Abstract: A comprehensive experimental and theoretical study of the low temperature properties of GdCoIn5 was performed. Specific heat, thermal expansion, magnetization and electrical resistivity were measured in good quality single crystals down to 4He temperatures. All the experiments show a second-order-like phase transition at 30 K probably associated with the onset of antiferromagnetic order. The magnetic susceptibility shows a pronounced anisotropy below T N with an easy magnetic axis perpendicular to the crystallographic ĉ-axis. Total energy GGA+U calculations indicate a ground state with magnetic moments localized at the Gd ions and allowed a determination of the Gd-Gd magnetic interactions. Band structure calculations of the electron and phonon contributions to the specific heat together with Quantum Monte Carlo calculations of the magnetic contributions show a very good agreement with the experimental data. Comparison between experiment and calculations suggests a significant anharmonic contribution to the specific heat at high temperature (View the MathML sourceT≳100K).

Abstract: 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
N´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 N´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 N´eel temperature in Co-115 systems and may shed light on the fact that
the Co-based 115 superconductors present the highest TC.