Thickness Effect on Thermal Stability by Phase Transition of Single Crystal Hematite Nanorings

Abstract

Single-crystal hematite (α- Fe 2 O 3) nanorings with three different thicknesses were synthesized by a hydrothermal method. The results of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) show that the nanorings are single-crystal and have relatively uniform outer diameters of 160nm, and heights of about 100nm. Magnetic measurements up to 920K have been performed on hydrothermally synthesized α- Fe 2 O 3 nanorings and nanoparticles using a quantum design vibrating sample magnetometer. A high temperature phase transition of thermal stability (α- Fe 2 O 3 to Fe 3 O 4) occurs when magnetic measurement was performed under high vacuum (< 9.5 × 10-5 Torr). The phase transition temperature is 670K for nanorings with thickness of ∼30nm, 718K for nanorings with thickness of ∼50nm, 678K for nanorings with thickness of ∼65nm, and 640K for ∼35nm nanoparticles. This data show better thermal stability of nanorings with the thickness of ∼50nm than the other two kinds of nanoring samples The Néel temperature (T N ) of α- Fe 2 O 3 nanorings with the thickness of ∼50nm is determined to be 937.2K by magnetic measurement for the first time, about 22.8K below the bulk value. The small reduction of the T N of the α- Fe 2 O 3 nanorings is consistent with the finite-size scaling theory.