Size effect of DyF nanoparticles on Curie temperature.

A series of DyF nanoparticles samples were synthesized by a hydrothermal treatment in an autoclave at 140 °C, 160 °C, 200 °C, and 230 °C for 24 h. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and temperature dependence of magnetic susceptibility. It was found that DyF particles possessed an ellipsoidal shape and size varying from 16 to 225 nm. Moroever, the Curie temperature shifts to lower temperatures when the particle size decreases. For the first ime, the critical exponent of the correlation length ( = 1.51 ± 0.25) and critical size ( = 1.2 ± 0.6 nm) for the dipole ferromagnet DyF has been determined experimentally by the finite-size-scaling theory.