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

Single-crystal hematite (alpha-Fe2O3) 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 160 nm, and heights of about 100 nm. Magnetic measurements up to 920 K have been performed on hydrothermally synthesized alpha-Fe2O3 nanorings and nanoparticles using a quantum design vibrating sample magnetometer. A high temperature phase transition of thermal stability (alpha-Fe2O3 to Fe3O4) occurs when magnetic measurement was performed under high vacuum (< 9: 5 x 10(-5) Torr). The phase transition temperature is 670 K for nanorings with thickness of similar to 30 nm, 718 K for nanorings with thickness of similar to 50 nm, 678 K for nanorings with thickness of similar to 65 nm, and 640 K for similar to 35 nm nanoparticles. This data show better thermal stability of nanorings with the thickness of similar to 50 nm than the other two kinds of nanoring samples The Neel temperature (T-N) of alpha-Fe2O3 nanorings with the thickness of similar to 50 nm is determined to be 937.2 K by magnetic measurement for the first time, about 22.8 K below the bulk value. The small reduction of the TN of the alpha-Fe2O3 nanorings is consistent with the finite-size scaling theory.