Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline Tb 2 Ti 2 O 7 , a pyrochlore antiferromagnet in which the Tb 31 moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, Tb 2 Ti 2 O 7 remains paramagnetic down to ϳ0.07 K, rather than ordering into a conventional Néel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at ϳ50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.

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The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF =22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy. * Address after September 01, 1996: Dept. of Physics, University of Waterloo, Waterloo, Ontario, N2L-3G1, CANADA Antiferromagnets which are frustrated or diluted can exhibit novel electronic and magnetic behaviour. Recently, there has been considerable interest in the behaviour of systems where the natural antiferromagnetic coupling between ions is frustrated by the geometry of the lattice. In two dimensions, Heisenberg spins on triangular and corner sharing triangular (kagomé) lattices are simple examples of geometric frustration, while in three dimensions, the most well studied systems have a pyrochlore structure, in which the magnetic ions occupy a lattice of corner sharing tetrahedra. A system of Heisenberg spins interacting via nearest-neighbor antiferromagnetic couplings on the pyrochlore lattice displays a classical ground state with macroscopic degeneracy, since the lowest energy spin configuration requires only that Σ 4 i=1 S i = 0 for each tetrahedron. This feature led Villain to argue that these systems remain in a cooperative paramagnetic state with only short range spin-spin correlations for all T>0 [1] and this has been confirmed by Monte Carlo simulations [2]. Possibly, the most interesting feature of the ground state of pyrochlore [3] and kagomé [4,5] lattice antiferromagnets is the prediction of a dispersionless spin-wave branch ("zero modes"). These zero modes manifestly affect the thermodynamics of these classical systems as demonstrated by Monte Carlo simulations, where the low temperature specific heat, C v , falls below the classical value k B expected from equipartition of energy [2,5,6]. Also, again because of these zero modes, the kagomé and pyrochlore antiferromagnets display large spin fluctuations down to T= s.In this letter we report an investigation of the low temperature magnetic properties of pyrochlores Y 2 Mo 2 O 7 and Tb 2 Mo 2 O 7 using the technique of muon spin rotation/relaxation (µSR ), which is sensitive to spin fluctuation rates in the range 10 4 − 10 11 s −1 [13], below that detectable with neutron scattering. We find that, despite its nominally disorder free structure, the magnetic behaviour in Y 2 Mo 2 O 7 is close to that observed in conventional random spin glasses. Specifically, a large static internal magnetic field with a very broad distribution develops below T F , such that no coherent muon spin precession is observed. At the same time, the muon spin relaxation rate 1/T 1 decreases according to a power l...

For some time now, there has been considerable experimental and theoretical effort to understand the role of the normal-state "pseudogap" phase in underdoped high-temperature cuprate superconductors. Recent debate has centered on the question of whether the pseudogap is independent of superconductivity. We provide evidence from zero-field muon spin relaxation measurements in YBa2Cu3O6+x for the presence of small spontaneous static magnetic fields of electronic origin intimately related to the pseudogap transition. Our most significant finding is that, for optimal doping, these weak static magnetic fields appear well below the superconducting transition temperature. The two compositions measured suggest the existence of a quantum critical point somewhat above optimal doping.

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.