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Magnetic properties of high Tc superconductors
Physica C 153-155 (1988) 1535-1536 North-Holland, Amsterdam
MAGNETIC PROPERTIES OF HIGH Tc SUPERCONDUCTORS T.C. SHIELDS, I.R. HARRIS and J.S. ABELL Department of Metallurgy and Materials, University of Birmingham, Birmingham, England B 15 2TT Magnetic susceptibility and oxygen desorption have been measured at elevated temperatures in a Curie-Faraday microbalance. Room temperature susceptibility and magnetisation at 77K are found to be sensitive to thermal history. 1.Introduction It has been suggested [1] that the normal state magnetic susceptibility of the Yttrium Barium Copper Oxide type superconductors is sensitive to the microscopic oxygen configuration. That is to say, both the oxygen stoichometry and the relative occupancy of the oxygen sites influence the magnetic susceptibility. Preliminary studies of the room temperature susceptibility of various samples (Table 1) show that the normal state susceptibility is very sensitive to thermal history and preparation route. An attempt has been made in this study to investigate the normal state magnetic susceptibility at elevated temperatures as the oxygen stoichometry decreases. Magnetization curves at 77K have also been obtained to try and ascertain the influence of oxygen content on the low temperature magnetic properties. TABLE1 HEAT T R E A T M E N T SUSCEPTIBILITY (XIOE-~ ~mu/g)f+A(I~05) Air reacted 40 hours 0.798 Oxygen reacted 40 hours 0.604 Oxygen sintered 8hours at 950°C 0.707 Quenched from 950°C 0.618 Quenched from 650°C 0.626 Quenched from 220°C 0.685 Liquid sintered at 1000°C 0.964 2.Experimental The samples were prepared by reacting the appropriate amounts of Yttrium Oxide, Barium Carbonate and Copper Oxide together in air for 24 hours at 950°C after ball milling in cyclohexane for half an hour, and then air quenching followed by compaction. The sintering procedure consisted of two stages. Firstly liquid sintering in air at 1000°C for 24 hours was performed followed by an air quench. Finally the discs were oxygen annealed at 950°C for 8 hours and cooled at a rate of 2°/rain to room temperature. The high temperature susceptibility data and the magnetization curves at 77K were obtained using a Curie-Faraday microbalance. Fields of up to 4.5 KGauss were provided by an electromagnet, and a non inductively wound furnace was employed to heat the sample to a maximum of 850°C. The whole system could be evacuated, filled with helium and the sample surrounded by a liquid nitrogen bath to cool to 77K. 3.Results The high temperature behaviour of two samples was studied. Sample A was heated in air continuously at a rate 0921-4534/88/$03.50 ©ElsevierSciencePublishersB.V. (North-HollandPhysicsPublishingDivision)
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Temperature (°C) FIGURE 1. Weight loss versus temperature for samples A and B. of 10°/min and cooled at the same rate. Sample B was heated in steps of approximately 40°C to a maximum of 850°C, with the heating rate between each step being 10°/min and cooled at a rate of 2°/rain between steps. The weight change of both samples was monitored by means of the microbalance. The high temperature susceptibility values of sample B were also obtained, and at each temperature before the measurements were taken the sample was allowed to attain equilibrium . This was achieved by waiting for the weight change and hence the oxygen content to remain constant. The time for this to occur was typically half an hour. Plots of dw/w (dw=weight change of sample; w=original weight of sample) versus temperature for samples A and B are shown in figure 1. On heating both curves are seen to exhibit similar behaviour. The samples start to show a weight decrease due to the loss of oxygen at about 300°C and the gradient of the curves becomes constant between 500°C and 550°C. There is however a change in the slope of the graphs which occurs at 675°C. This temperature corresponds to the orthorhombic to tetragonal phase transition which has been observed, in air by other workers [2,3,4,5] to be at roughly this temperature. The apparent large increase in weight loss at 800°C for sample A is due to the sample being left at this temperature for half an hour in order for its oxygen content to reach a constant value. On cooling, sample B shows a slight deviation from the heating curve and the final oxygen content is reduced from the initial value. However the curve for sample A reveals only a fractional reabsorption of oxygen because of its large cooling rate (10°/min). This sample was also observed to be non-superconducting at 77K from the magnetization measurements.
1536
T.C. Shields et al. / Magnetic properties of high-Tc superconductors
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Temperature (°C) FIGURE 2. Susceptibility versus temperature for sample B, also showing the room temperature value of sample A after heating (+). The high temperature susceptibility data of sample B is shown in figure 2 along with the room temperature value of sample A after heating. The initial values are generally higher than those of previous samples (see Table 1) and this could be due to the presence of a second phase at the grain boundaries introduced during the liquid sintering process. The initial part of the curve on heating shows a sharp decrease in susceptibility which reaches a uniform slope at the point at which oxygen starts to leave the material (work is currently under way to determine the susceptibilty behaviour between room temperature and Tc).It should be noted that there is no change in slope at the orthorhombic to tetragonal transition temperature. On cooling, the relationship shows no deviation from the heating curve until about 450°C (despite the deviation in tl weight loss curve of Figure 1) when there is a negative deviation. This reduced normal state susceptibility could be due to the decrease in the oxygen content compared to the initial state, as there is also a further decrease in the room temperature susceptibility of sample A (quickly cooled). The magnetization curves for sample B before and after heating are shown in figure 3, and both show typical imperfect type 2 behaviour as observed by other workers on the same material [6,7,8]. The irreversible nature of the curves is due to flux trapping on the defects present in the material. The reduction in the magnitude of the magnetization could again be caused by the decrease in oxygen content which appears to result in an overall reduction in the diamagnetism of the material.
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Applied Field (KGauss) FIGURE3. Magnetization curves at 77K for sample B. 4.CONCLUSION The normal state magnetic susceptibility of the yttrium barium copper oxide superconductors appears to be sensitive to the oxygen content. There is also evidence for the influence of oxygen content on the magnetization behaviour of the samples. Further work is needed to investigate the behaviour at lower temperatures and to produce samples of varying oxygen stoichometry in order to study their high temperature and low temperature magnetization behaviour. It is also intended to investigate the effect of gases such as nitrogen on the superconducting properties of these materials, and the Curie-Faraday balance provides an ideal medium for this, both at high and low temperatures. REFERENCES (1) R.J.Cava et al. Nature Vol 329 (1987) p423. (2) J.D.Jorgensen et al. Phys. Rev. B, Vol 36 no 7 (1987) p3608. (3) E.Takayama-Muromachi et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L665. (4) Y.K.Huang et al. Jnl. Less Common Metals. Vol 136 (1987) p169. (5) K.Yukino et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L869. (6) P.Pureur and J.Schaf. Jnl. Magn. Magnetic. Mat. Vol 69 (1987) L215. (7) T.Hioki et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L636. (8) M.Rosenberg et al. Z. Phys. B. Vol 69 (1987) p151.
MAGNETIC PROPERTIES OF HIGH Tc SUPERCONDUCTORS T.C. SHIELDS, I.R. HARRIS and J.S. ABELL Department of Metallurgy and Materials, University of Birmingham, Birmingham, England B 15 2TT Magnetic susceptibility and oxygen desorption have been measured at elevated temperatures in a Curie-Faraday microbalance. Room temperature susceptibility and magnetisation at 77K are found to be sensitive to thermal history. 1.Introduction It has been suggested [1] that the normal state magnetic susceptibility of the Yttrium Barium Copper Oxide type superconductors is sensitive to the microscopic oxygen configuration. That is to say, both the oxygen stoichometry and the relative occupancy of the oxygen sites influence the magnetic susceptibility. Preliminary studies of the room temperature susceptibility of various samples (Table 1) show that the normal state susceptibility is very sensitive to thermal history and preparation route. An attempt has been made in this study to investigate the normal state magnetic susceptibility at elevated temperatures as the oxygen stoichometry decreases. Magnetization curves at 77K have also been obtained to try and ascertain the influence of oxygen content on the low temperature magnetic properties. TABLE1 HEAT T R E A T M E N T SUSCEPTIBILITY (XIOE-~ ~mu/g)f+A(I~05) Air reacted 40 hours 0.798 Oxygen reacted 40 hours 0.604 Oxygen sintered 8hours at 950°C 0.707 Quenched from 950°C 0.618 Quenched from 650°C 0.626 Quenched from 220°C 0.685 Liquid sintered at 1000°C 0.964 2.Experimental The samples were prepared by reacting the appropriate amounts of Yttrium Oxide, Barium Carbonate and Copper Oxide together in air for 24 hours at 950°C after ball milling in cyclohexane for half an hour, and then air quenching followed by compaction. The sintering procedure consisted of two stages. Firstly liquid sintering in air at 1000°C for 24 hours was performed followed by an air quench. Finally the discs were oxygen annealed at 950°C for 8 hours and cooled at a rate of 2°/rain to room temperature. The high temperature susceptibility data and the magnetization curves at 77K were obtained using a Curie-Faraday microbalance. Fields of up to 4.5 KGauss were provided by an electromagnet, and a non inductively wound furnace was employed to heat the sample to a maximum of 850°C. The whole system could be evacuated, filled with helium and the sample surrounded by a liquid nitrogen bath to cool to 77K. 3.Results The high temperature behaviour of two samples was studied. Sample A was heated in air continuously at a rate 0921-4534/88/$03.50 ©ElsevierSciencePublishersB.V. (North-HollandPhysicsPublishingDivision)
200
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~: 100
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200
400
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800
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Temperature (°C) FIGURE 1. Weight loss versus temperature for samples A and B. of 10°/min and cooled at the same rate. Sample B was heated in steps of approximately 40°C to a maximum of 850°C, with the heating rate between each step being 10°/min and cooled at a rate of 2°/rain between steps. The weight change of both samples was monitored by means of the microbalance. The high temperature susceptibility values of sample B were also obtained, and at each temperature before the measurements were taken the sample was allowed to attain equilibrium . This was achieved by waiting for the weight change and hence the oxygen content to remain constant. The time for this to occur was typically half an hour. Plots of dw/w (dw=weight change of sample; w=original weight of sample) versus temperature for samples A and B are shown in figure 1. On heating both curves are seen to exhibit similar behaviour. The samples start to show a weight decrease due to the loss of oxygen at about 300°C and the gradient of the curves becomes constant between 500°C and 550°C. There is however a change in the slope of the graphs which occurs at 675°C. This temperature corresponds to the orthorhombic to tetragonal phase transition which has been observed, in air by other workers [2,3,4,5] to be at roughly this temperature. The apparent large increase in weight loss at 800°C for sample A is due to the sample being left at this temperature for half an hour in order for its oxygen content to reach a constant value. On cooling, sample B shows a slight deviation from the heating curve and the final oxygen content is reduced from the initial value. However the curve for sample A reveals only a fractional reabsorption of oxygen because of its large cooling rate (10°/min). This sample was also observed to be non-superconducting at 77K from the magnetization measurements.
1536
T.C. Shields et al. / Magnetic properties of high-Tc superconductors
800
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heating AFTER
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RUN
•
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mmmm
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0.4[ r.~
0.3 " 0
'
'
'
'
200
400
600
800
0
1000
Temperature (°C) FIGURE 2. Susceptibility versus temperature for sample B, also showing the room temperature value of sample A after heating (+). The high temperature susceptibility data of sample B is shown in figure 2 along with the room temperature value of sample A after heating. The initial values are generally higher than those of previous samples (see Table 1) and this could be due to the presence of a second phase at the grain boundaries introduced during the liquid sintering process. The initial part of the curve on heating shows a sharp decrease in susceptibility which reaches a uniform slope at the point at which oxygen starts to leave the material (work is currently under way to determine the susceptibilty behaviour between room temperature and Tc).It should be noted that there is no change in slope at the orthorhombic to tetragonal transition temperature. On cooling, the relationship shows no deviation from the heating curve until about 450°C (despite the deviation in tl weight loss curve of Figure 1) when there is a negative deviation. This reduced normal state susceptibility could be due to the decrease in the oxygen content compared to the initial state, as there is also a further decrease in the room temperature susceptibility of sample A (quickly cooled). The magnetization curves for sample B before and after heating are shown in figure 3, and both show typical imperfect type 2 behaviour as observed by other workers on the same material [6,7,8]. The irreversible nature of the curves is due to flux trapping on the defects present in the material. The reduction in the magnitude of the magnetization could again be caused by the decrease in oxygen content which appears to result in an overall reduction in the diamagnetism of the material.
•
0
a
a
a
•
•
•
•
'
!
!
m
1
2
3
4
5
Applied Field (KGauss) FIGURE3. Magnetization curves at 77K for sample B. 4.CONCLUSION The normal state magnetic susceptibility of the yttrium barium copper oxide superconductors appears to be sensitive to the oxygen content. There is also evidence for the influence of oxygen content on the magnetization behaviour of the samples. Further work is needed to investigate the behaviour at lower temperatures and to produce samples of varying oxygen stoichometry in order to study their high temperature and low temperature magnetization behaviour. It is also intended to investigate the effect of gases such as nitrogen on the superconducting properties of these materials, and the Curie-Faraday balance provides an ideal medium for this, both at high and low temperatures. REFERENCES (1) R.J.Cava et al. Nature Vol 329 (1987) p423. (2) J.D.Jorgensen et al. Phys. Rev. B, Vol 36 no 7 (1987) p3608. (3) E.Takayama-Muromachi et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L665. (4) Y.K.Huang et al. Jnl. Less Common Metals. Vol 136 (1987) p169. (5) K.Yukino et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L869. (6) P.Pureur and J.Schaf. Jnl. Magn. Magnetic. Mat. Vol 69 (1987) L215. (7) T.Hioki et al. Jap. Jnl. App. Phys. Vol 26 no 5 (1987) L636. (8) M.Rosenberg et al. Z. Phys. B. Vol 69 (1987) p151.