Appearance of a Large Magnetization at Elevated Temperatures in Nearly Antiferromagnetic $\alpha$-Fe2O3

概要

論文の詳細を見る
A significant magnetization of about 10--20 emu/g has been observed under a magnetic field of 5 kOe in nearly antiferromagnetic $\alpha$-Fe2O3 when heated at 350--500 °C in air. The present outstanding effect that contrasts with the temperature dependence of the Curie's law has been investigated on the basis of various temperature-dependent measurements of magnetization, diffuse reflectance spectra, electrical conductivity, Seebeck potential, X-ray and neutron diffractions, differential scanning calorimetry, thermogravimetry, and X-ray photoelectron spectra. The color of $\alpha$-Fe2O3 is found to drastically change from vivid red to black at about 380 °C, and this is closely correlated with the appearance of the magnetization. The color change is due to the formation of oxygen defects called a color center (i.e., $F$-center). Then, the absence of the intervening O2- ions greatly disturbs the superexchange interaction that is responsible for the anti-parallel spin alignment in the Fe--O--Fe (cation--anion--cation) system. As a result, the uncoupled spins can be aligned in parallel due to the direct exchange interaction between various nearest-neighboring Fe3+ ions, resulting in the appearance of the large magnetization at elevated temperatures.
2011-01-25