A series of the RE (RE = Gd, Dy, Ho, Tm and Yb) as well as Y and In substituted magnetites (Fe₃O₄): Fe_2.85RE_0.15O₄, Fe_2.85Y_0.15O₄ and Fe_2.55In_0.45O₄ were obtained by high energy ball milling. A mixture of crystalline powders of A₂O₃ (A = Y, In, Gd, Dy, Ho, Tm and Yb) and Fe₂O₃ were used as starting materials to produce Fe_2.85RE_0.15O₄ Fe_2.85Y_0.15O₄ and Fe_2.55In_0.45O₄ spinels. Mechanochemical treatment was performed in a planetary ball mill (Fritsch Pulferisette 5) for 20 hours.

To analyze magnetic behavior of the spinels studied, it is important to define, as precise as possible, the microstructure of the investigated samples. Therefore, we used transmission electron microscopy (TEM) image to investigate particle size and morphology. The crystallite size and strain and their anisotropy in samples were determined by refinements of the TCH-pV parameter and cubic harmonic function methods (incorporated in the Fullprof computer program). The microstructure of Fe₃O₄, Fe_2.85Y_0.15O₄ and Fe_2.55In_0.45O₄ was obtained by the Rietveld crystal structure refinement. The RMSS increase in the following order: Fe₃O₄ < Fe_2.85Y_0.15O₄ < Fe_2.55In_0.45O₄. This fact could be explained by the influence of Y³⁺ (»5 %) and In³⁺ (»15 %) on the crystal lattice strain. Crystallite size dimensions increase in following order Fe_2.85Y_0.15O₄ < Fe₃O₄ < Fe_2.55In_0.45O₄ what can be explained thought the cation radii influence on the crystallite size. The X-ray line broadening anisotropy due to crystallite size effect is significant for Fe_2.85Y_0.15O₄ (65±15 Å) and Fe_2.55In_0.45O₄ (176±39 Å). The X-ray line broadening anisotropy due to strain effect is small in all investigated samples. TEM analysis revealed that the particles are isotropic, with a relatively uniform size distribution and mean particle size of ~ 20 nm. In addition, the particle size is bigger than the crystallite size obtained by Rietveld method. It can conclude that the particles are composed of one or two crystallite.

Changes in coercivity H_C, and saturation magnetization M_S of magnetite with substitution, was studied by M(H) measurements at 2 K by a SQUID magnetometer. The Y substituted Fe₃O₄ has the largest H_C and the lowest M_S. The changes in magnetic parameter values are due to difference in magnetic anisotropy, microstructure and structure parameters. Observed high field irreversibility (at 20 kOe) in ZFC/FC magnetization vs temperature is а consequence of high anisotropy.

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