Less than 50% sublattice polarization in an insulating S=3/2 kagome antiferromagnet at low T

We have found weak long range antiferromagnetic order in the quasi-two-dimensional insulating oxide $ KCr_3(OD)_6(SO_4)_2$ which contains Cr$^{3+}$ S=3/2 ions on a kagom{e} lattice. In a sample with $approx$ 76% occupancy of the chromium sites the ordered moment is 1.1(3)$mu_B$ per chromium ion which is only one third of the N{e}el value $gmu_BS=3mu_B$. The magnetic unit cell equals the chemical unit cell, a situation which is favored by inter-plane interactions. Gapless quantum spin-fluctuations ($Delta/k_B <0.25$K) with a bandwidth of 60K >> $T_N$ = 1.6K are the dominant contribution to the spin correlation function, $S(Q,omega)$ in the ordered phase.