Variable-temperature Moessbauer spectroscopy of nano-sized maghemite and Al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, gamma -(Fe (sub 1-x) Al (sub x) ) (sub 2) O (sub 3) , have been prepared by thermal decomposition of Al-lepidocrocite (gamma -Fe (sub 1-x) Al (sub x) OOH), 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. Moessbauer 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 (T (sub 1/2) ), and the temperature below which the doublet ceases to exist (T (sub 0) ). These two parameters (T (sub 1/2) and T (sub 0) ) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % Al) and to 100 K and 48 K (18 mole % Al), respectively. The average hyperfine fields at 6 K undergo a steep drop in going from the Al-free sample (H(combining macron) (sub hf) = 506 kOe) to the sample with 4 mole % Al (H(combining macron) (sub hr) = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: tau E (sub Q.A) = 0.86 + or - 0.04 mm/s and tau E (sub Q.B) = 0.65 + or - 0.04 mm/s for the 4 mole % Al sample. The characteristic Moessbauer temperature, determined from the temperature dependence of the average isomer shift, was found to be in the range of 500-600 K.