||The discovery in 2001 of superconductivity in some heavy fermion compounds of the RMIn5 (R = 4f or
5f elements; M = Co, Rh, Ir) family, has triggered an enormous amount of research into understanding the
physical origin of superconductivity and its relation with magnetism. Although many properties have been
clarified, there are still crucial questions that remain unanswered. One of these questions is the particular role
of the transition metal in determining the value of critical superconducting temperature (TC). In this work, we
analyze an interesting regularity that is experimentally observed in this family of compounds, where the lowest
NÂ´eel temperatures are obtained in the Co-based materials. We focus our analysis on the GdMIn5 compounds
and perform density-functional-theory-based total-energy calculations to obtain the parameters for the exchange
coupling interactions between the magnetic moments located at the Gd3+ ions. Our calculations indicate that
the ground state of the three compounds is a C-type antiferromagnet determined by the competition between
the first- and second-neighbor exchange couplings inside GdIn3 planes and stabilized by the couplings across
MIn2 planes. We then solve a model with these magnetic interactions using a mean-field approximation and
quantum Monte Carlo simulations. The results obtained for the calculated NÂ´eel and Curie-Weiss temperatures,
the specific heat, and the magnetic susceptibility are in very good agreement with the existent experimental data.
Remarkably, we show that the first-neighbor interplane exchange coupling in the Co-based material is much
smaller than in the Rh and Ir analogs which leads to a more two-dimensional magnetic behavior in the former.
This result explains the observed lower NÂ´eel temperature in Co-115 systems and may shed light on the fact that
the Co-based 115 superconductors present the highest TC.