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

Abstract

Nanoparticles of 10 mol% Fe2O3 doped in silica glass (Fe10) samples prepared by a sol–gel methodfollowed by calcination at various temperatures in the range700–1000 °C are studied by x-ray, transmission electron microscopy and magnetic methods, includingelectron paramagnetic resonance (EPR). X-ray studies reveal the presence of bothγ-Fe2O3 and α-Fe2O3 nanocrystals in varying proportion, the latter being more abundant in samples subject tocalcination at higher temperature. Nanocrystals have mean sizes in the range 10–40 nm andare larger in the samples calcined at higher temperatures. The relatively narrow EPR linehaving its origin in superparamagnetism is observed at room temperature and is transformedto an asymmetrically broad ferromagnetic resonance signal at 77 K. Mössbauer spectra of the700 °C calcined sample at room temperature show two doublet structures due toγ-Fe2O3 and α-Fe2O3 nanoparticlessignifying their superparamagnetic character, whereas those of samples calcined at higher temperatures(≤1000 °C) displaytwo sextets indicating that the nanoparticles are magnetically ordered. At higher calcination temperatures(≥900 °C) the hyperfine lines become asymmetrically broadened and the average hyperfine fielddiminishes in a way typical for interacting magnetic nanoparticles. Zero-field-cooledmagnetization and hysteresis studies of Fe10 samples calcined at higher temperatures(≥800 °C) in the temperature range 5–300 K reveal unusual ferromagnetic behaviour with substantialmagnetization and large coercivities at low temperature (5 K). The presence of a highirreversibility field and shifted hysteresis loop in the Fe10 sample calcined at1000 °C at 5 K has been verified from field-cooled magnetization versus the magnetic field curve. Onlyα-Fe2O3 nanoparticles generatedin the Fe2O3:SiO2 sample calcinedat 1000 °C exhibit a Morintransition like bulk α-Fe2O3 crystals but at a muchlower temperature ∼100 K. Spin–flop like transitions have been observed for the first time at temperatures abovethe Morin temperature, possibly induced by an external dc magnetic field of appropriatemagnitude in conjunction with inter-nanoparticle exchange.