Field-induced spin-flop transitions of interacting nanosized α-Fe2O3 particles dispersed in a silica glass matrix

Nanoparticles of 10 mol% Fe2O3 doped in silica glass (Fe10) samples prepared by a sol-gel method followed by calcination at various temperatures in the range 700-1000 deg. C are studied by x-ray, transmission electron microscopy and magnetic methods, including electron paramagnetic resonance (EPR). X-ray studies reveal the presence of both γ-Fe2O3 and α-Fe2O3 nanocrystals in varying proportion, the latter being more abundant in samples subject to calcination at higher temperature. Nanocrystals have mean sizes in the range 10-40 nm and are larger in the samples calcined at higher temperatures. The relatively narrow EPR line having its origin in superparamagnetism is observed at room temperature and is transformed to an asymmetrically broad ferromagnetic resonance signal at 77 K. Moessbauer spectra of the 700 deg. C calcined sample at room temperature show two doublet structures due to γ-Fe2O3 and α-Fe2O3 nanoparticles signifying their superparamagnetic character, whereas those of samples calcined at higher temperatures (≤1000 deg. C) display two sextets indicating that the nanoparticles are magnetically ordered. At higher calcination temperatures (≥900 deg. C) the hyperfine lines become asymmetrically broadened and the average hyperfine field diminishes in a way typical for interacting magnetic nanoparticles. Zero-field-cooled magnetization and hysteresis studies of Fe10 samples calcined at higher temperatures (≥800 deg. C) in the temperature range 5-300 K reveal unusual ferromagnetic behaviour with substantial magnetization and large coercivities at low temperature (5 K). The presence of a high irreversibility field and shifted hysteresis loop in the Fe10 sample calcined at 1000 deg. C at 5 K has been verified from field-cooled magnetization versus the magnetic field curve. Only α-Fe2O3 nanoparticles generated in the Fe2O3:SiO2 sample calcined at 1000 deg. C exhibit a Morin transition like bulk α-Fe2O3 crystals but at a much lower temperature ∼100 K. Spin-flop like transitions have been observed for the first time at temperatures above the Morin temperature, possibly induced by an external dc magnetic field of appropriate magnitude in conjunction with inter-nanoparticle exchange