VARIABLE-TEMPERATURE MOSSBAUER SPECTROSCOPY OF NANO-SIZED MAGHEMITE AND A1-SUBSTITUTED MAGHEMITES

Abstract--Synthetic aluminum-substituted maghemite samples, "g-(Fe ~ _xAlx)203, have been prepared by thermal decomposition ofAl-lepidocrocite (3,-Fe~_xAlxOOH), with x = 0, 0.04, 0.06, 0.14 and 0.18. The particles are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. MSssbauer spectra were collected at 6 K and from 80 K up to 475 K at steps of 25 K. In a wide range of temperatures the spectra of the non-substituted sample consist of a superposition of a broad sextet and a superparamagnetic doublet, whereas for the Al-maghemites this range is much smaller. From the temperature variation of the fractional doublet area two different parameters were defined: the temperature corresponding to a 50/50 doublet-sextet spectrum (T1/2), and the temperature b low which the doublet ceases to exist (To). These two parameters (T1/2 and To) decreas