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The Y 2 Co 7-x Ni x B 3 compounds crystallize in a hexagonal structure of the Ce 2 Co 7 B 3 -type for x ≤ 4. Magnetic measurements were performed in the temperature range 4 -800 K and in fields up to 8 T. The compounds with x ≤ 3 are ferromagnetically ordered. Y 2 Co 3 Ni 4 B 3 is found to be paramagnetic in the studied temperature range. The experimental data were compared with the results obtained from band structure calculations using the TB-LMTO method in the ASA approximation. The magnetic behavior of cobalt in these compounds is analyzed within the theory of spin fluctuations.

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The GdxCe1−xCo4Si compounds with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 crystallize in the hexagonal structure of the CaCu5 type, with the space group P6/mmm. CeCo4Si exhibits anomalously low values of the elementary cell volume as well as of the Curie temperature and the mean cobalt moment in the RCo4Si series (R = rare earth or yttrium). All compounds exhibit a ferrimagnetic-type ordering. Band structure calculations using the ab initio tight-binding linear muffin tin orbital method in the atomic sphere approximation were performed for RCo4Si, R =  Ce, Gd. The computations provide evidence that the induced Ce moments are oriented antiparallel to the Co ones. The effect of Si and B atoms on the R–Co exchange interactions and on the magnetic behaviour of Co in RCo4B/Si compounds is discussed comparatively.