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Critical currents in superconducting HoBa2Cu3O7−α films
Physica B 165&166 (1990) 1417-1418 North-Holland
CRITICAL CURRENTS IN SUPERCONDUCTING HoBa2Cu307-a FILMS Da-Ming ZHU Department of Physics, University of Illinois, 1110 W. Green Street, Urbana, Illinois 61801, USA. WeiYan Guan* Texas Center for Superconducting, University of Houston, Houston, Texas 77204, USA. Superconducting thin films of HoBa2Cu307-a have been prepared by magnetron sputtering technique. The x-ray diffraction indicated that the 1-2-3 film is composed of well-oriented grains with Coaxes normal to the Zr02(Y) substrate. We have studied the temperature and field dependence of critical currents dansity of these films. The results can be treated within a mechanism controlled by flux creep.
The flux creep model predict that the temperature and field dependence of critical current density Jc(B,t) of superconductors should vary as Jc(B,t) = Np U (B 0) [1- a(B)t - bt2] 1.07(o B)1/2
(1)
for t=TlTc«1, where Np is the density of pinning centers, U i~ the pinning potential and <1>0 is the flux quantum (1). According to Equation (1), in the fixed magnetic field, Jc(t) follows a power law and at a constant temperature the Jc(B) follows a 8 1/ 2 behavior, if small effects due to the magnetic field dependence of a(B) and U(B,O) are neglected. The film of HoBa2Cu307-a have been deposited onto Zr02(Y) substrates by magnetron sputtering from target of sintered material in an oxygen-argon atmosphere. The films were deposited at substrate temperature 780°C. It was found that the material grows up in the oxygen difficient tetragonal phase. In situ heat treatment at 450°C in pure 02 atmosphere generates the orthorhombic structure. Optimum preparation conditions were a low discharge voltage (",100V) and at as high gas pressures as possible (6x1Q-1 Torr). This is a simple way to circumvent the bombardment effect of negative ions on the film surface during film growth. The films grow highly textured as proved by x-ray diffraction, which indicates that the 1-2-3 films is composed of well-oriented grains with Coaxes normal to the substrate. The films prepared by above technique show the full superconducting transition around 90K with a sharp transition. *On leave from Institute of Physics, Academia Sinica, Beijing, China. 0921-4526/90/$03.50
© 1990 -
The transport crffical currents were measured using a standard four probe technique. The electric contacts were obtained by evaporating gold strips and through indium press. No significant heating effects at the contacts were observed with current below 1.0A. The critical current density was defined as the current caused a 1llV/cm voltage drop across the sample. The film was mounted on a copper block on an insert of a helium cryostat. Temperature was measured by a carbon glass thermometer which was calibrated against a capacitor thermometer in magnetic fields up to 8T. The magnetic field was provided by a superconducting magnet with a maximum field strength of 5T. The film was orientated so that the field direction is parallel to the film surface (B// ab planes). In FIG.1 we give a plot of temperature dependence of normalized critical current JclJc(4.2K) on a log-log scale at H=OG, 396G, 4.6KG, 48KG. The results provide evidence that the critical current of HoBa2Cu307-a film is limited by flux creep. Equation (1) fits Jc(t) data well except near Tc. FIG 2 shows the critical current density of HoBa2Cu307-a film vs t at H=396G. Solid line is fits of Jc data with Equation (1): Jc = Jco(1-t-0.19t2). Mannhart et al (1) determined the critical current density of YBa2Cu307 films from transport measurements within single (001) grains. The results provide evidence that the basal-plane Jc is limited by flux creep. Equation (1) fits Jc(t) data well with a=0.722 and b=0.377 at zero field. However, Senoussi et al (2) carried out magnetic hysteretic measurements of single crystals of YBa2CU307-a. The temperature dependence of the
Elsevier Science Publishers B.V. (North-Holland)
D.-M. Zhu, W.- Y. Guan
1418
Jc(T) was deduced from magnetic hysteretic loops. The magnetic critical current density in the Cu-O basal planes decreases exponentially with temperature. Gyorgy et al (3) reported the Jc(T) also deduced from magnetization measurements for the BiSrCaCuO and YBaCuO samples. The values of Jc at H=9KG decrease exponentially with T. Yunhiu Xu et al (4) obtained an exponential temperature dependence of Jc for REBa2Cu307-a samples. They calculated Jc(T) from ac losses data deduced from the magnetic measurements.
... . x.
'. x.
nism controlled by flux creep. However, all magnetic critical current measurements give an exponential temperature dependence. Normalized transport critical current Jc/Jc(H=O) versus the applied field H are represented on a loglog scale in FIG. 3. The field dependence is JcaH-n with n=1 in the field region between 1DOG and 3KG at moderate temperature T=15,30, 45, 60K. Our results have an exponent (n=1) larger than 0.5, which is deduced from flux creep model (Equation 1). It should be noted that even for single grains the field exponent n is larger than 0.5 (See the raw Jc(S) data in FIG. 2 of Ref 1). Senoussi et al (2) find that the magnetic critical current density in the Cu-O basal planes of YBa2Cu307-a single crystals decreases exponentially with applied field for T~50K. The exponential law of Jc(T) and Jc(S) obtained from magnetic measurements cannot be explained by equations (1) and (2) deduced from the critical state model. We note that during the period of magnetic measurements, part of the magnetization is lost due to thermally activated flux motion. With an increasing field or temperature, Jc decreases faster in the magnetization measurements. Possibly this is the reason for the difference commonly observed between critical currents derived from transport and magnetization measurements. 1
--.----.--...
~--~--~-...--,
~
+
"".,L---~----:':---~---~,oo T(l}
FIG.1
Kol'1llal1zed cr'itlul current densHil!!s Jc/Jc(4.2k)
Ye~US
tm.per.ture
c
"
.1
CJ () b
15K 30K
45K 60K
+ + + +
.010'
B=396G
10'
10'
10'
10'
10'
H(Guass) FIG.3. NOl'lTlllil.e4 critic.1 CUfTern deQsilies J(:/1I:(H-o) versus the: applied m.gnuic rlCld. The solid line n::pll:SeDlS Jc II. H,t,
104 L...Ot
~
__LJ
REFERENCES
.1
I =I (Tc=90K) To AG.2. The temperature dependence of Je of HoBa2Cu307.a film. The Jc(l) can be described by nu~ creep model: Jc=Jco(l+0.1912)
We note that all transport measurements give a temperature dependence of Jc characterized by Equation (1) which can be treated within a mecha-
(1) J. Mannhart et aI., Phys. Rev. Lett. 61 (1988) 2476; M. Tinkham, Helv. Phys. Acta 61 (1988) 443. (2) S. Senoussi et aI., Phys. Rev.. S 37 (1988) 9792. (3) E.M. Gyorgy et aI., Appl, Phys. Lett. 53 (1988) 2223. (4) Yunhui Xu et al., Appl. Phys. Lett 54 (1989) 1699.
CRITICAL CURRENTS IN SUPERCONDUCTING HoBa2Cu307-a FILMS Da-Ming ZHU Department of Physics, University of Illinois, 1110 W. Green Street, Urbana, Illinois 61801, USA. WeiYan Guan* Texas Center for Superconducting, University of Houston, Houston, Texas 77204, USA. Superconducting thin films of HoBa2Cu307-a have been prepared by magnetron sputtering technique. The x-ray diffraction indicated that the 1-2-3 film is composed of well-oriented grains with Coaxes normal to the Zr02(Y) substrate. We have studied the temperature and field dependence of critical currents dansity of these films. The results can be treated within a mechanism controlled by flux creep.
The flux creep model predict that the temperature and field dependence of critical current density Jc(B,t) of superconductors should vary as Jc(B,t) = Np U (B 0) [1- a(B)t - bt2] 1.07(o B)1/2
(1)
for t=TlTc«1, where Np is the density of pinning centers, U i~ the pinning potential and <1>0 is the flux quantum (1). According to Equation (1), in the fixed magnetic field, Jc(t) follows a power law and at a constant temperature the Jc(B) follows a 8 1/ 2 behavior, if small effects due to the magnetic field dependence of a(B) and U(B,O) are neglected. The film of HoBa2Cu307-a have been deposited onto Zr02(Y) substrates by magnetron sputtering from target of sintered material in an oxygen-argon atmosphere. The films were deposited at substrate temperature 780°C. It was found that the material grows up in the oxygen difficient tetragonal phase. In situ heat treatment at 450°C in pure 02 atmosphere generates the orthorhombic structure. Optimum preparation conditions were a low discharge voltage (",100V) and at as high gas pressures as possible (6x1Q-1 Torr). This is a simple way to circumvent the bombardment effect of negative ions on the film surface during film growth. The films grow highly textured as proved by x-ray diffraction, which indicates that the 1-2-3 films is composed of well-oriented grains with Coaxes normal to the substrate. The films prepared by above technique show the full superconducting transition around 90K with a sharp transition. *On leave from Institute of Physics, Academia Sinica, Beijing, China. 0921-4526/90/$03.50
© 1990 -
The transport crffical currents were measured using a standard four probe technique. The electric contacts were obtained by evaporating gold strips and through indium press. No significant heating effects at the contacts were observed with current below 1.0A. The critical current density was defined as the current caused a 1llV/cm voltage drop across the sample. The film was mounted on a copper block on an insert of a helium cryostat. Temperature was measured by a carbon glass thermometer which was calibrated against a capacitor thermometer in magnetic fields up to 8T. The magnetic field was provided by a superconducting magnet with a maximum field strength of 5T. The film was orientated so that the field direction is parallel to the film surface (B// ab planes). In FIG.1 we give a plot of temperature dependence of normalized critical current JclJc(4.2K) on a log-log scale at H=OG, 396G, 4.6KG, 48KG. The results provide evidence that the critical current of HoBa2Cu307-a film is limited by flux creep. Equation (1) fits Jc(t) data well except near Tc. FIG 2 shows the critical current density of HoBa2Cu307-a film vs t at H=396G. Solid line is fits of Jc data with Equation (1): Jc = Jco(1-t-0.19t2). Mannhart et al (1) determined the critical current density of YBa2Cu307 films from transport measurements within single (001) grains. The results provide evidence that the basal-plane Jc is limited by flux creep. Equation (1) fits Jc(t) data well with a=0.722 and b=0.377 at zero field. However, Senoussi et al (2) carried out magnetic hysteretic measurements of single crystals of YBa2CU307-a. The temperature dependence of the
Elsevier Science Publishers B.V. (North-Holland)
D.-M. Zhu, W.- Y. Guan
1418
Jc(T) was deduced from magnetic hysteretic loops. The magnetic critical current density in the Cu-O basal planes decreases exponentially with temperature. Gyorgy et al (3) reported the Jc(T) also deduced from magnetization measurements for the BiSrCaCuO and YBaCuO samples. The values of Jc at H=9KG decrease exponentially with T. Yunhiu Xu et al (4) obtained an exponential temperature dependence of Jc for REBa2Cu307-a samples. They calculated Jc(T) from ac losses data deduced from the magnetic measurements.
... . x.
'. x.
nism controlled by flux creep. However, all magnetic critical current measurements give an exponential temperature dependence. Normalized transport critical current Jc/Jc(H=O) versus the applied field H are represented on a loglog scale in FIG. 3. The field dependence is JcaH-n with n=1 in the field region between 1DOG and 3KG at moderate temperature T=15,30, 45, 60K. Our results have an exponent (n=1) larger than 0.5, which is deduced from flux creep model (Equation 1). It should be noted that even for single grains the field exponent n is larger than 0.5 (See the raw Jc(S) data in FIG. 2 of Ref 1). Senoussi et al (2) find that the magnetic critical current density in the Cu-O basal planes of YBa2Cu307-a single crystals decreases exponentially with applied field for T~50K. The exponential law of Jc(T) and Jc(S) obtained from magnetic measurements cannot be explained by equations (1) and (2) deduced from the critical state model. We note that during the period of magnetic measurements, part of the magnetization is lost due to thermally activated flux motion. With an increasing field or temperature, Jc decreases faster in the magnetization measurements. Possibly this is the reason for the difference commonly observed between critical currents derived from transport and magnetization measurements. 1
--.----.--...
~--~--~-...--,
~
+
"".,L---~----:':---~---~,oo T(l}
FIG.1
Kol'1llal1zed cr'itlul current densHil!!s Jc/Jc(4.2k)
Ye~US
tm.per.ture
c
"
.1
CJ () b
15K 30K
45K 60K
+ + + +
.010'
B=396G
10'
10'
10'
10'
10'
H(Guass) FIG.3. NOl'lTlllil.e4 critic.1 CUfTern deQsilies J(:/1I:(H-o) versus the: applied m.gnuic rlCld. The solid line n::pll:SeDlS Jc II. H,t,
104 L...Ot
~
__LJ
REFERENCES
.1
I =I (Tc=90K) To AG.2. The temperature dependence of Je of HoBa2Cu307.a film. The Jc(l) can be described by nu~ creep model: Jc=Jco(l+0.1912)
We note that all transport measurements give a temperature dependence of Jc characterized by Equation (1) which can be treated within a mecha-
(1) J. Mannhart et aI., Phys. Rev. Lett. 61 (1988) 2476; M. Tinkham, Helv. Phys. Acta 61 (1988) 443. (2) S. Senoussi et aI., Phys. Rev.. S 37 (1988) 9792. (3) E.M. Gyorgy et aI., Appl, Phys. Lett. 53 (1988) 2223. (4) Yunhui Xu et al., Appl. Phys. Lett 54 (1989) 1699.