Sample processing pressure-dependence of weak links in polycrystalline YBa2Cu3O7

1 Magnusson et al./Physica

2370

C 282-287

(1997) 2369-2370

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10“ 0.01

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0.1

1 10 f [Hz1

100

Figure 1. Spectral density of the magnetic flux noise, S*(j), versus frequency for the temperatures, from top to bottom, T = 75 K, 65 K and 45 K. The inset shows S*(f) at 75 K for Hdc = 0 G (top) and 0.05 G (bottom).

mum at 75 K. Above the maximum the data extracted for different frequencies coincide (corresponding to a l/f noise spectrum). At and below the peak, fSa(w)/T decreases with increasing frequency. If the fluctuation-dissipation theorem holds these curves should show the same characteristics as the out-of-phase component of the ac susceptibility. In the inset of figure 2 a direct measurement of x”(w) is shown for comparison. These measurements were made in a field regime where the response is linear [5], thereby providing the necessary pre-conditions for applying eq. 1. There is a good agreement between the noise- and susceptibility data, confirming the validity of the fluctuation-dissipation theorem at zero-magnetic field in the sample. If the magnetic noise is due to motion of vortices, the noise power is expected to increase if the number of fluctuating vortices is increased by the application of a weak magnetic field [6]. In the inset of figure 1, it is seen that the noise is suppressed when a small field is applied. This rules out field induced vortices as the dominant noise source. Contrary the results favour a description in terms of fluctuating localised spontaneous magnetic moments. If the magnetic moments are polarised by the applied field, the noise

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TqoKl Figure 2. The out-of-phase component of the ac susceptibility derived from the noise power according to eq. 1 at 0.2 Hz (diamonds), 2 Hz (squares) and 20 Hz (circles). The lines are guides to the eye. The inset shows the temperature dependence of x”(w) for H& = 0 G and H,,= 0.1 G. The curves correspond from left to right to 0.17 Hz, 1.7 Hz and 17 Hz.

power will decrease in magnitude when the polarising energy is of the same order as the thermal energy. Moreover, such a model is in accordance with the experimental observation that the fluctuation-dissipation theorem is valid. REFERENCES 1. 2.

3. 4. 5. 6.

P. Svedlindh et al., Physica C. 162-164 (1989) 1365. W. Braunisch et al., Phys. Rev. Lett, 68 (1992) 1908; W. Braunisch et al., Phys. Rev. B, 48 (1993) 4030. M. Sigrist and T. M. Rice, Rev. Mod. Phys. 67 (1995) 503. J. Magnusson et al., Phys. Rev. B, 52 (199.5) 7675. J. Magnusson et al., t.o be published. M. J. Ferrari et al., .J. of Low Temp. Phys., 94 (1994) 15.

PHYSICA 8 ELSEVIER

Physica C 282-287 (1997) 2369-2370

Zero-field flux noise in granular Johan

Magnusson,

Department

Per Nordblad

of Materials

Bi$r&aCu208

*

and Peter Svedlindh

Science,

Uppsala

University,

Box 534, S-751 21 Uppsala,

Sweden

We report zero-magnetic field flux noise measurements on a granular BizSrzCaCuzOs sample displaying the paramagnetic Meissner effect (PME). The spectral noise density, S+(f), was measured in the frequency range It 0.04-400 Hz and for temperatures 0.5 < T/Tc < 1 (T, = 85 K) using a custom made SQUID magnetometer. is found that the noise density scales as Se N l/f” with a monotonously decreasing from a N 1.3 at the lowest temperature to a z 1 as T, is approached. The noise density exhibits its largest magnitude around T/T, = 0.9. Using the fluctuation-dissipation theorem, good a greement is found between the measured noise power and The results favour a model with fluctuating spontaneous the out-of-phase component of the ac susceptibility. magnetic moments.

Some (HTSC)

samples show

of

high-T,

a positive

superconductors

field-cooled

magnetisa-

tion when cooled in sufficiently small magnetic fields [1,2]. The effect is often called paramagnetic Meissner effect (PME). One of the possible explanations involves creation of spontaneous ofbital currents in so called ?r loops [3]. In this model, the energy of one such loop has two minimaseparated by an energy barrier. In a magnetic field the orbital moments produced by the orbital currents are polarised giving a positive magnetisation. Magnetic relaxation measurements have shown unusual behaviour of the zero-field-cooled and field-cooled magnetisation [4]. The objective of the present study is to measure the zero-field behaviour of the sample. We use the spontaneous magnetic flux noise as the probe. The BiaSrnCaCuzOs sample was prepared by reaction sintering BizOs, SrCOs, CaCOs and CaO at temperatures 790 “C - 860 “C. The size of the sample was 2.5 x 2.1 x 1.7 mm3 and the critical temperature was T, = 85 K. Noise measurements were performed in a noncommercial SQUID magnetometer in which the sample space was magnetically shielded by pmetal and superconducting screens. The residual background field was lower than 0.5 mOe and the background noise level lower than lo-’ @/Hz *Financial support from the Swedish Natural Science Research Council (NFR) and the Carl Trygger foundation is acknowledged. 0921-4534/97/$17.00 0 Elsevier Science B.\! PI1 SO921-4534(97)01326-9

All rights reserved.

at frequencies above 1 Hz. A superconducting NbTi third order gradiometer with a diameter of 5 mm was connected to the input terminals of the SQUID sensor. The sample was placed in one of the middle sections of the gradiometer with its longest side parallel to the axis of the coils. The output of the SQUID was connected to an HP 35670A dynamic signal analyser. The spectral noise density, S+(f), was measured in the frequency range 0.04-400 Hz and for temperatures 0.5 < T/Tc < 1. In figure 1 the frequency dependence of the spectral noise density is shown. The background noise power, obtained at 95 K, has been subtracted from the measured noise spectra. In the measured frequency range the noise scales as S*(f) - l/fo. The exponent a is found to decrease monotonously from a N 1.3 at the lowest temperature studied to a N 1 as T, is approached. The noise has its largest magnitude around 75 K. The fluctuation-dissipation theorem gives the following relation between the magnetic noise power, S@(w), and the out-of-phase component, x”(w), of the ac susceptibility: h(w)

= 2kTm,

w = 2*f, W

where k is the Boltzmann constant and T the temperature. In figure 2, fSQ(w)/T is plotted versus temperature. The quantity fS+.(w)/T has a maxi-