Uncommon magnetic ordering in the quantum magnet Yb₃Ga₅O₁₂

The antiferromagnetic structure of Yb₃Ga₅O₁₂ is identified by neutron diffraction experiments below the previously-known transition at Tλ=54 mK. The magnetic propagation vector is found to be k=(1/2, 1/2, 0), an unusual wave-vector in the garnet structure. The associated complex magnetic structure highlights the role of exchange interactions in a nearly isotropic system dominated by dipolar interactions and finds echos with exotic structures theoretically proposed. Reduced values of the ordered moments may indicate significant quantum fluctuations in this effective spin-1/2 geometrically frustrated magnet.