Determination of the magnetic structure of Yb3 Pt4 :k=0 local-moment antiferromagnet

We have used neutron-diffraction measurements to study the zero-field magnetic structure of the intermetallic compound Yb3 Pt4, which was earlier found to order antiferromagnetically at the Néel temperature TN =2.4K, and displays a field-driven quantum-critical point at 1.6 T. In Yb3 Pt4, the Yb moments sit on a single low-symmetry site in the rhombohedral lattice with space group R 3̄. 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 TN, we found new diffracted intensity due to a 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 Yb3 Pt4 is 0.81 μB /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.