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

Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1$\times$10$^{-5}$ torr) during the magnetic measurement up to 1000 K leads to a complete reduction of $\alpha$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. This precludes 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 the N\'eel temperature of the $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles for the first time. 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 small reduction of the N\'eel temperature of the $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles is consistent with a finite-size scaling theory. Our current results also show that coating nanoparticles with SiO$_{2}$ can effectively protect nanoparticles from oxidation or reduction, which is important to technological applications.