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Low field AC susceptibility of high Tc Y-Ba-Cu-O superconductors
Physica C 153-155 (1988) 1531-1532 North-Holland, Amsterdam
LOW FIELD AC SUSCEPTIBILITY OF HIGH Tc
Y-Ba-Cu-O SUPERCONDUCTORS+
J.S.MUNOZ,A.SANCHEZ,T.PUIG~D-X CHEN*and K.V.RAO* Department of Physics,Autonoma University Barcelona, 08193 BELLATERRA, Spain. *Department of Solid State Physics,Royal I n s t i t u t e of Technology,lO044 Stockholm,Sweden. Low f i e l d ac s u s c e p t i b i l i t y measurements on high Tc Y-Ba-Cu-O superconductors of several nominal compositions have been carried out in order to explain t h e i r d i s s i p a t i v e behaviour. I t appears that eddy currents and Meissner e f f e c t are the mechanisms involved. A d i s s i p a t i o n f a c t o r defined as X"/X' shows this c l e a r l y . I. INTRODUCTION Since the discovery of superconductors of high c r i t i c a l temperature a large world-wide flood of research in superconductivity is taking p l a c e ( l , 2 ) . I t is now well established that these oxide-based ceramic materials such as YIBa^Cu~07+x have an oxigen d e f i c i e n t perovskite structure and that rare earth s u b s t i t u t i o n of Y and Ba does not suppress superconductivity (3). The study of t h e i r magnetic properties are fundamental to understand the behaviour of these mat e r i a l s . The complex magnetic ac s u s c e p t i b i l i t y below and close to Tc appears to be a very useful technique to gain more information as to the nature of the mechanism responsible fo r the behaviour observed. In t h i s paper the imaginary (out of phase) and the real (in phase) parts of complex low f i e l d s u s c e p t i b i l i t y in several samples of the system Y-Ba-Cu-O are reported and t h e i r behaviour analysed in terms of d i s s i p a t i v e effects. Since there have been indications f o r the coexistence of two contributions to the magnetic s u s c e p t i b i l i t y even in single phase compounds i t w i l l be i n t e r e s t i n g to examine the ac f i e l d dependence of both X' and X" as a function of temperature. 2. EXPERIMENTALDETAILS 2.1 Samples preparation The samples investigated have nominal compositions Y-Ba2-Cu3-O7 x (D);Y-Ba-Cu~-O~ (E) and Y2-Ba2-Cu3 -07_ x - (F) and wer~ pPe~ared from the appropiate amounts of Y203 (99.99 %) BaCO3 (99.9) and CuO (99.9). The powders,after mixing and grinding,were heated at 850 eC for I0 hours in a i r and cooled slowly. The gray-black powder was pressed up to I0 bar in a hardened steel dies.The p e l l e t s were annealed f o r 6 h at I000 ~C in a i r and cooled in an oxigen flow of 0.25 I/min. From the sintered p e l l e t s small bars were3cut. The apparent density ranged from 3 to 5 g/cm . The remaining pieces of the p e l l e t s were regrinded in an agate mortar f o r identification.
2.2.Sample I d e n t i f i c a t i o n Part of the remaining powders were used f o r x-ray i d e n t i f i c a t i o n . The d i f f r a c t i o n patterns (determined with Cu K m r a d i a t i o n ) show that a l l the compounds have a common p h a s e , i d e n t i f i e d as the the YIBa^Cu~O~ ,orthorhombic. Sample D is single ph~se~ J / - x Samples E and F gave another phase which appears to be tetragonal. Energy dispersive x-ray analysis have also been performed on sample D and E confirming the s t a r t i n g composition to within I0 %. We expect to r e f i n e these spectra by choosing better standards. The samples were also examined under a scanning electron microscope. Photographs of the fresh fracture of the sintered p e l l e t s were taken to determine the mean grain size of the p a r t i c l e s , which ranged from 2 to I0 microns.
HQc---0.01
F
X"
0.0
A.U.
-0.4
-0.8
-1.2 0
20
40 60 T(K)
00
100
FIGURE 1 Plot of real and imaginary components of ac susceptibility.
+This work has been supported by Swedish agencies NFR and STU. 0921-4534/88/$03.50 @Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Oe f = I L H z
0.4
J.S. Mu~oz et aL / Low fieM A C susceptibifity
1532
2.3.Measurements Since the ac magnetic susceptibility gives more information than resistance measurements as to the behaviour of the superconductors close to the transition temperature,we made a set of measurements using a mutual inductance technique.The susc e p t i b i l i t y was measured upon warming the sample after being cooled in zero magnetic f i e l d . No corrections for the demagnetization factor or Earth magnetic f i e l d was done,so that we have determined the i n i t i a l susceptibility. The experiments were carried out at a fixed frequency (I000 Hz) and varying the rms f i e l d amplitud from O.Ol Oe to I0.0 Oe,at also fixed values. 3. RESULTSAND DISCUSSION The results of the real,X',and imaginary,X", components of the complex susceptibility are shown in Figure I. The values of X" have been scaled by 5and only the data of samples D and F have been plotted for the sake of c l a r i t y . I t is remarkable the sharp transition depicted by sample F from the minimum value of -I.2 to almost O.The peak in X" appears to coincide with the inflexion in X'.This pattern has already been reported (4,5) and a t t r i buted to the presence of two superconductive phases (6). This has also been observed in our single phase sample D. In order to elucidate whether the mechanism involved is the existence of weakly interconnected
SAMPLE E
the lowest temperature reached. Several models have been proposed to explain the properties of the real component X'(T) but l i t t l e has been said about the peak in the temperature dependenceof X"(T).Our data can be exp|ained in terms of a competing interaction where the competing processes are the accelerated eddy currents and the Meissner effect,which lowers the f i e l d amplitud preventing thereby the energy absorption. To see clearly these effects we have made a plot of a Dissipation Factor,the r a t i o X"/X',as a function of temperature shown in Figure 3. SAPPLE E t =1 k Hz
0.3
0.2 D.F. 0.1 e
0,0
f=lkHz
0.20
0 T(K) FIGURE 3
0.15 X 0.10 411 X"
<> 0
0.05
The outcome of the maximumis due to an increase in the supercurrent density (8)that leads to flux expulsion (Meissner).At the same time the eddy current is rapidly increasing becoming dominant as the temperature is lowered. The minimum observed in DF could be the starting of the dominant mechanism,i.e.the eddy currents.
X
~
0
~]
0 0.00 ,,, 0.00 010
!
0.25 Oe
0.50 4 fIX' 0 1 Oe
!
0.75
1.0(1
4. ACKNOWLEDGMENT One of us,A.S.would like to thank co-author Prof.K.V.Rao for his kind hospitality while he was at the Royal Institute of Technology,Stockholmw~ere part of this work was done. 5. REFERENCES
XO.10e
FIGURE 2 grains or Josephson links,as has been suggested (7) we have made a plot of 4~X" against 4RX'. We have observed some deviations.See the squared points. I t seems that the response of our samples give support to the coexistence of two mechanisms or suprconductive components as suggested by Goldfarb et al.(4).The curves in this figure have been obtained by normalizing to l the X' interval from nearly zero at Tc down to the minimum value at
(1) J.G.Bednorz and K.A.MUIler,Z.Phys.B64 (1986)189 (2) M.K.Wu et al.Phys.Rev.Lett.58(1987)908 (3) Gang Xiao et al. Solid State Commun.63(1987) 817. (4) R.B.Goldfarb et al.Cryogenics,27(1987)475. (5) D.-X.Chen et al.Applied Phys.Lett.in print. 6) J.Garcia et al. Journal of Magnetism and Magnetic Mat.69 (1987) 1225. 7)~7~hida and H.Mazaki,Journal App.Phys.52(l~T)] 8) E.Maxwell and M.Strongin,Phys.Rev.Let.lO(1963) 212
LOW FIELD AC SUSCEPTIBILITY OF HIGH Tc
Y-Ba-Cu-O SUPERCONDUCTORS+
J.S.MUNOZ,A.SANCHEZ,T.PUIG~D-X CHEN*and K.V.RAO* Department of Physics,Autonoma University Barcelona, 08193 BELLATERRA, Spain. *Department of Solid State Physics,Royal I n s t i t u t e of Technology,lO044 Stockholm,Sweden. Low f i e l d ac s u s c e p t i b i l i t y measurements on high Tc Y-Ba-Cu-O superconductors of several nominal compositions have been carried out in order to explain t h e i r d i s s i p a t i v e behaviour. I t appears that eddy currents and Meissner e f f e c t are the mechanisms involved. A d i s s i p a t i o n f a c t o r defined as X"/X' shows this c l e a r l y . I. INTRODUCTION Since the discovery of superconductors of high c r i t i c a l temperature a large world-wide flood of research in superconductivity is taking p l a c e ( l , 2 ) . I t is now well established that these oxide-based ceramic materials such as YIBa^Cu~07+x have an oxigen d e f i c i e n t perovskite structure and that rare earth s u b s t i t u t i o n of Y and Ba does not suppress superconductivity (3). The study of t h e i r magnetic properties are fundamental to understand the behaviour of these mat e r i a l s . The complex magnetic ac s u s c e p t i b i l i t y below and close to Tc appears to be a very useful technique to gain more information as to the nature of the mechanism responsible fo r the behaviour observed. In t h i s paper the imaginary (out of phase) and the real (in phase) parts of complex low f i e l d s u s c e p t i b i l i t y in several samples of the system Y-Ba-Cu-O are reported and t h e i r behaviour analysed in terms of d i s s i p a t i v e effects. Since there have been indications f o r the coexistence of two contributions to the magnetic s u s c e p t i b i l i t y even in single phase compounds i t w i l l be i n t e r e s t i n g to examine the ac f i e l d dependence of both X' and X" as a function of temperature. 2. EXPERIMENTALDETAILS 2.1 Samples preparation The samples investigated have nominal compositions Y-Ba2-Cu3-O7 x (D);Y-Ba-Cu~-O~ (E) and Y2-Ba2-Cu3 -07_ x - (F) and wer~ pPe~ared from the appropiate amounts of Y203 (99.99 %) BaCO3 (99.9) and CuO (99.9). The powders,after mixing and grinding,were heated at 850 eC for I0 hours in a i r and cooled slowly. The gray-black powder was pressed up to I0 bar in a hardened steel dies.The p e l l e t s were annealed f o r 6 h at I000 ~C in a i r and cooled in an oxigen flow of 0.25 I/min. From the sintered p e l l e t s small bars were3cut. The apparent density ranged from 3 to 5 g/cm . The remaining pieces of the p e l l e t s were regrinded in an agate mortar f o r identification.
2.2.Sample I d e n t i f i c a t i o n Part of the remaining powders were used f o r x-ray i d e n t i f i c a t i o n . The d i f f r a c t i o n patterns (determined with Cu K m r a d i a t i o n ) show that a l l the compounds have a common p h a s e , i d e n t i f i e d as the the YIBa^Cu~O~ ,orthorhombic. Sample D is single ph~se~ J / - x Samples E and F gave another phase which appears to be tetragonal. Energy dispersive x-ray analysis have also been performed on sample D and E confirming the s t a r t i n g composition to within I0 %. We expect to r e f i n e these spectra by choosing better standards. The samples were also examined under a scanning electron microscope. Photographs of the fresh fracture of the sintered p e l l e t s were taken to determine the mean grain size of the p a r t i c l e s , which ranged from 2 to I0 microns.
HQc---0.01
F
X"
0.0
A.U.
-0.4
-0.8
-1.2 0
20
40 60 T(K)
00
100
FIGURE 1 Plot of real and imaginary components of ac susceptibility.
+This work has been supported by Swedish agencies NFR and STU. 0921-4534/88/$03.50 @Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Oe f = I L H z
0.4
J.S. Mu~oz et aL / Low fieM A C susceptibifity
1532
2.3.Measurements Since the ac magnetic susceptibility gives more information than resistance measurements as to the behaviour of the superconductors close to the transition temperature,we made a set of measurements using a mutual inductance technique.The susc e p t i b i l i t y was measured upon warming the sample after being cooled in zero magnetic f i e l d . No corrections for the demagnetization factor or Earth magnetic f i e l d was done,so that we have determined the i n i t i a l susceptibility. The experiments were carried out at a fixed frequency (I000 Hz) and varying the rms f i e l d amplitud from O.Ol Oe to I0.0 Oe,at also fixed values. 3. RESULTSAND DISCUSSION The results of the real,X',and imaginary,X", components of the complex susceptibility are shown in Figure I. The values of X" have been scaled by 5and only the data of samples D and F have been plotted for the sake of c l a r i t y . I t is remarkable the sharp transition depicted by sample F from the minimum value of -I.2 to almost O.The peak in X" appears to coincide with the inflexion in X'.This pattern has already been reported (4,5) and a t t r i buted to the presence of two superconductive phases (6). This has also been observed in our single phase sample D. In order to elucidate whether the mechanism involved is the existence of weakly interconnected
SAMPLE E
the lowest temperature reached. Several models have been proposed to explain the properties of the real component X'(T) but l i t t l e has been said about the peak in the temperature dependenceof X"(T).Our data can be exp|ained in terms of a competing interaction where the competing processes are the accelerated eddy currents and the Meissner effect,which lowers the f i e l d amplitud preventing thereby the energy absorption. To see clearly these effects we have made a plot of a Dissipation Factor,the r a t i o X"/X',as a function of temperature shown in Figure 3. SAPPLE E t =1 k Hz
0.3
0.2 D.F. 0.1 e
0,0
f=lkHz
0.20
0 T(K) FIGURE 3
0.15 X 0.10 411 X"
<> 0
0.05
The outcome of the maximumis due to an increase in the supercurrent density (8)that leads to flux expulsion (Meissner).At the same time the eddy current is rapidly increasing becoming dominant as the temperature is lowered. The minimum observed in DF could be the starting of the dominant mechanism,i.e.the eddy currents.
X
~
0
~]
0 0.00 ,,, 0.00 010
!
0.25 Oe
0.50 4 fIX' 0 1 Oe
!
0.75
1.0(1
4. ACKNOWLEDGMENT One of us,A.S.would like to thank co-author Prof.K.V.Rao for his kind hospitality while he was at the Royal Institute of Technology,Stockholmw~ere part of this work was done. 5. REFERENCES
XO.10e
FIGURE 2 grains or Josephson links,as has been suggested (7) we have made a plot of 4~X" against 4RX'. We have observed some deviations.See the squared points. I t seems that the response of our samples give support to the coexistence of two mechanisms or suprconductive components as suggested by Goldfarb et al.(4).The curves in this figure have been obtained by normalizing to l the X' interval from nearly zero at Tc down to the minimum value at
(1) J.G.Bednorz and K.A.MUIler,Z.Phys.B64 (1986)189 (2) M.K.Wu et al.Phys.Rev.Lett.58(1987)908 (3) Gang Xiao et al. Solid State Commun.63(1987) 817. (4) R.B.Goldfarb et al.Cryogenics,27(1987)475. (5) D.-X.Chen et al.Applied Phys.Lett.in print. 6) J.Garcia et al. Journal of Magnetism and Magnetic Mat.69 (1987) 1225. 7)~7~hida and H.Mazaki,Journal App.Phys.52(l~T)] 8) E.Maxwell and M.Strongin,Phys.Rev.Let.lO(1963) 212