PAC study of magnetic critical behavior and line broadening in La2CuO4

Physica C 162-164 (1989) 1283-1284 North-Holland

PAC STUDY OF M A G N E T I C C R I T I C A L B E H A V I O R AND L I N E B R O A D E N I N G IN La2CuO 4

-~(I) j . SAYLOR,O) A. TEH, (I)J. I. BUDNICK,(2) M. FILIPKOWSKI,(2) and C. HOHENEMSI~v,.," B. CHAMBERLANDO) (1) Departmentof Physics, Clark University, Worcester,MA 01610 (2)Departmentof Physics and (3)Department of Chemistry, University of Connecticut, Storrs, CT 06268 We report perturbed angular correlations studies of lllln diffused into La2CuO4 and find a mean field value of I~= 0.50(4) in material with a N6el temperature of TN = 317 K. We also provide a calibration of the linewidth in La2.xSrxCuO4 supporting argument that TN = 317 K material is near stoichiometrie. 1. Introduction The phase diagram of La2CuO4+yplays a central role in the still evolving theories of high temperature superconductivity, and has been the object of much recent work. 1 Here we discuss perturbed W angular correlations (PAC) experiments using the 111In/111Cd probe to study the phase diagram of "stoichiometric" material. Our samples were doped with < 10 ppm of radioactive 1HIn in 02 gas at 1273 K, and annealed for 3-4 h in vacuo at 773 K. As shown in a recent paper, 2 they have susceptibility maxima near T N = 317(3) K. Because line broadening above and below T N is absent to within A0)/0) < 0.02, the samples are interpreted to be free of charge defects to within IAyl < 0.016, and in that sense are "stoichiometric." Analysis of the PAC data suggests 111In resides in the La site. 2 In this note we provide the first measurement of magnetic critical behavior in Tr~ = 317 K material, and provide a calibration of charge defect line broadening.

2. Magnetic critical behavior The temperature dependence of the local magnetization near T N may be discerned from the neutron, 1 laSR,3 and NQR 4 data, but has so far not yielded a value of the critical exponent 13, presumably because of sample size and inhomogeneity. In contrast, our PAC samples are sufficiently small 0921--4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

and homogeneous to permit following Bhf(T) to a reduced temperature of t --- 3x10 -3. The Fourier transform of the PAC signal just above T N is illustrated in Fig.la, and shows the characteristic three frequency pattern for an 1=5/2 nucleus in a non-cubic field G2(t) = 1 + (13/7)coS0)ot + (10/7) cos(2-~)0)ot + (5/7) cos(3-~)0)ot (1)

_

'--

t

I

t

'l

%


(a)

L

"

|

(b)

-~

0

200

400

600

800

(MRad/s) Fig. I. Fourier transform of PAC time spectra above (a) and below (b) "IN. yielding 0)0 = 247.7(1) Mrad/s and ~ = 0.040(1), implying an EFG asymmetry parameter of r I = 0.140(2). As shown in Fig. lb, below T N the 0)0 and 30)0 Fourier lines are split by a weak magnetic hyperfine field, Bhf(T), which as shown in Fig. 2,

1284

C. Hohenemser et al. / PAC study o f magnetic critical behavior 1.00

¢=

TN= 316.7(3) K 0.30

mJ= 0.10 0.03 10 -3

l

I

I

I

I

IIII

•

*

10 -2

*

•

1=*

r.-r

10

-I

reduced temperature ( TN Fig. 2. Power law fit of La2CuO4 staggered magnetization yielding a mean field exponent in the critical regime. may be fitted via the power law (2)

Bhf(T)/Bhf(O ) = D(1 - T/TN)I5

with ~ = 0.50(4) and T N = 316.7(3) K. Th¢ perhaps surprising result is that the value of 13 corresponds to mean-field theory rather than either the d--3 Heisenberg or Ising model, implying that ordering is dominated by long range interactions. A model for such behavior is ordering dominated by inter-planar interactions between the 2-d correlated spin regions observed in the neutron data. 1

we estimated that the observed linewidth 5o~/co < 0.02 for TN=317K La2CuO4+y implies ]Yl < 0.016. A more direct approach is measurement of line broadening in samples with known concentrations of charge defects. Using samples of Lal.xSrxCuO 4 for 0 < x < 0.03, with vacuum annealing comparable to our T N = 317 K "stoichiometric" samples, we have obtained spectra closely similar to those described by Eq. (1) and Fig. la, but with line broadening as illustrated in Fig. 3. Assuming that Sr involves a unit charge defect, this implies a relation between line broadening and charge defect concentration given by ~ / o ~ = 2.1(x + 0.007). A simple interpretation of the x intercept is that our samples contain a residual oxygen excess equivalent to a Sr defect content of x = 0.007(3). The fact that the PAC linewidth provides a direct estimate of the charge defect concentration means it can be used as a new way of solving the oxygen stoichiometry problem for L a 2 C u O 4 samples, a way which is free of the errors involved in direct chemical analysis.5

Acknowledgments Research support was received via NSF under DMR 87-23033, and via a Connecticut Department of Higher Education Grant.

3. Line broadening In earlier work, 2 based on experimentally derived EFGs for charge defects of other systems,

0.08

I

'

I

'

I

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0.00

-0.02

1. R. J. Birgeneau and G. Shirane, in: Physical Properties of High Temperature Superconductors, G. M. Ginsberg eel. (World Scientific Publishing, in press). 2.

°°4f --

References

J. Saylor, L. Takacs, C. Hohenemser, J. I. Budnick, and B. Chamberland, Phys. Rev. B, in press.

3. J. I. Budnick, B. Chamberland, D. P. Yang, Ch. Niedermayer, A. Golnick, E. Recknagel, M. Rossmanith, and A. Weidinger, Europhys. Lett., 5 (1988) 651. 0.00

0.02

0.04

Sr c o n t e n t (x)

Fig. 3. Calibration of PAC charge defect line broadening

4. Y. Kitaoka, S. Hirarnatsu, K. Ishida, T. Kohara and K.

Asayama, J. Phys. Soc. Jpn., 56 (1987) 3024. 5. D. C. Johnston, S. K. Sinha, A. J. Jacobson, and J. M. Newsam, Physica C153-155 (1988) 572.