Abstract: We report detailed thermal expansion and magnetostriction experiments on GdCoIn5 and GdRh(In1−xCdx )5
(x = 0 and 0.025) single-crystal samples that show a sudden change in the dilation at a field B for temperatures
below the Néel transition temperature TN . We present a first-principles model including crystal-field effects,
dipolar and exchange interactions, and the dependence of the latter with lattice distortions in order to fully
account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows
that a transition between metastable states occurs at the field B. It also indicates that two degenerate phases
should coexist at temperatures below TN , which may explain the lack of observation, in high-resolution
x-ray experiments, of an orthorhombic distortion at the Néel transition, even though the experimentally
determined magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling from our model
is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the
same phenomenology.

Abstract: We present a theoretical analysis of the magnetic phase diagram of CeTi1-xScxGe and GdFe1-xCoxSi as a function
of the temperature and the Sc and Co concentration x, respectively. CeScGe and GdCoSi, as many other RTX
(R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure
where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi0.75Sc0.25Ge are ferromagnetic,
CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned
ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The
antiferromagnetic transition temperature TN decreases with decreasing concentration x in both compounds and
for low enough values of x the compounds show a ferromagnetic behavior. Based on these observations we
construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization.
We find a good qualitative agreement between the model and the experimental data. Our results show that the
main magnetic effect of the Sc→Ti and Co→Fe substitution in these compounds is consistent with a change in
the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar
phenomenology for other magnetic RTX compounds with the same type of crystal structure.

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.

Abstract: Ce intermetallic compounds have been intensively studied in the last few decades due to the large variety of behaviours they exhibit. In particular, two ways of demagnetizing Ce have been experimentally tried within the ferromagnetic systems Ce(Pd1-xMx), namely through electronic concentration variations (M=Rh) and volume reductions (M=Ni). On the other hand, when Pd is substituted by Ag, a transformation from a ferromagnetic to an antiferromagnetic ground state has been observed. In this work, we analyse the evolution of the magnetism of Ce in the above-mentioned family of compounds by calculating the spin contribution to magnetism that results from spin polarised LDA calculations. (C) 2001 Elsevier Science B.V. All rights reserved.