Suppression of Antiferromagnetic Order by Light Hole Doping in La2Cu1-xLixO4: A 139La NQR Study

V OLUME 81, N UMBER 13 PHYSICAL REVIEW LETTERS 28 S EPTEMBER 1998 Suppression of Antiferromagnetic Order by Light Hole Doping in La 2 Cu 12x Li x O 4 : A 139 La NQR Study B. J. Suh, 1 P. C. Hammel, 1 Y. Yoshinari, 1 J. D. Thompson, 1 J. L. Sarrao, 1 and Z. Fisk 2 Condensed Matter and Thermal Physics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306 ( Received 22 October 1997 ) La nuclear quadrupole resonance measurements in lightly doped La 2 Cu 12x Li x O 4 have been performed to reveal the dependence of the magnetic properties of the antiferromagnetic CuO 2 planes on the character of the doped holes and their interactions with the dopant. A detailed study shows that the magnetic properties are remarkably insensitive to the character of the dopant impurity. This indicates that the added holes form previously unrecognized collective structures. [S0031-9007(98)07260-3] PACS numbers: 75.30.Kz, 74.72.Dn, 76.60.Gv, 76.60.Jx Full understanding of the character of holes added to cuprate planes and their interactions with the two- dimensional lattice of Cu spins remains a crucial and un- solved problem in the high temperature superconductors. While the detailed mechanism is poorly understood, the rapid suppression of the antiferromagnetic (AF) ordering temperature T N by doping is clearly related to the disrup- tive effects of mobile holes: &3% added holes whether from Sr substitution, addition of interstitial oxygen, or in- plane substitution of Li for Cu [1] suppresses T N to zero, yet ,30% isovalent substitution of Zn or Mg for Cu is required [2] to produce the same effect. Li- and Sr-doped holes have very different mobilities. For x or y o 0.025, the room temperature resistivity r of La 2 Cu 12x Li x O 4 ex- ceeds that of La 22y Sr y CuO 4 by over an order of magni- tude [1,3,4]; more strikingly, for Sr doping drydT . 0 for T * 100 K, in contrast to the negative slope found in Li-doped material for all x and T . It is well recognized that the 2D cuprates are inclined to- ward microscopic charge inhomogeneity [5–7]. Evidence for such an effect in lightly doped La 22y Sr y CuO 4 was ob- tained from a scaling analysis of the doping y and tem- perature T dependence of the static susceptibility xs y, T d [8] which indicated that the magnetic correlation length is limited to the dimensions of AF domains (finite-size scal- ing) formed by microsegregation of doped holes into hole- rich domain walls surrounding hole-free, AF domains. Interpretations involving charge stripes have also been pro- posed [9]. Castro Neto and Hone [10] have examined the influence of doping on the long wavelength properties of a 2D antiferromagnet in a model in which charged stripes cause the exchange coupling J to become anisotropic; this model reproduces the relationship between M s 0 (M s is the sublattice magnetization; M s 0 is that obtained by extrapo- lation of data for T . 30 K to T ­ 0) and T N as the two are suppressed by Sr doping in La 22y Sr y CuO 4 [11]. How- ever, using a similar model, van Duin and Zaanen [12] find that T N is suppressed much more rapidly than M s 0 with in- creasing anisotropy (doping). We have used 139 La nuclear quadrupole resonance (NQR) measurements to microscopically examine the 0031-9007y98y81(13)y2791(4)$15.00 effects of doped holes on the AF spin correlations in La 2 Cu 12x Li x O 4 s0.019 , x , 0.025d. We find that the magnetic behavior of lanthanum cuprate is remarkably insensitive to the detailed nature of the dopant, in spite of the differing charge transport associated with the two dopants. In addition to the similarly strong suppression of T N by doping, we find the identical correspondence between the suppression of M s 0 and T N by doping which has been observed in La 22y Sr y CuO 4 . Further we show, for the first time, that in the vicinity of T N the dynamical susceptibility, as reflected in the nuclear spin-lattice relax- ation rate 2W, follows a scaling law consistent with the finite-size scaling demonstrated in the static susceptibility by Cho et al. [8]. Finally, at low temperature, we find that two peculiar features are very similar in the two systems. These are the very strong peak in 2W (at a temperature T f ­ 10 16 K depending on x) that indicates freezing of spin degrees of freedom, which is accompanied at slightly higher temperatures s. 30 Kd by the abrupt recovery of M s sT d, almost to x ­ 0 values. Thus, while very small concentrations of added holes induce a range of characteristic magnetic properties which are entirely insensitive to the nature of the dopant, the transport properties are very sensitive to the dopant. Unable to understand these contrasting behaviors as arising from properties of individual holes, we conclude holes form collective structures. We will argue that holes form charged, antiphase domain walls [13] which surround mobile domains in which the phase of the AF order is reversed. Such mobile domains will suppress the time-averaged static moment thus suppressing AF order and M s . These domain structures will have contrasting interactions with in-plane vs out-of-plane dopants (e.g., stronger scattering by in-plane impurities) which explain the different transport behaviors, while the universal magnetic properties can be understood as long as the domains are sufficiently mobile that they move across a given site rapidly compared to a measurement time. Three powder samples of La 2 Cu 12x Li x O 4 (labeled A1, B1, and B2) were prepared from starting material con- taining concentrations x nom of Li as described elsewhere © 1998 The American Physical Society