We have determined the Curie temperature ${\mathit{T}}_{\mathit{c}}$ of ferrimagnetic ${\mathrm{MnFe}}_2$${\mathrm{O}}_4$ nanoscale particles by means of direct measurement of the magnetization. A considerable enhancement in ${\mathit{T}}_{\mathit{c}}$ compared to the bulk was observed for particles in the size range 7.5 to 24.4 nm. At d=7.5 nm, ${\mathit{T}}_{\mathit{c}}$ is 97 K higher than for the bulk material. The shift in Curie temperature is well described by the finite-size-scaling formula [${\mathit{T}}_{\mathit{c}}$(d)-${\mathit{T}}_{\mathit{c}}$(∞)]/${\mathit{T}}_{\mathit{c}}$(∞)=(d/${\mathit{d}}_0$${)}^{\mathrm{-}1/\nu }$ with ν=0.71±0.07 and ${\mathit{d}}_0$=2.0 nm.

• Received 3 June 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.3602