The magnetization of luminescent ${\text{Er}}^{3+}$-doped ${\text{PbF}}_2$ nanoparticles (formula ${\text{Er}}_{0.3}{\text{Pb}}_{0.7}{\text{F}}_{2.3}$) has been studied. Despite the high concentration of the doping ${\text{Er}}^{3+}$ ions and relatively large size (8 nm) of these nanoparticles we have found no deviation between field-cooled and zero-field-cooled magnetization curves down to $T=0.35\text{K}$, which points out an ultralow blocking temperature for the reversal of magnetization. We also have found strongly deviating magnetization curves $M\left(H/T\right)$ for different temperatures $T$. These results altogether show that the investigated nanoparticles are not superparamagnetic, but rather each ${\text{Er}}^{3+}$ ion in these nanoparticles is found in a paramagnetic state down to very low temperatures, which implies the breakdown of the Néel-Brown giant spin model in the case of these nanoparticles. Calculations of magnetization within a paramagnetic model of noninteracting ${\text{Er}}^{3+}$ ions completely support this conclusion. Due to the ultralow blocking temperature, these nanoparticles have a potential for magnetic field-induced nanoscale refrigeration with an option of their optical localization and temperature control.

• Received 23 March 2010

DOI:https://doi.org/10.1103/PhysRevB.82.094421