Magnetism and Spin Fluctuations of Frustrated YMn_2.

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Laves phase intermetallic compounds YMn_2 is an antiferromagnet with a complicated spin structure with a geometrical frustration of magnetic interaction. Substitution of Sc for Y collapses the antiferromagnetic ordering and gives rise to the paramagnetic ground state. The paramagnetic Y (Sc) Mn_2 exhibits giant spin fluctuations and strong enhancement of the low temperature specific heat coefficient. In this research, we study the ground state properties of this system and effects of impurity doping in order to know the role of spin frustrations. We also study the magnetic and other properties of 6-Mn-Al alloys in the same interests. We have got following results in this study.(1) It has been revealed that Y (sc) (Mn_<1-x>Al_x) _2 system becomes spin-glass by substitution of Al for Mn and the specific heat coefficient rapidly decreases with increasing Al concentration. The temperature dependence of electric resistivity of Al substituted alloys exhibits the Kondo type resistance minimum, indicating localization Mn moments. We interpreted these results as follow : A spin liquid state is formed as a result of frustration. Substitution of Al for Mn raise the degeneracy due to frustration, leading spin glass state.(2) Similar results have been obtained as above.(3) We have successfully set up the coherent type NMR spectrometer, which will contribute to obtain microscopic information to prove the present model.