The magnetoelastic paradox in GdAg2 and GdRu2Si2

For isotropic systems the only source of magnetostriction is believed to be the exchange striction (ES). In many cases large magnetostrictive effects have been observed in Gd^{3 +} based systems (S = frac(7, 2), L = 0), but in no case a significant symmetry breaking lattice distortion has been found. For ferromagnetic (fm) order this is in line with the ES model. However, for antiferromagnetic (afm) large symmetry breaking distortions of the lattice are expected from theory. This contrasts to all available experimental data on several Gd-based antiferromagnets. We demonstrate examples for this magnetoelastic paradox and discuss in detail experimental evidence for GdAg₂ and GdRu₂ Si₂.