Staggered magnetization in La2-xSrxCuO4 from La139 NQR and SR: Effects of Sr doping in the range 0
Journal
Physical Review B
Journal Volume
52
Journal Issue
10
Pages
7334-7345
Date Issued
1995
Author(s)
Borsa, F.
Carreta, P.
Cho, J. H.
Hu, Q.
Johnston, D. C.
Lascialfari, A.
Torgeson, D. R.
Gooding, R. J.
Salem, N. M.
Vos, K. J. E.
Abstract
We have used La139 nuclear quadrupole resonance (NQR) and positive muon spin rotation (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 (Néel) temperature TN(x). From these measurements, our previous La139 NQR measurements [F. C. Chou et al., Phys. Rev. Lett. 71, 2323, (1993)], and auxiliary La139 nuclear magnetic resonance (NMR) measurements of single-crystal La2CuO4, we have determined the Cu2+ staggered magnetization M°(x,T) below TN(x). Above 30 K, M°(x,T) at each T is found to be progressively depressed with increasing x (decreasing TN); the M°(x,0) at T=0, extrapolated from T>30 K, is found from both the NQR and SR measurements to follow the same empirical relation M°(x,0)/M°(0,0)=(1-x/xc)n with xc=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 doped 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 30 K, consistent with the phase separation phenomenology of Emery and Kivelson. Below 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 T0 and is therefore about the same as the T0 value observed for undoped La2CuO4. We interpret this effect as arising from localization of the doped holes below 30 K, whereby localized doped holes are much less effective in reducing M°than mobile holes. In support of this hypothesis, we find that the width of the La139 NQR 2Q 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 SR measurements to support the idea that the effective spin degrees of freedom associated with the localized doped holes undergo a continuous freezing at Tf(815 K)x, as opposed to a cooperative phase transition, superimposed on the preexisting AF long-range order. © 1995 The American Physical Society.
Type
journal article