Uncommon magnetic ordering in the quantum magnet Yb${}_3$Ga${}_5$O${}_{12}$

The antiferromagnetic structure of Yb${}_3$Ga${}_5$O${}_{12}$ is identified by neutron diffraction experiments below the previously-known transition at $T_{\lambda }=54$ mK. The magnetic propagation vector is found to be $\mathbf{k}=(1\mathrm{/}2,1\mathrm{/}2,0)$, 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.