Toth, S.; Lake, B.; Kimber, S.A.J.; Pieper, O.; Reehuis, M.; Islam, A.T.M.N.; Zaharko, O.; Ritter, C.; Hill, A.H.; Ryll, H.; Kiefer, K.; Argyriou, D.N.; Williams, A.J.: 120° helical magnetic order in the distorted triangular antiferromagnet α-CaCr2O4. Physical Review B 84 (2011), p. 054452/1-12
α-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+ ions, which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below TN=42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wave vector k=(0,∼ 1/3,0) and angles close to 120∘ between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the ac plane perpendicular to k. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest-neighbor interactions are equal, in spite of the fact that α-CaCr2O4 is distorted with two inequivalent Cr3+ ions and four different nearest-neighbor interactions. By simulating the magnetic order as a function of these four interactions, it is found that the special pattern of interactions in α-CaCr2O4 stabilizes 120∘ helical order for a large range of exchange interactions.