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

Abstract

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE=\mathrm{La}$, Eu, Gd, Yb and $T=\mathrm{Ag}$, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic ($RE=\mathrm{La}$, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. 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 $4f^7$ shells of Eu${}^{2+}$ or Gd${}^{3+}$. However, for GdAgMg we find clear evidence for two phase transitions indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.

12 pages, 7 figures included