| Budget Amount *help |
¥3,030,000 (Direct Cost: ¥2,700,000、Indirect Cost: ¥330,000)
Fiscal Year 2007: ¥1,430,000 (Direct Cost: ¥1,100,000、Indirect Cost: ¥330,000)
Fiscal Year 2006: ¥1,600,000 (Direct Cost: ¥1,600,000)
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| Research Abstract |
The magnetic behavior in the low temperature region and the ground state of the quantum spin system with magnetically frustrated remain open problem theoretically and experimentally. Pyrochlore compound Hg_2Cu_2FeS has Cu^2+ ions with S=1/2, which consists of a 3-dimensional linkage of corner-sharing Gu_4 tetrahedra, and the Weiss temperature θ_w estimated from the temperature (T) dependence of the magnetic susceptibility is〜150 K, indicating the interaction between the Cu^2+ pins is antiferromagnetic. From these facts, this system is regarded as an ideal example of the geometrically frustrated quantum spins systems. The measurements of the magnetic susceptibility and specific heat as well as neutron diffraction studies have been carried out on the polycrystalline samples of Hg_2Cu_2F_6S. No magnetic transition has been observed in the temperature range clown to 2 K, indicating that the magnetic frustration is not released down to the lowest temperature. To extract microscopic informat
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ion of the magnetic behavior ; we have also carried out ^19F-NMR, Above〜100K the Knight shift roughly obeys the following Curie Weiss law. With decreasing T from〜100K, the Knight shift has a broad maximum at around 80K, and with further decreasing T, the Knight shift decreases significantly. We cannot distinguish if the Knight shift (spin component of the magnetic susceptibility) approaches zero or not, as Tapproaches zero, because of the increase in the width of the NMR spectra. The magnetic correlation length ξ is estimated by comparing the observed spin-lattice relaxation rate divided T,1/ (T_1T) with the theoretical result obtained by Ganals, et. al. for a 1/2 spin system on the pyrochlore lattice. The values of 1/ (T_1T) and increase gradually with decreasing T, and as the temperature is lowered, ξ increases to a value approximately equal to the Cu-Cu atomic distance. It is interesting that these behaviors seem to correspond to the characteristics of spin system with the geometrical frustration. Because the broadening of the NMR spectra induced by the effects of impurities and/or defects is observed in the low temperature region, it is required to be synthesized the higher purity sample. However, it is not easy to see the intrinsic behavior without the disturbance of the extrinsic effects such as lattice defects and/or impurities, because just a small perturbation brings about rather serious efforts on systems with large degeneracy, which steins from their geometrical characteristics. Less
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