists in the ensemble, how the easy axes are spatialy distributed, and so on...). This model will describe analitically how the bifurcation point is dependent on the measuring field. The final step is to fit the model to the measured dependence. The result is a true blocking temperature value under the zero measuring field which allows you to extract other physical parameters of the superparamagnetic ensemble (anisotropy constant, mean size of the particles, interparticle interaction strength etc.).
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* Very often scientists use not exactly the bifurcation point but some other close lying points which are more convenient to determine (maximum of ZFC curve, or maximum of derivative of ZFC, difference between FC and ZFC etc.). Each concrete choise should be based on the concrerte analitical treatment, otherwise you will have an additionoal error in the extracted parameters. If you are interested in this, I can propose you to read my paper: http://dx.doi.org/10.1063/1.2920171
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