Noncollinear magnetic ordering in small chromium clusters

We investigate noncollinear effects in antiferromagnetically coupled cluste rs using the general, rotationally invariant form of local spin-density the ory. The coupling to the electronic degrees of freedom is treated with rela tivistic nonlocal pseudopotentials and the ionic structure is optimized by Monte Carlo techniques. We find that small chromium clusters (N less than o r equal to 13) strongly favor noncollinear configurations of their local ma gnetic moments due to frustration. This effect is associated with a signifi cantly lower total magnetization of the noncollinear ground states, amelior ating the disagreement between Stem-Gerlach measurements and previous colli near calculations for Cr-12 and Cr-13. Our results further suggest that the trend to noncollinear configurations might be a feature common to most ant iferromagnetic clusters. [S0163-1829(99)10929-9].