Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)

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.