Novel Frustrated Behavior in Quantum Heisenberg Antiferromagnets on the Pyrochlore Lattice: NMR Studies of R-2(OH)(3)Cl (R = Cu and Ni)

A pyrochlore lattice is a three-dimensional corner-sharing network of tetrahedra. Spin systems on pyrochlore lattices exhibit several exotic magnetic properties, such as spin ice, spin liquid and order by disorder, due to macroscopic degeneracy caused by geometrical frustration, depending oil whether the interactions are ferromagnetic or antiferromagnetic, whether the spin system is classical or quantum, the symmetry of the spin space and the kinds of additional interactions. We survey the magnetic behavior of spin systems on pyrochlore lattices and present nuclear-magnetic-resonance (NMR) studies oil the quantum Heisenberg antiferromagnets consisting of magnetic 3d transition metal ions Cu-2(OH)(3)Cl and Ni-2(OH)(3)Cl with spin 1/2 and 1, respectively. Cu-2(OH)(3)Cl exhibits successive phase transitions at 18.1 and 6.4 K. The NMR spectra and spin-lattice relaxation rates indicate curious static and dynamic properties that appear to conflict. The coexistence of freezing and quantum fluctuation of spins and the time scales of the fluctuations due to geometrical frustration play an important role in understanding the phenomena in these materials as well as a possible novel order parameter.