Sr impurity effects on the magnetic correlations of La2-xSrxCuO4

We examine the low-temperature magnetic properties of moderately doped La2-xSrxCuO4, paying particular attention to the spin-glass phase and the commensurate-incommensurate transition as they are affected by Sr impurity disorder. A model of the carriers, believed to be appropriate at low temperatures, is employed in the CuO2 planes that accounts for both the strong coupling of the hole's motion to the antiferromagnetically correlated spins and the pinning potential associated with the Sr impurities. This model has been shown to explain quantitatively several magnetic and transport features of the antiferromagnetic region of the phase diagram. Preliminary indications that this model also can explain the x greater than or equal to 0.02 region of the phase diagram follow from its success in accounting far the doping and temperature variation of the spin correlation length. Here we further scrutinize this model by attempting to explain various features of the spin texture of the spin-glass phase. New measurements with traveling-solvent float zone grown crystals of the low-temperature susceptibility show an increase of an anomalously small Curie constant with doping. This behavior is modeled in terms of our numerical simulation results that find small clusters of antiferromagnetically aligned regions separated by disordered domain walls produced by the impurities - the domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as 1/[2 xi(x, T=0)(2)], xi being the spin correlation length, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states produced by our simulations are commensurate in the spin-glass phase, consistent with experiment, and that this behavior will persist at higher temperatures where the holes should move along the domain walls effectively being expelled from the antiferromagnetically correlated domains. However, our results show that incommensurate correlations develop continuously around 5% doping with an incommensurate wave vector along [(+/- 1,0),(0,+/- 1)], consistent with recent measurements by Yamada et al. At this doping lever the domain walls and not the clusters start to become the dominant feature of the spin texture, and thus the Curie behavior should disappear in the incommensurate phase, again consistent with experiment. Thus, we find that this model is capable of describing the low-temperature disorder-induced magnetic spin morphologies of the La2-xSrxCuO4 system in the low and intermediate doping regions of the phase diagram. Coupling this understanding of the magnetic correlations with the observed transport features, it is clear that impurity effects strongly influence the physics of LaSrCuO.