where 𝑇 ! (∞) is the bulk Curie temperature, 𝜈 is a critical exponent, 𝑑 # is a characteristic length scale of the system, and 𝑑 is the smallest length scale of the system. Further mean-field and atomistic simulations also showed that the drop of 𝑇 ! can approach 20~30% of the bulk value in NPs around 1 nm. These findings indicate that it might no longer be applicable to simply use the bulk critical point to describe the phase-transition induced phenomenon, such as heat-assisted magnetic recording and the magneto-catalytic effect (MCE) .