Nanocrystalline alloys which are obtained after subsequent annealing of the amorphous precursor consists of nanocrystalline grains embedded within a residual amorphous matrix. Thus, they exhibit a two-phase magnetic behaviour which is strongly dependent on the volumetric fraction of the crystalline phase at elevated temperatures. We emphasise the high efficiency of ~(57)Fe Moessbauer spectrometry which is able to elucidate the different kinds of Fe atoms, particularly the crystalline phase, the amorphous residual matrix, and the interface zone between crystalline grains and the amorphous phase and which reveals these different magnetic high temperature behaviours in iron-based nanocrystalline alloys. The fitting procedures involving either distributions of hyperfine magnetic fields or both distributions of hyperfine magnetic fields and of quadrupolar splitting are first described and then discussed when analysing temperature dependencies of Mossbauer spectra, particularly in the vicinity as well as above the Curie point of the residual amorphous phase. Superparamagnetic and penetrating magnetisation effects are discussed on the basis of different contributions of the magnetic energy. Part II., the following paper concentrates on the distributions of hyperfine fields.
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