Appearance of a Large Magnetization at Elevated Temperatures in Nearly Antiferromagnetic α-Fe2O3

Abstract

A significant magnetization of about 10–20 emu/g has been observed under a magnetic field of 5 kOe in nearly antiferromagnetic α-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 α-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.