The Nd₃(Fe,Ti)₂₉-type intermetallic compounds (3:29) crystallise in the A2/m space group and consist of 2:17R-type and 1:12-type segments in a ratio 1:1. The study of the effect of Co substitution on the structural and magnetic properties of R₃(Fe_1-xCo_x)_29-yM_y (R = Y, Nd, Tb, Dy; M = Ti, V, Cr, Mn; 0≤x≤1; y=0.9-7.0) is presented. For the compounds with low Co content (0≤x≤0.4) the unit cell volume is decreasing as the Co content increases. ⁵⁷Fe Mössbauer spectroscopy has shown that the Co atoms avoid those Fe sites shared with the Ti atoms, i.e., the dumb-bell sites. The Curie temperature values, T_C, increase monotonically with x. For the compounds with high Co content (0.6≤x≤1) the situation is rather different. Starting with the 3:29 stoichiometry [Tb₃(Fe_1-xCo_x)_27.4V_1.6 and Dy₃(Fe_1-xCo_x)_27.8V_1.2; x= 0.6, 0.8, 1.0] a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. In Y₃(Fe_1-xCo_x)_29-yCr_y the monoclinic 3:29 structure is retained if the amount of the stabilizing element is increased. The lattice parameters are changing in a constant manner with the increase of Co showing that the replacement of Co for Fe is a continuous procedure. The change in the monoclinic angle shows an interesting difference compared to the compounds with x<0.3; it increases with Co indicating that high Co concentration (x>0.3) is pushing the structure to its limit. The T_C values of the compounds are reduced; this should be attributed to the larger Cr content. The same behavior was observed in the case of Nd based products with 0.6≤x≤1. The T_C values present a considerable difference (more than 100 K for x=0.6). Preliminary studies show that in the Nd₃(Fe_1-xCo_x)_29-yCr_y samples the main phase is a rhombohedral Th₂Zn₁₇-type structure. In all cases the considerable effect of Co substitution on the magnetocrystalline anisotropy will be discussed.

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