120∘helical magnetic order in the distorted triangular antiferromagnetα-CaCr2O4

Abstract

$\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ is a distorted triangular antiferromagnet. The magnetic Cr${}^{3+}$ ions, which have spin-3$/$2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below ${T}_{N}=42.6$ K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wave vector $\mathbf{k}=(0,\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}1/3,0)$ and angles close to ${120}^{\ensuremath{\circ}}$ between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the $ac$ plane perpendicular to $\mathbf{k}$. 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 $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ is distorted with two inequivalent Cr${}^{3+}$ 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 $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ stabilizes ${120}^{\ensuremath{\circ}}$ helical order for a large range of exchange interactions.