Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic RETMg intermetallics with RE = La, Eu, Gd, Yb and T = Ag, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic (RE = La, Yb) or they order either ferroor antiferromagnetically with transition temperatures ranging from about 13 to 81K. All of them are metallic, but the resistivity varies over 3 orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well-defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusions that generally the magnetic transition can be described by an ordering of localized S = 7/2 moments arising from the half-filled 4f shells of Eu or Gd. However, for GdAgMg we find clear evidence for two phase transitions indicating that the magnetic ordering sets in partially below about 125K and is completed via an almost first-order transition at 39K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.