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Anisotropic magnetization in single crystal Bi2Sr2CaCu2Oy
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Solid State Communications, Vol. 68, No. 3, pp.331-332, Printed in Great Britain.
1988.
0038-1098/88 $3.00 + .00 Pergamon Press plc
ANISOTROPIC MAGNETIZATION IN SINGLE CRYSTAL Bi2Sr2CaCu20y Chang-gen~ Cui, Hong Yin, Jin-long Zhang, Shan-lin Li, Ke-chen Wu, Ji-hong Wang, Yi-feng Yan (Institute of Physics, Academia Sinica, BeiJing, China)
(Received 17 August 1988 by W.Y. Kuan) Superconducting single crystals of BiSrCaCu0 with zero resistance temperature 86K have been prepared and the magnetization of the crystals has been measured in an applied magnetic field for two orientations of the crystals. Results show that the lower critical fields are Hc'/ =2.8KG, Hc,L =0.45KG at temperature 1.5K. An A anisotropy ratio of Hc,II/~{c, is about 6. Jc~'(H=O, 1.5K) is estimated to be 1.5x105A/cm 2.
Introduction Following the initial work by Michel et al}~ ]-Maeda et al~ ~ observed superconductivity up to IO5K in the multiphase Bi-Sr-Oa-Ou-O system. Recently single crystal growth in the same system has been reported by Yan et al~5]. The crystals exhibit superconductivity at 95K with zero resistance at 76.SK.Strong attention has been paid to investigating the crystal preparation and their properties. We have prepared large and high quality plate-like single crystals by improving the heat treatment conditions and measured magnetization in an applied magnetic field for two orientations of the single crystal. From these measurements, we also estimate the lower critical fields and the degree of anisotropy. Experiments Single crystals were grown by self-flux method. The raw materials were prepared first by grounding high purity reagents of CaC05, Sr(N05)2, Bi20 ~ and CuO. The mixture was then calcined in a resistance furnace at 865°C for 7hrs. The sintered powder was regrounded and the powder material was melt at I025oc for 16hrs. After that it was cooled down to 925°C at the rate of 80°C/hr and held for 8hrs, then slowly cooled to 835°C at the rate of 1°C/hr,and finally down to room temparature at the rate of 10°C/hr. In the final products, many platelike single crystals are randomly stacked. The single crystals can be easily cleaved after immersed in alcohol and they have irregular shape with typical dimensions of 5 x 5 m m C a n d O.05mm thick. The superconducting phase has been identified to be Bi2Sr2CaCulOy with an orthorhombic structure of a=5.14~, b=5.42~ and c=30.78~ respectively{4J. The temperature dependence of the resistance of the crystal was measured directly by a standard four-point method. By using silver paste, fine gold wires was affixed as electrod@s onto the surface of a single crystal sheet. The current used was IOOALA in the a-b plane and perpendicular to the c-axis. The temperature was determined by a calibrated Cu thermometer. The magnetization measurements were made on a magnetometer in which the magnetic field can be varied from O-8T continuously, in a bore diameter of 60mm, the inhomogeneity of the field
being less than 10-4. In order to obtain a signal strong enough to be detected, many sheets were adhered together by glue to a cylindrically shaped sample with dimension about#1.6xl.6mm5 for the measurement of magnetization. The temperature was computer controlled at 1.5K within an accuracy of XO.OIK. The experimental error of the magnetization measurement was less than 5x10-4emu. The magnetization as a function of temperature for the sample was also measured to observe DO screening and Meissner effect at a fixed field of 50G by using the same magnetometer. The temperature was determined by a calibrated Pt thermometer. Results and Discussions Fig. 1 is the temperature dependence of resistance of the crystal. It shows that superconducting transition temperature Tc(midpoint of the transition) is 88.5K and the transition width (10-90% of residual resistance) is 3K. The resistance became zer~ at 86K in the region of sensitivity of 2x10-~V. In Fig.2, we present the low field magnetization of the cylindrical sample at 50G with applied field parallel to the c-axle. After the sample had been cooled under zero field, the diamagnetic shielding was measured at a fixed field of 50G with increasing temperature, the diamagnetic signal is near 100°/o. The Meissner flux expulsion was measured at the Same field with decreasing temperature and expressed about 25~ of this diamagnetic shielding. But no significant difference between the flux expulsion and diamagnetic shielding was observed in a region of 6 5 K t o lOOK. The results of the low field magnetization also show that the temperature of superconducting onset is 86K. The magnetization loops from lOG to 60KG for the samples of the single crystal is shown for temperature 1.5K i ~ F i g . 3. We present data for two orientations of the crystal, namely, with the applied field parallel to the c-axis and perpendicular to the c-axis. There is a high degree of symmetry for each of the curves. In each orientation, the lower critical fields, Hc~ and Hc~ , can be identified as the point at which the virgin magnetization curve deviates from a linear dependence on the applied field. As a measure of the anisotropy of the crystal, the lower critical
331
332
ANISOTROPIC MAGNETIZATION IN SINGLE CRYSTAL Bi2Sr2CaCU20y
Vol. 68, No. 3 1.5K o
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Fig. 1. Temperature dependence of resistance of Bi2Sr2CaCu20y crystal.
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Fig.2. Magnetization versus temperature for a sample of single crystals of Bi2Sr2CaOu20y at 50G with field direction parallel to c-axis. Both IX] screening (open circles) and Meissner effect (solid circles) curves are shown.
Fig.5. Magnetization hysteresis ~oops at 1.5K for a sample of single crystals of Bi2sr2CaCu2Oy. The orientation of the applied fiela (H) with respect to the crystallographic c-axis is indicated for each curve. fields obtained directly from the graph are about Hc~'=2.SKG and Hct=O.45KG , yielding an anisotropy ratio of 6.2. Because the shape of the sample can be regarded as a cylinder under a field parallel to c-axis and a collection of many parallel plates under a field perpendicular to c-axis, we take the two demagnetization factors as approximate I. A critical current J~ is induced in the a-b planes when H is parallel to the c-axis. In contrast, a J~ is induced perpendicular to the a-b planes when H is perpendicular to the c-axis. The difference in the magnetude of the width of Is the two loops shows that Jc and J ~ are different with high anisotropy and J~ is estimated to be 1.5x1OSA/cm 2 by the formular Jc=5OM/R. In conclusion, we have investigated the behavior of the single crystals of Bi2Sr2CaCu20y , and shown that the magnetization in the superconducting state is highly anisotropic. The lower critical fields, Hc~'and HcA,are 2.SKG and 0.45KG respectively. References lil C. Michel et al., Zeit. Phys. B68, 421(1987). H. Maeda et al., Jpn Appl. Phys. 27(1988) Y.F. Yah et al., to be appeared in Inter. Modern Phys. Lett.B. (4) Z.H. Mai et al., to be published.
Solid State Communications, Vol. 68, No. 3, pp.331-332, Printed in Great Britain.
1988.
0038-1098/88 $3.00 + .00 Pergamon Press plc
ANISOTROPIC MAGNETIZATION IN SINGLE CRYSTAL Bi2Sr2CaCu20y Chang-gen~ Cui, Hong Yin, Jin-long Zhang, Shan-lin Li, Ke-chen Wu, Ji-hong Wang, Yi-feng Yan (Institute of Physics, Academia Sinica, BeiJing, China)
(Received 17 August 1988 by W.Y. Kuan) Superconducting single crystals of BiSrCaCu0 with zero resistance temperature 86K have been prepared and the magnetization of the crystals has been measured in an applied magnetic field for two orientations of the crystals. Results show that the lower critical fields are Hc'/ =2.8KG, Hc,L =0.45KG at temperature 1.5K. An A anisotropy ratio of Hc,II/~{c, is about 6. Jc~'(H=O, 1.5K) is estimated to be 1.5x105A/cm 2.
Introduction Following the initial work by Michel et al}~ ]-Maeda et al~ ~ observed superconductivity up to IO5K in the multiphase Bi-Sr-Oa-Ou-O system. Recently single crystal growth in the same system has been reported by Yan et al~5]. The crystals exhibit superconductivity at 95K with zero resistance at 76.SK.Strong attention has been paid to investigating the crystal preparation and their properties. We have prepared large and high quality plate-like single crystals by improving the heat treatment conditions and measured magnetization in an applied magnetic field for two orientations of the single crystal. From these measurements, we also estimate the lower critical fields and the degree of anisotropy. Experiments Single crystals were grown by self-flux method. The raw materials were prepared first by grounding high purity reagents of CaC05, Sr(N05)2, Bi20 ~ and CuO. The mixture was then calcined in a resistance furnace at 865°C for 7hrs. The sintered powder was regrounded and the powder material was melt at I025oc for 16hrs. After that it was cooled down to 925°C at the rate of 80°C/hr and held for 8hrs, then slowly cooled to 835°C at the rate of 1°C/hr,and finally down to room temparature at the rate of 10°C/hr. In the final products, many platelike single crystals are randomly stacked. The single crystals can be easily cleaved after immersed in alcohol and they have irregular shape with typical dimensions of 5 x 5 m m C a n d O.05mm thick. The superconducting phase has been identified to be Bi2Sr2CaCulOy with an orthorhombic structure of a=5.14~, b=5.42~ and c=30.78~ respectively{4J. The temperature dependence of the resistance of the crystal was measured directly by a standard four-point method. By using silver paste, fine gold wires was affixed as electrod@s onto the surface of a single crystal sheet. The current used was IOOALA in the a-b plane and perpendicular to the c-axis. The temperature was determined by a calibrated Cu thermometer. The magnetization measurements were made on a magnetometer in which the magnetic field can be varied from O-8T continuously, in a bore diameter of 60mm, the inhomogeneity of the field
being less than 10-4. In order to obtain a signal strong enough to be detected, many sheets were adhered together by glue to a cylindrically shaped sample with dimension about#1.6xl.6mm5 for the measurement of magnetization. The temperature was computer controlled at 1.5K within an accuracy of XO.OIK. The experimental error of the magnetization measurement was less than 5x10-4emu. The magnetization as a function of temperature for the sample was also measured to observe DO screening and Meissner effect at a fixed field of 50G by using the same magnetometer. The temperature was determined by a calibrated Pt thermometer. Results and Discussions Fig. 1 is the temperature dependence of resistance of the crystal. It shows that superconducting transition temperature Tc(midpoint of the transition) is 88.5K and the transition width (10-90% of residual resistance) is 3K. The resistance became zer~ at 86K in the region of sensitivity of 2x10-~V. In Fig.2, we present the low field magnetization of the cylindrical sample at 50G with applied field parallel to the c-axle. After the sample had been cooled under zero field, the diamagnetic shielding was measured at a fixed field of 50G with increasing temperature, the diamagnetic signal is near 100°/o. The Meissner flux expulsion was measured at the Same field with decreasing temperature and expressed about 25~ of this diamagnetic shielding. But no significant difference between the flux expulsion and diamagnetic shielding was observed in a region of 6 5 K t o lOOK. The results of the low field magnetization also show that the temperature of superconducting onset is 86K. The magnetization loops from lOG to 60KG for the samples of the single crystal is shown for temperature 1.5K i ~ F i g . 3. We present data for two orientations of the crystal, namely, with the applied field parallel to the c-axis and perpendicular to the c-axis. There is a high degree of symmetry for each of the curves. In each orientation, the lower critical fields, Hc~ and Hc~ , can be identified as the point at which the virgin magnetization curve deviates from a linear dependence on the applied field. As a measure of the anisotropy of the crystal, the lower critical
331
332
ANISOTROPIC MAGNETIZATION IN SINGLE CRYSTAL Bi2Sr2CaCU20y
Vol. 68, No. 3 1.5K o
o HIIC
0
lOG
•
04
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Fig. 1. Temperature dependence of resistance of Bi2Sr2CaCu20y crystal.
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Fig.2. Magnetization versus temperature for a sample of single crystals of Bi2Sr2CaOu20y at 50G with field direction parallel to c-axis. Both IX] screening (open circles) and Meissner effect (solid circles) curves are shown.
Fig.5. Magnetization hysteresis ~oops at 1.5K for a sample of single crystals of Bi2sr2CaCu2Oy. The orientation of the applied fiela (H) with respect to the crystallographic c-axis is indicated for each curve. fields obtained directly from the graph are about Hc~'=2.SKG and Hct=O.45KG , yielding an anisotropy ratio of 6.2. Because the shape of the sample can be regarded as a cylinder under a field parallel to c-axis and a collection of many parallel plates under a field perpendicular to c-axis, we take the two demagnetization factors as approximate I. A critical current J~ is induced in the a-b planes when H is parallel to the c-axis. In contrast, a J~ is induced perpendicular to the a-b planes when H is perpendicular to the c-axis. The difference in the magnetude of the width of Is the two loops shows that Jc and J ~ are different with high anisotropy and J~ is estimated to be 1.5x1OSA/cm 2 by the formular Jc=5OM/R. In conclusion, we have investigated the behavior of the single crystals of Bi2Sr2CaCu20y , and shown that the magnetization in the superconducting state is highly anisotropic. The lower critical fields, Hc~'and HcA,are 2.SKG and 0.45KG respectively. References lil C. Michel et al., Zeit. Phys. B68, 421(1987). H. Maeda et al., Jpn Appl. Phys. 27(1988) Y.F. Yah et al., to be appeared in Inter. Modern Phys. Lett.B. (4) Z.H. Mai et al., to be published.