Experimental and theoretical studies based on mass spectrometry, collision-induced dissociation, and ab initio calculations are performed on the formation and stability of FeOn+ clusters, as well as on their structural, electronic, and magnetic properties. In the mass spectra, clusters with an even number of oxygen atoms show increased stability, most prominently for FeO10+. The extra stability of this cluster is confirmed by measurements of fragmentation cross sections through crossed molecular beam experiments. In addition, the calculations indicate a structural phase transition at this size, and most importantly, the FeOn+ clusters show unique magnetic features, exhibiting isoenergetic low-spin (LS) and high-spin (HS) ground states. In the LS state, the magnetic moments of the O atoms adopt an antiferromagnetic alignment with respect to the magnetic moment of Fe+, whereas in the HS state, the alignment is ferromagnetic. FeO10+ is the largest thermodynamicaly stable complex, with the highest magnetic moment among the FeOn+ clusters (13 mu(B) in HS).