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We report elastic, quasielastic (fdE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin Suctuations in as-grown and doped La2Cu04. Four samples have been studied: (A) as-grown La2Cu04 with Tn 195 K, (8) oxygenated La2Cu04 with Tjv=100 K, (C) LatCup. psLippsOg, and (D) La~.p7Srp. p3Cupp5Lipps04. A11 crystals exhibit variable-rangehopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetie instantaneous correlations in the CuO2 sheets with the correlation length varying from -200 A in crystal A to -14 A in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and 8 the 20 correlation length becomes sullciently large with decreasing temperature that the interplanar coupling is able to derive a transition to 30 long-range order. The spin dynamics have been studied in detail in crystals A and 8 and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no signi6cant E~O component for temperatures~Tã nd instead the 20 response function is highly inelastic. The effective dispersion of the spin excitations is 0.4 eV k This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped LatCuO& in which the pairing is magnetic in origin. I. H4i. RODUi-l IONThe discovery of superconductivity in doped La2Cu04 by Bednorz and Muller' has stimulated an enormous number of experimental and theoretical studies of this and related materials. It is broadly viewed that in order to understand the novel high-temperature superconductivity, it will be necessary to elucidate fully the behavior across the phase diagram as a function of temperature, doping, etc.It is already known that the phase diagram is remarkably rich with competing or attendant phases and phase transitions including a tetragonal-orthorhombic structural transition, 2 5 three&imensional (3D) antiferromagnetic ordering, s 7 a crossover from a 3D Niel to a 2D quantum spin Quid states and possibly a low-temperature 3D spinglass state, " an insulator-metal transition, ' and, of course, the superconducting transition. 'In any model for the superconductivity, the spindegrees of freedom of the Cu2+ ions are essential and, indeed, in many models, the magnetism is the progenitor of the high-temperature superconductivity. ' t3 It is therefore essential that the microscopic static and dynamic spin fluctuations in both pure and doped La2Cu04 be elucidated in detail. Such experiments can only be done properly utilizing large, high quality homogeneous single crystals and thus these measurements place great demands on crystal-growth techniques. ' 's In this paper we report the results of a detailed neutron scattering study of the spin Buctuations in four crystals: (A) as-grown La2Cu04, (B) oxygenated La2Cu04, (C) La2CuppsLip I-04, and (D) Lap.97Srp.p3Cup. 95Llp.p504 The 6rst two crystals were grown at Nippon Telegraph and Tele hone...

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Measurements are reported of the magnetoresistance (MR) for fields up to 23 T in LazCu04 single crystals, which order antiferromagnetically at T&-240 K, and in which the conductivity at low temperature is characterized by hopping in localized states. Using the MR, the phase diagram of the spin-Aop transition, observed when the magnetic field is applied parallel to the zero-field staggered magnetization, is mapped out. Two transitions of the background Cu + spins are observed, which are governed by the symmetric and antisymmetric anisotropic components of the superexchange tensor. The antiferromagnetic propagation vector changes from r~~a at zero field to r~~c at the highest fields. This subtle change in the ordering of the Cu' spins is accompanied by a large enhancement of the interlayer hopping conductivity up to a factor 2. We show that the magnetoconductance is proportional to the three-dimensional staggered moment with r~~c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting La2, (Sr,Ba)"Cu04.