A μ+SR study of the magnetic properties of La1.85Sr0.15CuO4

Physiea C 153-155 (1988) 755-756 North-Holland, Amsterdam

A #+SR STUDY OF THE MAGNETIC PROPERTIES OF Lal.ssSr0asCu04

S. BARTH +, P. BIRRER*, D. CATTANF*, J. CORS**, M. DECROUX**, O. FISCHER**, F.N. GYGAX*, B. HITTI*, E. LIPPELT" and A. SCHENCK* *) Institut ffir Mittelenergiephysik, ETH Ziirich, c/o PSI, CH-5234 Villigen; **) D~partement de Physique de la Mati~re Condens~e, Universit~ de Geneva, CH-1211 Gen~ve 4; +) Laboratorium f'dr FestkSrperphysik, ETH-HSnggerberg, CH-8093 Zfirich

The magnetic properties of two polycrystalllne samples of Lal.ssSr0.1sCuO4 have been studied by muon spin rotation spectroscopy. Below Tc two precessing signals indicating different magnetic environments were observed in both samples. This is explained qualitatively as the consequence of the anisotropic field penetration into the parallel and perpendicular layers of the crystal. 1.

INTRODUCTION Muon Spin Rotation spectroscopy (1) was used to investigate the magnetic properties of two Lal.ssSr0.asCuO4 samples. The average magnetic field at the muon site B, is deduced from the p+ precession frequency. B is given by,

B = H,,t + (41r - N ) M

(1)

where M is the induced magnetization M=xHe~, and N is the demagnetizing factor. Field distributions around the average result in the depolarization of the muon spins observed in the #SR signal, the depolarization function can be approximated by a Gaussian P(t)=EXP(-a2t2), with the relaxation rate given by, cr = ~'u

~/-~zi2B2>

{¢o~'

(3)

AL is the London penetration depth. The independence of Eq. 3 from p is the consequence of a regular triangular lattice. 2.

EXPERIMENTAL RESULTS The measurements were performed at the Paul Scherrer Institute (PSI) using a conventional #SR spectrometer. The samples were sintered Lal.ssSr0.asCuO4 of cylindrical shape prepared as described in ref. (3). The external field was applied along the cylindrical axis. For a preliminary account of our results see ref. (4).

0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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In the mixed state of a type II superconductor the external field penetrates the sample in flux lines of density p=B/(ao (q~o is the flux quantum) arranged in a triangular array. Muons are sensitive to the vortex lattice and are subject to a distribution of fields, the second moment of which (excluding very small and large values of p) is given by (2),

< AB 2 > ~ 0.6 \ 4 ~ U

In all our data from both samples, below Tc two components were observed in the ftSR signal. This is best seen in the Fourier spectrum taken at 2000 Oe after zero field cooling (zfc) to 7.3 K, Fig. 1. Above Tc only one component was observed.

2/+ 26 28 30 FREOUENCY (MHz I

32

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FIGURE 1 Figs. 2 a,b,c show the field dependence of the relaxation rates, (B/~-H~t) and amplitudes respectively, of the two components at 15 K. These data were obtained after zfc to 15 K, for first ascending field scans up to 2000 Oe and then descending scans to zero. Looking at Fig. 2a we see that the slow damping rate is independent from the applied field and magnetic history. On the other hasld the fast damping rate of the ascending field scans is field dependent up to 1500 Oe, while that of the descending scans is field independent. One might attribute the behaviour of the fast damped component of the ascending field scans to the formation of a regular vortex lattice, which is attained only at higher fields, and the field independence of the descending scans to a regular vortex lattice, as discussed earlier (Eqs. 2 and 3). However, the Fourier spectrum (Fig. 1) is not what is expected for a regular vortex lattice. This is not surprising considering the strong anisotropy and granularity of these systems (5).

756

S. Barth et al. / A ll +S R study of the magnetic properties of La1.ssSro.15CuO 4

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As seen in Fig. 2b, ( B ~ - H ~ t ) of the slow damped component is about 0.5 Oe larger than the external field, while that of the fast damped component is dependent on the field history. However, no field free volume was seen, as should be present in the Meissner state. From Fig. 2c it is clear that the amplitudes of the two components at low fields are field dependent, the slow damped component decreases while the fast damped component increases with rising external field. The sum of the two components is constant, and its value implies that all positive muons implanted in the sample contribute to the signal. The relaxation rate as a function of temperature (measured at 2000 Oe) are shown in ref. (6). The zero field cooling (zfc) and field cooling (fc) data were measured on two different samples. We used the fe data to estimate the London penetration depth at 0 K; it is found to be on the order of 3100/~ and 6000 & for the fast and slow damped components, respectively.

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DISCUSSION Any model to explain the data should consider in detail the microscopic structure of the lattice. We calculated the relaxation rate for a / t + located at the center of an octahedral site surrounded by four Cu and two La ions. The result is consistent with the value of cr measured above Tc. Considering the layer structure of the crystal, a magnetic field applied parallel or perpendicular to the planes will have a different local effect. Two different magnetic fields at the muons site throughout the lattice will result in two precession frequencies. Since a quantitative analysis has not been done, the numbers reported for the penetration depths should be considered only as estimates. We hope #+SR measurements on single crystals at different orientations with respect to the external field can be carried out soon.

20

REFERENCES (1) A. Schenek, llMuon Spin Rotation Spectroscopy (Adam Hilger, Bristol, 1985). (2) A.M. Portis (private communication, 1987). (3) M. Decroux et al., Europhysics Letters 3, 1035 (1987). (4) F.N. Gygax et al., Europhysics Letters 4_, 473 (1987). (5) IV[. Cello (private communication, 1987). (6) A. Schenck (this conference).

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