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Scaling of the hysteretic magnetization in epitaxial Bi2Sr2CaCu2Ox films
Physica C 235-240 (1994) 2843-2844
PHYSICA [~
North-Holland
Scaling of the Hysteretic Magnetization in Epitaxial Bi2Sr2CaCu2Ox Films. S.F.Kim, S.Labdi, Z.Z.Li, and H.Raffy. Laboratoire de Physique des Solides, Bat. 510, Universit6 Paris-Sud, 91405 Orsay, France. Scaling behaviour of the hysteretic magnetization, or associated with it the critical current density, of high quality epitaxial Bi2Sr2CaCu20 x thin films was studied in the temperature range 5-45K and over three decades of the magnetic field H, 50G-50kG. Our study shows that there exists at least three different regions in the magnetic phase diagram with different scaling behaviour.
Recent studies of high-T c superconductors have demonstrated that in some cases the magnetic field dependence of the flux pinning force, or the critical current, determined at different temperatures can be scaled to a single curve [1-3]. In this paper we present data on the m a g n e t i z a t i o n m e a s u r e m e n t s of epitaxial Bi2Sr2CaCu2Ox thin films. Since the width of the magnetization curve can be associated with the critical current density jc through the Bean model, the shape of the curves thus p r o v i d e s a direct v i s u a l i z a t i o n of the magnetic field dependence of jc. Our extensive study over a wide region of the magnetic field reveals the presence of at least three regions in the magnetic phase diagram H-T with different scaling behaviour. The experiments were performed on caxis o r i e n t e d epitaxial thin films of Bi2Sr2CaCu20 x prepared by an in-situ growth technique (see, for instance, [4]). The high quality of the samples were confirmed by Xray diffraction studies, the energy dispersive X-ray spectroscopy and the scanning electron microscopy. Several thin films were used in the magnetization measurements. All films s h o w sharp s u p e r c o n d u c t i n g transition measured by a standard dc resistive four probe method with Tc'~80K. Magnetization m e a s u r e m e n t s M(H) w e r e p e r f o r m e d by u s i n g a SQUID magnetometer in magnetic fields up to 50 kG 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights SSDI 0921-4534(94)01987-8
and in temperature range 5-45K for H applied perpendicular to the film plane. Typical magnetization curves of an epitaxial B i 2 S r 2 C a C u 2 O x thin film at different temperatures are presented in Fig.1. It should be noted that the shape of the magnetization curves for the thin films are quite regular and monotonous, and does not show an anomalous behaviour with a peakeffect, as it is often observed in single crystals of Bi2Sr2CaCu20 x [5,6]. The detailed analysis shows that the behaviour of jc(H,T) is different at high and low temperature regions. The characteristic temperature which divides the two regions is about T O -- 25K. At low temperatures, i.e. at T < T o , the experimental data (with the exception of low field data H<2kG, see below) can be scaled to a single curve only by using 0.l
'
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1 ~ 6 K
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-20
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'
0 2'0 40 60 H(kG) FIGURE 1. Typical magnetization curves at selected temperatures. reserved.
-.40
2844
S.E Kim et al./Phvsica C235-240 (1994) 2843 2844
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0 1 H/H* FIGURE 2. Universal behaviour of the magnetization at temperatures below 25K. r e d u c e d values of field h = H / H * , i.e. the critical current density can be represented in the following form, jc(T,H)=I3f(H/H* ), where 13is a constant, f(h) is some function, and H* is a scaling parameter. This striking result is presented in Fig.2. The function f(h) can be approximated by f(h)=h'0"5(1-h) 2. As can be seen, diverging values for jc in the limit of zero field are given by the above expression. This is of course not the case, and the experimental data display that below H<2kG the critical current shows saturation, i.e. jc(H,T)=jc(0,T ). For high temperature region T>25K we find the scaling behaviour for the reduced values of magnetization m=AM/AM(H=0) and field h = H / H * (Fig.3). This m e a n s that for the critical current density we have, jc(H,T)=jc(0,T)g(H/H*), w h e r e g(h)-exp(c~h). The scaling p a r a m e t e r H* can be associated with the irreversibility field, i.e. the field at which jc diminishes to zero. We find that H*=Hoexp(-T/To), where To~9K for all the samples. The comparison of these data with that measured on the same kind of films by the t r a n s p o r t m e t h o d [4] indicates the importance of strong thermal fluctuation effects, such as creep effect. Indeed, the consequence of the fast relaxation is that the actual current as measured by the width of the magnetic hysteresis loop fails well below its critical value. However at low temperatures
0
i
i
0.5
1
H/H*
•
1.5
FIGURE 3. Scaling of the reduced magnetization versus the reduced field at temperatures above 25K. the thermally activated flux creep appears to play an important role only at high magnetic fields, n a m e l y near the irreversibility field H*, while at T>25K the thermal activation becomes dominant over the whole magnetic field region. REFERENCES
1. J.D.Hettinger, A.G.Swanson, W.J.Skocpol, J.S.Brooks, J.M. Graybeal, P.M. Mankiewich, R.E. Howard, B.L. Straughn, and E.G.Burkhardt, Phys.Rev.Lett. 62 (1989) 2044. 2. S.Labdi, H.Raffy, O . L a b o r d e , a n d P.Monceau, Physica C 197 (1992) 274. 3. L.Civale, M.W.McElfresh, A.D.Marwick, F.Holtzberg, C.Feild, J.R.Thompson, and D.K.Christen, Phys.Rev.B 43 (1991) 13732. 4. H.Raffy, S.Labdi, Z.Z.Li, H.Rifi, S.F.Kim, S.Megtert, O.Laborde, and P.Monceau, P r o c e e d i n g s of the 7th I n t e r n a t i o n a l Workshop on Critical Currents, Alpbach (Austria), ed. H . W . W e b e r , World Scientific Publishers 1994 (to be published). 5. K.Kadowaki and T.Mochiku, Physica C 195 (1992) 127. 6. G.Yang, P.Shang, S.D.Sutton, I.P.Jones, J.S.Abell, and C.E.Gough, Phys.Rev.B, 48 (1993) 4054. In this paper the authors show t h a t the a n o m a l o u s m a g n e t i z a t i o n disappears in B i 2 S r 2 C a C u 2 O x single crystals after a vacuum annealing.
PHYSICA [~
North-Holland
Scaling of the Hysteretic Magnetization in Epitaxial Bi2Sr2CaCu2Ox Films. S.F.Kim, S.Labdi, Z.Z.Li, and H.Raffy. Laboratoire de Physique des Solides, Bat. 510, Universit6 Paris-Sud, 91405 Orsay, France. Scaling behaviour of the hysteretic magnetization, or associated with it the critical current density, of high quality epitaxial Bi2Sr2CaCu20 x thin films was studied in the temperature range 5-45K and over three decades of the magnetic field H, 50G-50kG. Our study shows that there exists at least three different regions in the magnetic phase diagram with different scaling behaviour.
Recent studies of high-T c superconductors have demonstrated that in some cases the magnetic field dependence of the flux pinning force, or the critical current, determined at different temperatures can be scaled to a single curve [1-3]. In this paper we present data on the m a g n e t i z a t i o n m e a s u r e m e n t s of epitaxial Bi2Sr2CaCu2Ox thin films. Since the width of the magnetization curve can be associated with the critical current density jc through the Bean model, the shape of the curves thus p r o v i d e s a direct v i s u a l i z a t i o n of the magnetic field dependence of jc. Our extensive study over a wide region of the magnetic field reveals the presence of at least three regions in the magnetic phase diagram H-T with different scaling behaviour. The experiments were performed on caxis o r i e n t e d epitaxial thin films of Bi2Sr2CaCu20 x prepared by an in-situ growth technique (see, for instance, [4]). The high quality of the samples were confirmed by Xray diffraction studies, the energy dispersive X-ray spectroscopy and the scanning electron microscopy. Several thin films were used in the magnetization measurements. All films s h o w sharp s u p e r c o n d u c t i n g transition measured by a standard dc resistive four probe method with Tc'~80K. Magnetization m e a s u r e m e n t s M(H) w e r e p e r f o r m e d by u s i n g a SQUID magnetometer in magnetic fields up to 50 kG 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights SSDI 0921-4534(94)01987-8
and in temperature range 5-45K for H applied perpendicular to the film plane. Typical magnetization curves of an epitaxial B i 2 S r 2 C a C u 2 O x thin film at different temperatures are presented in Fig.1. It should be noted that the shape of the magnetization curves for the thin films are quite regular and monotonous, and does not show an anomalous behaviour with a peakeffect, as it is often observed in single crystals of Bi2Sr2CaCu20 x [5,6]. The detailed analysis shows that the behaviour of jc(H,T) is different at high and low temperature regions. The characteristic temperature which divides the two regions is about T O -- 25K. At low temperatures, i.e. at T < T o , the experimental data (with the exception of low field data H<2kG, see below) can be scaled to a single curve only by using 0.l
'
'
0.05
'
]A'~
'
1 ~ 6 K
I
I--~-25K I
-0.05
-0.1
-60
'
'
-20
'
'
0 2'0 40 60 H(kG) FIGURE 1. Typical magnetization curves at selected temperatures. reserved.
-.40
2844
S.E Kim et al./Phvsica C235-240 (1994) 2843 2844
0.1
i
'
'o
'
,,
0.05
:. ~*~
0
.
.
.
-0.05
.
I
: ?0K o
15K
•
20K
o
~ ~
.
0.1
° 35K ~a~
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~0.01
"-"
-2
o
'
'
40K
fi%$
8 -0.1
30K
X
'
-1
0 1 H/H* FIGURE 2. Universal behaviour of the magnetization at temperatures below 25K. r e d u c e d values of field h = H / H * , i.e. the critical current density can be represented in the following form, jc(T,H)=I3f(H/H* ), where 13is a constant, f(h) is some function, and H* is a scaling parameter. This striking result is presented in Fig.2. The function f(h) can be approximated by f(h)=h'0"5(1-h) 2. As can be seen, diverging values for jc in the limit of zero field are given by the above expression. This is of course not the case, and the experimental data display that below H<2kG the critical current shows saturation, i.e. jc(H,T)=jc(0,T ). For high temperature region T>25K we find the scaling behaviour for the reduced values of magnetization m=AM/AM(H=0) and field h = H / H * (Fig.3). This m e a n s that for the critical current density we have, jc(H,T)=jc(0,T)g(H/H*), w h e r e g(h)-exp(c~h). The scaling p a r a m e t e r H* can be associated with the irreversibility field, i.e. the field at which jc diminishes to zero. We find that H*=Hoexp(-T/To), where To~9K for all the samples. The comparison of these data with that measured on the same kind of films by the t r a n s p o r t m e t h o d [4] indicates the importance of strong thermal fluctuation effects, such as creep effect. Indeed, the consequence of the fast relaxation is that the actual current as measured by the width of the magnetic hysteresis loop fails well below its critical value. However at low temperatures
0
i
i
0.5
1
H/H*
•
1.5
FIGURE 3. Scaling of the reduced magnetization versus the reduced field at temperatures above 25K. the thermally activated flux creep appears to play an important role only at high magnetic fields, n a m e l y near the irreversibility field H*, while at T>25K the thermal activation becomes dominant over the whole magnetic field region. REFERENCES
1. J.D.Hettinger, A.G.Swanson, W.J.Skocpol, J.S.Brooks, J.M. Graybeal, P.M. Mankiewich, R.E. Howard, B.L. Straughn, and E.G.Burkhardt, Phys.Rev.Lett. 62 (1989) 2044. 2. S.Labdi, H.Raffy, O . L a b o r d e , a n d P.Monceau, Physica C 197 (1992) 274. 3. L.Civale, M.W.McElfresh, A.D.Marwick, F.Holtzberg, C.Feild, J.R.Thompson, and D.K.Christen, Phys.Rev.B 43 (1991) 13732. 4. H.Raffy, S.Labdi, Z.Z.Li, H.Rifi, S.F.Kim, S.Megtert, O.Laborde, and P.Monceau, P r o c e e d i n g s of the 7th I n t e r n a t i o n a l Workshop on Critical Currents, Alpbach (Austria), ed. H . W . W e b e r , World Scientific Publishers 1994 (to be published). 5. K.Kadowaki and T.Mochiku, Physica C 195 (1992) 127. 6. G.Yang, P.Shang, S.D.Sutton, I.P.Jones, J.S.Abell, and C.E.Gough, Phys.Rev.B, 48 (1993) 4054. In this paper the authors show t h a t the a n o m a l o u s m a g n e t i z a t i o n disappears in B i 2 S r 2 C a C u 2 O x single crystals after a vacuum annealing.