  The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by
resonant x-ray diffraction at the Gd LII and LIII edges, followed by a
renormalization group analysis for this and other related Gd-based compounds,
namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the
isostructural Ce-based heavy-fermion superconductors. The ground state of
GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation
vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the
a-direction and antiparallel alignment along b and c. A comparison between this
magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n
family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is
determined by a competition between first-(J1) and second-neighbor(J2)
antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an
antiparallel alignment along the three directions (the so-called G-AFM
structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM).
In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is
in a frontier between these two ground states, which may explain its
non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close
to the paramagnetic transition at TN = 39 K was also studied in detail. A
typical second-order transition with the ordered magnetization critical
parameter beta = 0.35 was experimentally found, and theoretically investigated
by means of a renormalization group analysis.
