Unquenched Geometric Frustration Effect on Spiral Spin Correlation in Magnetically Ordered Phase

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We studied magnetic phase diagram and $x$-dependence of spiral spin correlation in magnetically ordered phase of (Co1-xZnx)Cr2O4 by magnetization and neutron scattering experiments. The spiral correlation survives in a wide $x$ range from 0 to 0.9. As $x$ increases, the correlation length decreases from $\xi\sim 23$ Å at $x=0$, and saturates at $\xi\sim 2.5$ Å at around $x=0.75$, which is close to the nearest neighboring Cr3+-Cr3+ distance. The pitch is almost constant from $x=0$ to 0.75, then abruptly shortens above around $x=0.75$. The correlation is isotropic at $x=0.25$ but anisotropic at $x=0.75$. 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 $x=0.75$, possibly as the precursor of dynamic spin hexamer caused by high geometric frustration.
2009-08-15