Morin Temperature and Néel Temperature of Hematite Nanocrystals

Abstract

Thermodynamic analytic models have been established first to describe the size dependences of Morin temperature TM and Néel temperature TN in hematite nanocrystals based on size-dependent cohesive energy model. The models indicate that both Morin temperature and Néel 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 ξ0 and shift exponent λ in the finite size scaling theory thermodynamically for the first time. The calculated ξ0 and λ at infinite size respectively correspond to the result of the mean field theory and that obtained through fitting experimental results. Moreover, the shift exponent λ is found to be size-dependent. It decreases with increasing of the size and then reaches to a plateau in contrast with TM(D) and TN(D) functions.