Noncollinear magnetic ordering in small chromium clusters

We investigate noncollinear effects in antiferromagnetically coupled clusters using the general, rotationally invariant form of local spin-density theory. The coupling to the electronic degrees of freedom is treated with relativistic nonlocal pseudopotentials and the ionic structure is optimized by Monte Carlo techniques. We find that small chromium clusters $(N<~13)$ strongly favor noncollinear configurations of their local magnetic moments due to frustration. This effect is associated with a significantly lower total magnetization of the noncollinear ground states, ameliorating the disagreement between Stern-Gerlach measurements and previous collinear calculations for ${\mathrm{Cr}}_{12}$ and ${\mathrm{Cr}}_{13}.$ Our results further suggest that the trend to noncollinear configurations might be a feature common to most antiferromagnetic clusters.