  We have used neutron diffraction measurements to study the zero-field
magnetic structure of the intermetallic compound Yb3Pt4, which was earlier
found to order antiferromagnetically at the Neel temperature TN=2.4 K, and
displays a field-driven quantum critical point at 1.6 T. In Yb3Pt4, the Yb
moments sit on a single low-symmetry site in the rhombohedral lattice with
space group R-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 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 towards 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 Yb3Pt4 is 0.81 uB/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.
