Nanoparticles of Li _0.5 ZnFe _1.5 O ₄ (LiZn ₂ Fe ₃ O ₈ ) with the spinel structure were prepared by a sol–gel auto-combustion method at two different annealing temperatures.

Magnetic properties correlated to structural transformations, from metastable hexagonal ${\mathrm{CaCu}}_5$-type phases to equilibrium tetragonal 1:12 phases, are studied systematically for nanocrystalline $\mathrm{Sm}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_{11}\mathrm{Ti}$ alloys $(x<~2).$ The Rietveld analysis gives a stoichiometry ratio equal to 1:10, for the out of equilibrium hexagonal precursor, which is described with three crystallographic transition-metal T sites: $3g$ is fully occupied, and $6l$ occupation is limited to hexagons surrounding the T dumbbell pairs $2e.$ The substitution of Co, in both structures, contributes to reduce the negative exchange interactions and results in Curie temperature enhancement. The hyperfine parameter assignment, on the basis of the Wigner-Seitz volume calculation, in the tetragonal structure points towards the Co $8f$ preferential occupation. The mean hyperfine field increase, with Co content, in the two phases, might be correlated to the compensation of the core-electron polarization field by the $4s$ polarization field produced by the $3d$ moment. The coercitive field of 0.6 T measured at room temperature suggests that nanocrystalline ${\mathrm{SmFe}}_9{\mathrm{Co}}_2\mathrm{Ti},$ with a Curie temperature equal to 790 K, is a semihard material and is a potential candidate for magnetic recording.