Less than 50% sublattice polarization in an insulatingS=32kagoméantiferromagnet atT≈0

We have found weak long-range antiferromagnetic order in the quasi-two-dimensional insulating oxide ${\mathrm{KCr}}_3{\mathrm{}\left(\mathrm{OD}\right)\mathrm{}}_6{(\mathrm{S}\mathrm{O}}_4{)}_2$ which contains ${\mathrm{Cr}}^{3+}$ $S=3/2\mathrm{}$ ions on a kagomé lattice. In a sample with $\approx 76\%$ occupancy of the chromium sites the ordered moment is $1.1\left(3\right)\mathrm{}{\mu }_B$ per chromium ion which is only one-third of the Néel value $g{\mu }_{B}S=3\mathrm{}{\mu }_B.$ The magnetic unit cell equals the chemical unit cell, a situation which is favored by interplane interactions. Gapless quantum spin fluctuations $(\Delta {/k}_B<0.25\mathrm{}\mathrm{K})$ with a bandwidth of 60 K $\gg T_N=1.6\mathrm{}\mathrm{K}$ are the dominant contribution to the spin correlation function $\mathcal{S}(Q,\omega )$ in the ordered phase.