Crystal and magnetic structures of $R_2$Ni${}_2$In compounds ($R$ = Tb and Ho)

Abstract

Crystal and magnetic structures of $R_2$Ni${}_2$In ($R$ = Tb and Ho) have been studied by powder neutron diffraction at low temperatures. The compounds crystallize in an orthorhombic crystal structure of the Mn${}_2$AlB${}_2$-type. At low temperatures, the magnetic moments localized solely on the rare earth atoms form antiferromagnetic structures. The Tb magnetic moments, equal to 8.65(6) ${\mu }_B$ and parallel to the $c$-axis, form a collinear magnetic structure described by the propagation vector $\mathbf{k}=[\frac{1}{2},\frac{1}{2},\frac{1}{2}]$. This magnetic structure is stable up to the Néel temperature equal to 40 K. For Ho${}_2$Ni${}_2$In a complex, temperature-dependent magnetic structure is detected. In the temperature range 3.5-8.6 K, an incommensurate magnetic structure, described by the propagation vector ${\mathbf{k}}_1=[0.76,0,0.52]$ is observed, while in the temperature interval 2.2-3.1 K the magnetic order is described by two propagation vectors, namely, ${\mathbf{k}}_2=[\frac{5}{6},0.16,\frac{1}{2}]$ and its third harmonics $3{\mathbf{k}}_2=[\frac{5}{2},0.48,\frac{3}{2}]$. Below 2 K, a coexistence of all magnetic structures detected at higher temperatures is observed. For all magnetic phases, the Ho magnetic moments are parallel to the $c$-axis. The low temperature heat capacity data confirm a first order transition near 3 K.