  We report measurements of the thermal expansion for two Eu$^{+2}$- and two
Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase
transitions. These materials show sharp positive (EuAgMg and GdAuMg) and
negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal
expansion coefficient $\alpha$ which become smeared and/or displaced in an
external magnetic field. Together with specific heat data we determine the
initial pressure dependences of the transition temperatures at ambient pressure
using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure
dependences indicating strong spin-phonon coupling, in particular for GdAgMg
and EuAuMg where a quantum phase transition might be reached at moderate
pressures of a few GPa.
