Magnetism and magnetic fluctuations in La_{1 - x}Sr_xCuO₄ for x = 0 (2D antiferromagnet), 0.04 (3D spin glass) and x = 0.15 (superconductor)

We review recent studies of the static and dynamic magnetic fluctuations in La_{2 - x}Sr_xCuO₄. In La₂CuO₄(T_N = 325K) the instantaneous two-dimensional (2D) spin correlations have been studied over the temperature range 340K < T ≤ 820K. We find quantitative agreement for the correlation length over the complete temperature range with no adjustable parameters with predictions for the 2D quantum non-linear sigma model in the renormalized classical regime. We have carried out bulk magnetization measurements in La_1.96Sr_0.04CuO₄ which has neither antiferromagnetic long range order nor superconductivity. All of the features that characterize a canonical spin glass transition have been seen: irreversibility, remnant magnetization and scaling behavior. Finally, neutron inelastic scattering experiments have been performed on homogeneous single crystals of La_1.85Sr_0.15CuO₄ with T_c = 37.3K (at onset), higher than any previously studied single crystals. The temperature dependence of the low energy incommensurate peak intensity at (π(1 ± δ), π) and (π, π(1 ± δ)) exhibits a pronounced maximum near 3T_c. In contrast to the results reported on lower T_c crystals, the intensity for energies below 3.5 meV dramatically decreases as the temperature decreases below T_c, vanishing into the background below ~ 15K. The behavior is consistent with predictions based on a d_{x² - y²} superconducting order parameter.