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In the paper, the particle morphology is considered and the slices of phase diagrams of nanosystems agreeable to the synthesis conditions are constructed according to the data obtained earlier by authors, as well as new results of the study of nanostructured Fe-Co, Fe-Ni, Co-Ni, Fe-Co-Ni, Fe-Pt, Cu-Ni and Ni-Cd powders. It is found that all considered polymetallic systems have common nature of the particle size spatial organization, i.e., 7-20 nm nanocrystals (for different systems) form highly compact aggregates (40-100 nm) which put together into loose porous agglomerates (up to 200-250 nm) and then into unconsolidated micron size formation of cloud type. It is classified uncovered features of nanostructured polymetallic phase diagrams in comparison with phase diagrams of bulk systems. Magnetic properties of nanosystems are studied.

Nanostructured Fe–Co–Ni powders, with crystallite sizes of 10–35 nm for different compositions, are obtained in the whole range of compositions (step change in the components content is of 10 wt.% and, in the case of near the phase boundaries, 3–5 wt.%) by the reduction of freshly co‐precipitated mixed hydroxides with hydrazine hydrate in a strongly alkaline medium. The isothermal section of the phase diagram corresponding to the synthesis temperature (363–368 K) is constructed using reflection modeling based on XRD data of the samples series with different compositions. Its features are founded, characterized, and discussed: the phase diagram of the system in the nanostructured and nonequilibrium state is compared with the phase diagram of the system in the bulk state. In the region of the FCC‐solid solution, the “lattice parameter–composition” dependence was described by the flat surface equation (three‐dimensional analog of Vegard's law). Some magnetic characteristics are measured for the Fe–Co–Ni systems containing 10 wt.% Ni and variable Co/Fe ratio. Some features of the superparamagnetic component are observed above the blocking temperature; they are related to the crystallite size and the phase composition of the samples.

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