Superconductivity and magnetism of La2CuO4+δ by μSr

Physica C 162-164 (1989) 147-148 North-Holland

SUPERCONDUCTIVITY

AND MAGNETISM

O F La2CuO4+6 B Y # S R

E. J. ANSALDO*, 3.H. Brewert, T.M. Risemant, J.E. Schirber:~, E.L. Venturini:L B. Morosin~, D.S. Ginley~, and B. Sternlieb* *Department of Physics, University of Saskatchewan, Saskatoon, SASK., Canada S7N 0K3; SUniversity of British Columbia, Vancouver, BC, Canada V6T 2A3; ~tSandia National Laboratories, Albuquerque, NM 87185, USA; *Columbia University, New York, NY 10027, USA The two phases constituting La~CuO4+~ have been determined and studied in a series of redundant spin rotation and relaxation measurements in transverse and zero external fields. 60 % of the sample volume was found to be superconducting with penetration depth A(0)=4200/~ (powder average). The other phase displayed similar magnetic ordering to that of La2CuO4 .

Given its microscopic nature, the #SR technique has provided unique information on the magnetic and superconducting behaviour of La2CuO4-v and the doped system La1.ssSr0.15Cu04.1-9 We have applied the technique to the "superoxide" La2CuO4+s which is known to consist of two phases, to determine in detail the microscopic effect of the extra oxygen. This is also a good test case of the extent to which the technique may be reliably applied in more complex situations, such as inhomogeneous La2_xSr~CuO4 4,9 The sample was prepared, as described before, 1°'11 by high-pressure annealing of La2CuOa (3 kbar at 500°C) in oxygen. This resulted in a nominal oxygen excess of ~=0.13, but surface effects may imply its true value to be somewhat lower. Susceptibility measurements showed that at least 30 % of the material is superconducting, and Jorgensen et aL have shown, by means of neutron diffraction, that such samples consist of two nearly identical orthorhombic phases. 12 To characterize the microscopic magnetic parameters of the different phases, measurements were carried out at zero magnetic field (ZF) and weak (100 Oe) transverse field (WTF) to explore the magnetic ordering, and at higher transverse field (HTF, 2.5 kOe) to study the superconducting phase. Figure 1 is an example of the effect of internal fields on the precession signal for HTF at low temperature. 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

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FIGURE 1. Example of the 3-signal fits to a typical HTF asymmetry spectrum. Only the first 4 #s of data are shown for clarity of the diagram. The random internal fields m the ordered (AFM) phase give rise to a rapidly decaying modulation of the signal, readily measured in WTF, and the signal remaining at longer times (after ca. 1 #s) is due only to the coherent precession of the spin of the muons in the paramagnetic environment. The polarization of this fraction is damped by the magnetic field distribution in the vortex state and by interaction with nuclear dipoles. The paramagnetic signal includes contributions from the superconducting (SC) phase as well as from muons stopped in the sample holder (about 15% of the total), since the sample was comparatively small. The inhomogeneous broadening due to the onset of superconductivity

E.J. Ansaldo et aL / Superconductivity and magnetism of LaeCuO++a

148

is clearly seen (Figure 2) tbr the fraction f,c of muons in

be inhomogeneous even for x in the optimal range 0.15

the SC phase below 40K. In zero applied field the muon

to 0.2,9'~3and where the ordering is not simple static

polarization precesses in the internal field [-I int due to

AFM for the magnetic fraction, care must be taken to

the ordered moments in the AFM state. The observa-

insure that a great majority of the muons stops in the

tion of a discrete frequency with little depolarization, an

sample material, and that the analysis hypotheses are

example of which is shown in Figure 3, indicates that

confirmed by consistency and redundancy in the exper-

the internal field at the muon site is unique and quasi-

iments (using ZF as well as T F in the same geometry)

static. This signal dissapears between 275 and 300K,

when using the technique to study the important issue

similar to the La2CuO4 case.

of the co-existence of superconductivity and magnetic ordering.

if++++

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FIGURE 2. Relaxation rate vs temperature for the SC fraction. The onset of the SC vortex lattice structure is

FIGURE 3. Fourier transform of the ZF signal. Mag-

evident below 40K. The HTF data were analysed independently us-

nitude and T-dependence are very similar to those of La2CuO4.

ing three components, namely AFM, SC, and background. The relative amplitudes of the three signals

REFERENCES

were the same within errors as for the W T F , and independent of temperature. This yielded a value of f~c=0.6

1. G. Aeppli et al., Phys. Rev. B 35 (1987)7129.

for the oxygen-rich SC fraction of the sample, with

2. W. J. Kossler et al., Phys. Rev. B 35 (1987)7133.

Tc=37(1)K, and a penetration depth A(0)= 4200 /~ .

3. D. R. Harshman et al., Phys. Rev. B 38 (1988)852.

In the London picture, A(0) c((n,/m*) "~, which implies a value of 1021 cm -3 for the superconducting carrier

4. J. I. Budnick et al., Phys. Le~t. A 124 (1987)103.

density n+, assuming an effective mass m*/m~=7 from

6. Y. J. Uemura et al., Phys. Rev. Left. 62 (1989)2317.

5. Y. J. Uemura et al., Phys. Rev. Lett. 59 (1987)1045.

the Lal.ssSr0.15CuO4 case, t in rough agreement with the

7. J. I. Budnick et al., Europhys. Left. 5 (1988)651.

hole number density produced by the extra oxygen in the SC phase. In conclusion, besides determining ba-

8. Y. J. Uemura et al., Physics C 153-155 (1988)767.

sic properties of La2CuO4+6 , we have shown that #SR

9. A. Weidinger et al., Phys. Rev. Left. 62 (1989)102. 10. J. E. Schirber et al., Physics C 152 (1988)121.

yields reliable results in mixed AFM/SC samples, even

11. J. W. Rogers et al., Phys. Rev. B 38 (1988)5021.

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12. J. D. Jorgensen et al., Phys. Rev. B38(1988)11377.

star background due to the smallness of the sample. In

13.

more complex cases, such as La2_~Sr~CuO4 which may

Harshman et aL, to be published.

R. F. Kiefl et al., These Proceedings and D. R.