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

作者：Yui Yuhki^*; Ito Shimpei; Mizuguchi Jin; Ishikawa Yoshihisa; Kiyanagi Ryoji; Noda Yukio

来源：Japanese Journal of Applied Physics, 2011, 50(1): 013003.

DOI：10.1143/JJAP.50.013003

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 degrees 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 degrees 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 O-2(-) 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.

• 出版日期2011-1