  The compounds, Gd6Co1.67Si3 and Tb6Co1.67Si3, recently reported to form in a
Ce6Ni2Si3-derived hexagonal structure (space group: P6_3 / m) and to order
magnetically below 295 and 190 K respectively, have been investigated by
detailed magnetization (M) studies in the temperature interval 1.8-330 K as a
function of magnetic field (H). The points of emphasis are: We observe multiple
steps in the M(H) curve for the Tb compound at 1.8 K while increasing H, but
these steps do not appear in the reverse cycle of H. At higher temperatures,
such steps are absent. However, this 'staircase' behavior of M(H) is not
observed for the Gd compound at any temperature and the isothermal
magnetization is not hysteretic unlike in Tb compound. From the M(H) data
measured at close intervals of temperature, we have derived isothermal entropy
change (Delta S) and it is found that Delta S follows a theoretically predicted
H^2/3-dependence.
