Unquenched geometric frustration effect on spiral spin correlation in magnetically ordered phase

We studied magnetic phase diagram and x-dependence of spiral spin correlation in magnetically ordered phase of (Co _1-xZn _x)Cr ₂O ₄ by magnetization and neutron scattering experiments. The spiral correlation survives in a wide × range from 0 to 0.9. As × increases, the correlation length decreases from ξ ∼ 23 Å at × - 0, and saturates at ξ ∼ 2:5 Å at around × - 0:75, which is close to the nearest neighboring Cr ₃₊-Cr ₃₊ distance. The pitch is almost constant from × - 0 to 0.75, then abruptly shortens above around × - 0:75. The correlation is isotropic at × = 0:25 but anisotropic at × = 0:75. We discuss that the spiral correlation is of short-range owing to unquenched geometric frustration, and begins to collapse with the minimization of correlation length above around × - 0:75, possibly as the precursor of dynamic spin hexamer caused by high geometric frustration.