Suppression of the Néel temperature in hydrothermally synthesized alpha-Fe2O3 nanoparticles
Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized $α$-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 $α$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. This precludes the determination of the Néel temperature for the $α$-Fe$_{2}$O$_{3}$ nanoparticles. In contrast, coating $α$-Fe$_{2}$O$_{3}$ nanoparticles with SiO$_{2}$ stabilizes the $α$-Fe$_{2}$O$_{3}$ phase up to 930 K, which allows us to determine the Néel temperature of the $α$-Fe$_{2}$O$_{3}$ nanoparticles for the first time. The Néel temperature of the 60-nm $α$-Fe$_{2}$O$_{3}$ nanoparticles is found to be 945 K, about 15 K below the bulk value. The small reduction of the Néel temperature of the $α$-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.