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Zeroand longitudinal-field muon-spin relaxation (it+ SR) measurements have been performed on La2-Sr"Cu04 alloys in both single-crystal and sintered powder forms above and below their magnetic transition temperatures, Tf. The p+ precession frequency v depends only weakly on x and Tf, an observation which together with resistivity data implies classical freezing of magnetic moments in the regime where the carriers are localized. For x 0.05, critical dynamics are observed near Tf. The p SR technique is shown to be very sensitive to ferromagnetically aligned pairs of Cu + moments; the population of such pairs increases greatly with x. Over the past year or so, various cuprates have been found to be superconducting with transition temperatures (T, 's) as high as -90 K. ' Bulk investigations of related compounds, such as La2Cu04 s, have also been performed in the hope that they may provide a basis for understanding the superconducting materials. Neutron scattering experiments have confirmed the existence of antiferromagnetism and its sensitivity to oxygen stoichiometry in LazCu04 s. In the present paper, we describe a detailed study of magnetic order in the La2, Sr,Cu04 series, precursors to the superconducting systems that are obtained for x ) 0.06. We have chosen to use the muon-spin relaxation (p+ SR) technique because

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