Hyperfine Induced Magnetic Ordering in a S=1/2 Quantum Magnets on Kagomé Lattice

Susceptibility and specific heat measurements have been carried out down to 23 mK on polycrystalline [Cu 3 (titmb) 2 (OCOCH 3 ) 6 ]⋅H 2 O {titmb = 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6 trimethylbenzene} with S =1/2 antiferromagnet on Kagomé lattice. The results show an unexpected ferromagnetic long range ordering at 56 mK. The entropy change from 10 K to 23 mK is much larger than the entropy corresponding to electron spin S =1/2 of Cu 2+ . The large excess entropy reduction strongly suggests a novel magnetic ordering of the coupled Cu electron and nuclear spin ( I =3/2) system. We propose that a frustrated ground state changes to the complex ferromagnetic ground state with help of the enhanced indirect nuclear spin interaction through the hyperfine interaction.