Samples of γ -Fe 2 O 3 nano-particles with a mean size of 4.0(3) nm and with different hydration and surfactant degrees were prepared by sol-gel methods. Morphology and structural data were obtained by transmission electron microscopy and x-ray diffraction, whereas the surface effects and hyperfine interactions were analysed mainly by Mössbauer spectroscopy. The relative number of surface iron positions was found to be proportional to the amount of OH − and SO 2− 4 groups on the particle surface, which in turn is strictly dependent on the preparation conditions. Strong relaxation processes versus temperature were evidenced in the analysed systems. New criteria for the evaluation of the blocking temperature via Mössbauer measurements are proposed. The results are in good agreement with blocking temperatures obtained by magnetic measurements. Moreover, it was shown that the interparticle magnetic interactions decrease with the number of iron surface states.
The phase formation during thermal annealing of Fe/Al multilayer
thin films prepared by electron-beam evaporation, with an overall
atomic concentration ratio of Fe:Al = 1:1, has been studied
by Rutherford backscattering spectrometry (RBS), x-ray diffraction
spectroscopy (XRD), and conversion-electron Mössbauer spectroscopy
(CEMS). At the annealing temperature of 473 K some degree of atomic
mixing between Fe and Al layers is revealed only by CEMS. At 573 K
a large degree of atomic mixing is indicated also by RBS, leading to
the nucleation and growth of the B2 FeAl intermetallic phase, as
detected by means of XRD and CEMS. At 673 K all Fe atoms have
reacted and the multilayer film is transformed into a defective B2
phase. Annealing at higher temperature increases the structural
order of the B2 phase. We suggest that the observed phase formation
occurs in three stages: (1) formation of a thin intermixed
layer between Fe and Al in the as-deposited sample; (2) Al migration
into the initial intermixed layer; (3) B2 phase growth at the
interface between the intermixed layer and the Fe layer.
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