Using the fully unconstrained version of the density-functional method SIESTA with the generalized gradient approximation for exchange and correlation, we compute the structures and magnetic configurations of the lowest-energy isomers of the free-standing cluster ${\text{Fe}}_2{\text{Cr}}_4$. The two most stable isomers of distorted octahedral geometry exhibit magnetic frustration but collinear magnetic order, in contrast to the behavior of extended Cr/Fe frustrated systems. Confinement effects and structural relaxation are shown to be the origin of this difference, which illustrates the importance of finite-size effects on magnetism.

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