Following diagram show Hysteresis loops of CoFe2O4 Spherical nanoparticles at 300k & 70K (liquid Nitrogen). Why such decrease in hysteresis loop and Hc have been seen with reduce temperature?
At very low temperatures, quantum mechanical effects become more pronounced. These effects can lead to a reduction in magnetic interactions between atoms, making it easier for the magnetic domains to respond to an external magnetic field. As a result, coercivity tends to decrease.