Suppression of the Néel temperature in hydrothermally synthesized alpha-Fe2O3 nanoparticles

Jun Wang; Wei Wu; Fan Zhao; Guo-meng Zhao

doi:10.1063/1.3554624

paper

Suppression of the Néel temperature in hydrothermally synthesized alpha-Fe2O3 nanoparticles

arXiv:1010.5294 · doi:10.1063/1.3554624

Abstract

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.

5 pages, 5 figures