Dear Korobi,

What do you mean by irreversible temperature? Are you thinking in the temperature below the blocking temperature? If you are thinking in this later case, where the timescale of the

experiment is too short for the sample to reach thermal equilibrium value of magnetization, then you obtain a hysteresis loop M-H where there are irreversible heat release. Is this what you tried to say? I recommend you a paper where you can follow the different superparamagnetic behaviours

https://www.researchgate.net/publication/326581341_Beyond_the_blocking_model_to_fit_nanoparticle_ZFCFC_magnetisation_curves

Irreversible temperature is the blocking temperature of particles with the highest energy barrier while normal blocking temperature gives an average Tb value of all the particles. The difference between blocking temperature and irreversible temperature represents to the width of blocking temperature distribution. Korobi Konwar