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Magnetic measurements up to 1000 K have been performed on hydrothermallysynthesized $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles (60 nm) using a QuantumDesign vibrating sample magnetometer. A high vacuum environment(1$\times$10$^{-5}$ torr) during the magnetic measurement up to 1000 K leads toa complete reduction of $\alpha$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. Thisprecludes the determination of the N\'eel temperature for the$\alpha$-Fe$_{2}$O$_{3}$ nanoparticles. In contrast, coating$\alpha$-Fe$_{2}$O$_{3}$ nanoparticles with SiO$_{2}$ stabilizes the$\alpha$-Fe$_{2}$O$_{3}$ phase up to 930 K, which allows us to determine theN\'eel temperature of the $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles for the firsttime. The N\'eel temperature of the 60-nm $\alpha$-Fe$_{2}$O$_{3}$nanoparticles is found to be 945 K, about 15 K below the bulk value. The smallreduction of the N\'eel temperature of the $\alpha$-Fe$_{2}$O$_{3}$nanoparticles is consistent with a finite-size scaling theory. Our currentresults also show that coating nanoparticles with SiO$_{2}$ can effectivelyprotect nanoparticles from oxidation or reduction, which is important totechnological applications.

Suppression of the N\'eel temperature in hydrothermally synthesized alpha-Fe2O3 nanoparticles