STAGGERED MAGNETIZATION IN LA(2-X)SR(X)CUO4 FROM LA-139 NQR AND MU-SR - EFFECTS OF SR DOPING IN THE RANGE 0-LESS-THAN-X-LESS-THAN-0.02

摘要

We have used La-139 nuclear quadrupole resonance (NQR) and positive muon spin rotation (mu SR) measurements to probe the weakly doped antiferromagnetic (AF) region (x<0.02) of the La2-xSrxCuO4 system below the three-dimensional (3D) AF ordering (Neel) temperature T-N(x) From these measurements, our previous La-139 NQR measurements [F. C. Chou et al.,Phys. Rev. Lett. 71, 2323, (1993)], and auxiliary La-139 nuclear magnetic resonance (NMR) measurements of single-crystal La2CuO4, we have determined the CU2+ Staggered magnetization M(x,T) below T-N(x) Above similar to 30 K, M(x,T) at each T is found to be progressively depressed with increasing x (decreasing T-N); the M(x,0) at T=0, extrapolated from T>30 K, is found from both the NQR and mu SR measurements to follow the same empirical relation M(x,0)/M(0,0)=(1-x/x(c))(n) with x(c)=0.023 and n=0.236. To model the extrapolated values, we assume that the doped holes are mobile and are situated in 1D ''rivers'' of charge which form walls in the CuO2 planes and which separate and uncouple undoped AF domains; an infinite number of these layers are stacked along the c axis. Using linear spin-wave theory and the interplanar and intraplanar CU2+ AF exchange interactions found from neutron-scattering measurements on La2CuO4, we calculated M(x,0) for this hoped hole and CU2+ spin configuration and find good agreement with the above extrapolated T=0 values. Thus, in the AF region x<0.02 of the phase diagram of La2-xSrxCuO4, these results support the previous hypothesis that microsegregation of the (mobile) doped holes into domain walls occurs above similar to 30 K, consistent with the phase separation phenomenology of Emery and Kivelson. Below similar to 30 K, an anomalous increase in M(x,T) is observed in La2-xSrxCuO4, particularly for the larger x values approaching x=0.02, such that M(x,T) is nearly independent of x as T --> 0 and is therefore about the same as the T --> 0 value observed for undoped La2CuO4. We interpret this effect as arising from localization of the doped holes below similar to 30 K, whereby localized doped holes are much less effective in reducing Mi than mobile holes. In support of this hypothesis, we find that the width of the La-139 NQR 2 nu(Q) line in La2-xSrxCuO4 is nearly independent of temperature below 4.2 K and increases linearly with x for x<0.02. We present new mu SR measurements to support the idea that the effective spin degrees of freedom associated with the localized doped holes undergo a continuous freezing at T-j approximate to(815 K)x, as opposed to a cooperative phase transition, superimposed on the preexisting AF long-range order. [References: 24]