Relationship between polymorphic structures and magnetic properties of La$_{2-x}A'_{x}$Ni$_{7}$ compounds ($A$' = Sm, Gd)

Abstract

In this study, the crystal structure and magnetic properties of La$_{2-x} A'_{x}$Ni$_{7}$ compounds with magnetic rare earth elements ($A$' = Sm, Gd) have been investigated combining X-ray powder diffraction and magnetic measurements. These intergrowth compounds crystallize in a mixture of 2$H$ hexagonal (Ce$_{2}$Ni$_{7}$-type) and 3$R$ rhombohedral (Gd$_{2}$Co$_{7}$-type) polymorphic structures which are related to the stacking of [$AB_{5}$] and [$A_{2}B_{4}$] subunits along the $c$-axis.The average cell volume decreases linearly versus $A$' content, whereas the $c/a$ ratio reaches a minimum at $x$ = 1, due to geometric constraints upon $A$' for La substitution between the two different subunits. The magnetic properties strongly depend on the structure type and the $A$' content. Hexagonal La$_{2}$Ni$_{7}$ is a weak antiferromagnet (wAFM) at low field and temperature and undergoes metamagnetic transitions towards weak ferromagnetic state (wFM) under applied field. Under an applied field of 0.1 T, La$_{2-x}A'_{x}$Ni$_{7}$ intermetallic compounds display two different transition temperatures $T_{1}$ and $T_{2}$ that both increase with $x$. $T_{1}$ is associated with a wFM-wAFM transition in the 2$H$ phase for $A$'= Sm, whereas $T_{2}$ is related to the Curie temperature of both 2$H$ and 3$R$ phases. A metamagnetic behaviour is observed between $T_{1}$ and $T_{2}$ with transition field $\mu_{0}H_{Trans}$ between 2 and 3.5 T for compounds with $A$' = Sm. The La$_{2-x}Sm_{x}$Ni$_{7}$ compounds ($x$ > 0} behave as hard magnets with a large coercive field $\mu_{0}H_{C}$ at low temperature ($\mu_{0}H_{C}$ > 9 T at 5 K for $x$ = 2), whereas the La$_{2-x} Gd_{x}$Ni$_{7}$ compounds ($x$ > 0) are soft ferrimagnets with a linear increase of the saturation magnetization versus Gd content.