Finite-size scaling law of the Néel temperature in hematite nanostructures

We report high-temperature magnetic properties of single-crystalline hematite (α-Fe2O3) nanostructures with different shapes. Magnetic measurements under a high vacuum (<9.5 × 10−6 Torr) up to 920 K were used to characterize thermal stability of the nanostructures. The onset temperature of the α-Fe2O3 to Fe3O4 phase transformation and the transformed fraction were found to depend strongly on the shape of the nanostructure. The data demonstrate that the phase transformation mainly occurs at the (001) surfaces. The high thermal stability of the nanoring and nanotube samples allows us to accurately measure their Neel temperatures. The Neel temperatures of the nanoring and nanotube samples were found to decrease with decreasing the mean wall-thickness of the nanoring/nanotube assembly. The data confirm the two-dimensional finite-size scaling law for the Neel temperature.