Morin Temperature and Neel Temperature of Hematite Nanocrystals

Thermodynamic analytic models have been established first to describe the size dependences of Morin temperature T-M and Neel temperature T-N in hematite nanocrystals based on size-dependent cohesive energy model. The models indicate that both Morin temperature and Neel temperature reduce with decreasing size D or increasing magnetic proximity effect constant m at the interface, where the size effect is the principle factor while the proximity effect is secondary. Agreements between the model predictions and the corresponding experimental results are found to be reasonable, which enables us to determine the values of correlation length xi(0) and shift exponent lambda in the finite size scaling theory thermodynamically for the first time. The calculated xi(0) and lambda at infinite size respectively correspond to the result of the mean field theory and that obtained through fitting experimental results. Moreover, the shift exponent lambda is found to be size-dependent. It decreases with increasing of the size and then reaches to a plateau in contrast with T-M(D) and T-N(D) functions.