The discovery in 2001 of superconductivity in some heavy fermion compounds of the RMIn$_5$ (R=4f or 5f elements, M=Co, Rh, Ir) family, has triggered enormous amount of research pointing to understand the physical origin of superconductivity and its r
elation with magnetism. Although many properties have been clarified, there are still crutial 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 analyse an interesting regularity that is experimentally observed in this family of compounds, where the lowest Neel temperatures are obtained in the Co-based materials. We focus our analysis on the GdMIn$_5$ 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 Gd$^{3+}$ 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 GdIn$_3$ planes and stabilized by the couplings across MIn$_2$ 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 Neel 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 analogues due to a more two dimensional behaviour in the former. This result explains the observed lower Neel temperature in Co-115 systems and may shed light on the fact that the Co-based 115 superconductors present the highest Tc.
A comprehensive experimental and theoretical study of the low temperature properties of GdCoIn$_5$ was performed. Specific heat, thermal expansion, magnetization and electrical resistivity were measured in good quality single crystals down to $^4$He
temperatures. All the experiments show a second-order-like phase transition at 30 K probably associated with the onset of antiferromagnetic order. 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 and Quantum Monte Carlo calculations of the magnetic contributions to the thermodynamic quantities reproduce quite well the experimental data.
Structural, magnetic and thermal measurements performed on CeCo{1-x}Fe{x}Si alloys are reported. Three regions can be recognized: i) Co-rich (x < 0.20) with a decreasing long range antiferromagnetic order which vanishes at finite temperature, ii) an
intermediate region (0.20 < x < 0.30) showing a broad magnetic anomaly (C_A) in specific heat and iii) the non-magnetic region progressively changing from a non-Fermi-liquid type behavior towards a Fermi liquid one as Fe concentration increases. The C_A anomaly emerges as an incipient contribution above T_N already at x = 0.10, which indicates that this contribution is related to short range correlations likely of quasi-two dimensional type. Both, T_N transition and C_A anomaly are practically not affected by applied magnetic field up to B ~ 10 Tesla.
