Less than 50% sublattice polarization in an insulating S=32 kagomé antiferromagnet at T≈0

We have found weak long-range antiferromagnetic order in the quasi-two-dimensional insulating oxide KCr3(OD)6(SO4)2 which contains Cr3+ S=3/2 ions on a kagomé lattice. In a sample with ≈76% occupancy of the chromium sites the ordered moment is 1.1(3)μB per chromium ion which is only one-third of the Néel value gμBS=3μB. The magnetic unit cell equals the chemical unit cell, a situation which is favored by interplane interactions. Gapless quantum spin fluctuations (Δ/kB<0.25 K) with a bandwidth of 60 K ≫TN=1.6 K are the dominant contribution to the spin correlation function S(Q,ω) in the ordered phase.