Determination of the magnetic structure ofYb3Pt4:k=0local-moment antiferromagnet

Abstract

We have used neutron-diffraction measurements to study the zero-field magnetic structure of the intermetallic compound ${\text{Yb}}_{3}{\text{Pt}}_{4}$, which was earlier found to order antiferromagnetically at the N\'eel temperature ${T}_{\text{N}}=2.4\text{ }\text{K}$, and displays a field-driven quantum-critical point at 1.6 T. In ${\text{Yb}}_{3}{\text{Pt}}_{4}$, the Yb moments sit on a single low-symmetry site in the rhombohedral lattice with space group $R\overline{3}$. The Yb ions form octahedra with edges that are twisted with respect to the hexagonal unit cell, a twisting that results in every Yb ion having exactly one Yb nearest neighbor. Below ${T}_{\text{N}}$, we found new diffracted intensity due to a $\mathbit{k}=0$ magnetic structure. This magnetic structure was compared to all symmetry-allowed magnetic structures and was subsequently refined. The best-fitting magnetic-structure model is antiferromagnetic and involves pairs of Yb nearest neighbors on which the moments point almost exactly toward each other. This structure has moment components within the $ab$ plane as well as parallel to the $c$ axis although the easy magnetization direction lies in the $ab$ plane. Our magnetization results suggest that besides the crystal-electric-field anisotropy, anisotropic exchange favoring alignment along the $c$ axis is responsible for the overall direction of the ordered moments. The magnitude of the ordered Yb moments in ${\text{Yb}}_{3}{\text{Pt}}_{4}$ is $0.81{\ensuremath{\mu}}_{\text{B}}/\text{Yb}$ at 1.4 K. The analysis of the bulk properties, the size of the ordered moment, and the observation of well-defined crystal-field levels argue that the Yb moments are spatially localized in zero field.