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Effect of sample temperature on the irradiation effects of energetic particles in the YBCO system
Vacuum/volume
46/number 3lpages 227 to 228/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00
Pergamon 0042-207X(94100050-6
Effect of sample temperature on the irradiation of energetic particles in the YBCO system S K Das, P K Ashwini
Kumar and S K Sarkar, National
Physical
Laboratory,
effects
New De/hi 1’10072, lndia
and D Kanjilal and L Senapati, received for publication
Nuclear
Science Center, New Delhi 110067, India
8 June 1994
‘“0 ions at 75 Me\/ incident energy have been used to study the irradiation effects on the normal state resistance variation with fluence. The studies conducted include the measurement of resistance variation with dose due to the ion used. The samples were the bulk sintered pellets of YBa,Cu,O,_, maintained at normal temperature (T - 300 K) and low temperature fT - 100 K). In the case of a sample irradiated at normal temperature there is a continuous decrease in resistance with dose, while in the other case there is a minimal change in the resistance vs fluence curve. These experimental results reveal that the sample temperature has a special effect on the damage mechanism.
Introduction The discovery of superconductivity above about 50 K in some ceramic cuprates’ led to a spurt2m4in research into these materials. Soon after, attempts were made to see the effects of energetic radiations on the normal and superconducting properties. With regard to this aspect, it has been well established’ that whenever an ion traverses through matter it loses its energy via two processes; namely, the first one by the ion-target electron interaction, called the electronic energy loss (Se), and the other one due to ion-target atom interaction, called the nuclear energy loss (Sn). With regard to the particular case of ionic irradiations on superconductor like YBa$u,O,, it has been shown that there is a threshold value for Se, above which the material undergoes a radical change in the properties, i.e. the normal state resistance may increase very sharply, the ion on its passage through the material leaves a permanent track and, also, the superconductor shows a sudden deterioration in its properties. While most of the experiments reported to date have focused attention on the iontarget interaction, with the sample at normal temperature or at low temperature, the present paper describes the results of work done with IhO on a YBaQr,O, bulk sample held at ambient temperature and also at low temperature (- 100 K) but greater than the T,. The results are explained on the basis of the effect of sample temperature on the role of ion irradiation with regard to damage creation.
Experimental Bulk samples of YBa,Cu,O, ~, were made by the standard solid state reaction techniqueh. The starting materials were YIO1,
BaC03 and CuO. The materials of proper stoichiometry were thoroughly mixed, ground and pelletised to circular discs measuring about 25 mm in diameter. The calcination was done at 900 C and the sintering was done at 930 C. The pellets were checked to be superconducting at about 90 K. For irradiation experiments the samples were cut into rectangular bars measuring 10 x 3 x 2 mm3. The samples were irradiated by 75 MeV oxygen beam from a 16 MV Tandem Accelerator (PELLETRON)’ at the Nuclear Science Center. The samples mounted on the glass epoxy were fixed on a cold finger cooled by liquid nitrogen. The glass epoxy mount ensured that the temperature of the material was above the transition temperature. The vacuum in the chamber during irradiation was maintained at about 5 x lo-’ mbar. A standard four probe technique using constant current source and nanovoltmeter was employed to measure the variation of resistance with fluence in situ. The data were taken at about 5 min after stopping the beam, so that the short lived transient effects were equilibrated. The ion fluence was varied between 1 x 10” cm-’ and 1 x 10” cm-‘. The fluence of the beam was determined by counting the number of particles bombarding the sample using a current integrator and a counter. The secondary electrons coming out of the sample were suppressed by an electrostatic suppressor. Results and discussions Figures 1 and 2 show the variation of normalised resistance (R,) with dose for the sample kept at ambient temperature and at low temperature. respectively. In both the cases the resistance has been decreased from its initial value as a result of irradiation. However, the striking difference between the two curves are : 227
S K Das er al: Irradiation
in the
Unirrodioted
YBCO system value of the resistance = 119 m. ohm
FLUENCE (/cm.cm)
Figure 1. Variation of normalised resistance oxygen for target at ambient temperature.
0.940
-
with fluence
of 75 MeV
value of the resistance = 165.4m.ohm
Unirradiated
a
z e
0.925
although the projectile, i.e. 0 ions. interacts with all the atoms in the target, namely Y, Ba, Cu and 0, during its passage, the probability of 0 ions getting displaced is highest because 0 is the lightest atom amongst the others and, also, they are loosely bound in the lattice compared to the other atoms. Since it is believed that an ordered arrangement of oxygen vacancy is crucial for even normal state metallic conductivity’. then from the previous argument the decrease in resistance strongly suggests an improvement of the ordered arrangement of oxygen vacancy as the major effect of the oxygen irradiation in our YBaQ,O, ~, bulk sample. Also, the possible incorporation of oxygen in the YBaQ,O, lattice at the end of the projectile’s range might be a factor in reducing the resistance of the samples from its initial value. At this point, it is worthwhile to compare the nature of the two variations in R,, vs fluence at two temperatures. The larger value of AR/R,,,,, for low temperature irradiation shows that there is a favorable situation for improving the vacancy ordering of the oxygen sites due to ion irradiation and at the same time freezing this situation, thus retaining the ordered vacancy state ; this is not the case at room temperature irradiation, since a part of the ordered vacancy arrangement due to irradiation again gets disordered due to a larger amplitude of lattice vibration. Secondly, the change in resistance due to irradiation at low temperature shows a very marginal increase of R,, at higher Auences. leading to the creation of other types of defects (point defects and cascades etc.). which have a deteriordtory effect on conductivity. From the nature of the two curves. it seems that up to a fluence of about IO” cm-’ the vacancy ordering phenomenon prevails over the other types of effects. The result of the present experiment may be compared with that of the work on a thin film irradiated by 25 MeV ‘“0 ions at 100 K (ref 6). In the case of the thin film. the resistance of the irradiated sample is always more than the unirradiated value, while in the present case of the bulk sample it is always less. This anomaly seems to be due to the fact that bulk samples used in the present study may have more intrinsic imperfections compared to the thin film. Acknowledgements
0
0920
I
-
10'2
I
IO'3
IO
I
I4
lOI3
FLUENCE (/cm.cm)
Figure 2. Variation of normalised oxygen at low temperature.
(9
for
low
temperature
resistance
irradiation
with fluence
the
resistance
of 75 MeV
shows
a
room temperature irradiation the nature of the resistance wrt fluence shows an initial flat region at lower doses, followed by a gradual decrease at higher doses ; (ii) the normalised change in resistance (AR/R,,,,,) is much more in the case of low temperature irradiation than the case at room temperature irradiation (at 1 x 10” cm-’ fluence, for low temperature irradiation AR/R,,,,, z -0.077 and at room temperature irradiation AR/ R “lllrrz -0.002). slight
trend
of increase
at higher
doses,
while
References
for
In order to explain the decrease in resistance from its initial value as a result of irradiation, one must remember the fact that
228
The authors are grateful to the Director. National Physical Laboratory and the Director. Nuclear Science Center, New Delhi for their continuous encouragement during the course of the work. One of us, SKD, is grateful to the Director, National Physical Laboratory, for the award of a senior research fellowship. The co-operation of the Pelletron accelerator crew is gratefully acknowledged.
‘J G Bednorz and K A Mull&r, Z Ph~,sik, B64, 189 (1988). ‘T Terai, T Masegi. T Takahashi, Y Enomoto and S Kubo, Jpn. .I Appl Ph,vs, 29, L2053 (1990). ‘A D Marwick. G J Clark, Nucl Instrum Meth, B37/38,910 (1989). 4D Bourgoult, S Boufford, M Toulmonde. D Groult, J Provost, F Studert. N Ngyuen and B Raveau, Phys REV, B39,6549 (I 989). ‘M Toulmonde, Nucl. Ins/rum Me/h, B19/20, 120 (1987). ‘B Hensel, B Roas, S Henke, R Hopfengartner, M Lippert. J P Strobel. M Vildic, G Saemann Ischenko and S Klaumunzer, Phy.s RN, 42, 4135 (1990). ‘D Kanjilal, S Chopra, M M Narayanan. I S Iyer. V Jha. R Joshi and S K Dutta. ,%‘:uclIns/rum Me/h, A328, 97 (1993). ‘G J Clark, A D Marwick. F Legoues. R B Laibowitz. R Koch and P Madakson, Nucl Instrum Melh, B32, 405 (1988).
46/number 3lpages 227 to 228/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00
Pergamon 0042-207X(94100050-6
Effect of sample temperature on the irradiation of energetic particles in the YBCO system S K Das, P K Ashwini
Kumar and S K Sarkar, National
Physical
Laboratory,
effects
New De/hi 1’10072, lndia
and D Kanjilal and L Senapati, received for publication
Nuclear
Science Center, New Delhi 110067, India
8 June 1994
‘“0 ions at 75 Me\/ incident energy have been used to study the irradiation effects on the normal state resistance variation with fluence. The studies conducted include the measurement of resistance variation with dose due to the ion used. The samples were the bulk sintered pellets of YBa,Cu,O,_, maintained at normal temperature (T - 300 K) and low temperature fT - 100 K). In the case of a sample irradiated at normal temperature there is a continuous decrease in resistance with dose, while in the other case there is a minimal change in the resistance vs fluence curve. These experimental results reveal that the sample temperature has a special effect on the damage mechanism.
Introduction The discovery of superconductivity above about 50 K in some ceramic cuprates’ led to a spurt2m4in research into these materials. Soon after, attempts were made to see the effects of energetic radiations on the normal and superconducting properties. With regard to this aspect, it has been well established’ that whenever an ion traverses through matter it loses its energy via two processes; namely, the first one by the ion-target electron interaction, called the electronic energy loss (Se), and the other one due to ion-target atom interaction, called the nuclear energy loss (Sn). With regard to the particular case of ionic irradiations on superconductor like YBa$u,O,, it has been shown that there is a threshold value for Se, above which the material undergoes a radical change in the properties, i.e. the normal state resistance may increase very sharply, the ion on its passage through the material leaves a permanent track and, also, the superconductor shows a sudden deterioration in its properties. While most of the experiments reported to date have focused attention on the iontarget interaction, with the sample at normal temperature or at low temperature, the present paper describes the results of work done with IhO on a YBaQr,O, bulk sample held at ambient temperature and also at low temperature (- 100 K) but greater than the T,. The results are explained on the basis of the effect of sample temperature on the role of ion irradiation with regard to damage creation.
Experimental Bulk samples of YBa,Cu,O, ~, were made by the standard solid state reaction techniqueh. The starting materials were YIO1,
BaC03 and CuO. The materials of proper stoichiometry were thoroughly mixed, ground and pelletised to circular discs measuring about 25 mm in diameter. The calcination was done at 900 C and the sintering was done at 930 C. The pellets were checked to be superconducting at about 90 K. For irradiation experiments the samples were cut into rectangular bars measuring 10 x 3 x 2 mm3. The samples were irradiated by 75 MeV oxygen beam from a 16 MV Tandem Accelerator (PELLETRON)’ at the Nuclear Science Center. The samples mounted on the glass epoxy were fixed on a cold finger cooled by liquid nitrogen. The glass epoxy mount ensured that the temperature of the material was above the transition temperature. The vacuum in the chamber during irradiation was maintained at about 5 x lo-’ mbar. A standard four probe technique using constant current source and nanovoltmeter was employed to measure the variation of resistance with fluence in situ. The data were taken at about 5 min after stopping the beam, so that the short lived transient effects were equilibrated. The ion fluence was varied between 1 x 10” cm-’ and 1 x 10” cm-‘. The fluence of the beam was determined by counting the number of particles bombarding the sample using a current integrator and a counter. The secondary electrons coming out of the sample were suppressed by an electrostatic suppressor. Results and discussions Figures 1 and 2 show the variation of normalised resistance (R,) with dose for the sample kept at ambient temperature and at low temperature. respectively. In both the cases the resistance has been decreased from its initial value as a result of irradiation. However, the striking difference between the two curves are : 227
S K Das er al: Irradiation
in the
Unirrodioted
YBCO system value of the resistance = 119 m. ohm
FLUENCE (/cm.cm)
Figure 1. Variation of normalised resistance oxygen for target at ambient temperature.
0.940
-
with fluence
of 75 MeV
value of the resistance = 165.4m.ohm
Unirradiated
a
z e
0.925
although the projectile, i.e. 0 ions. interacts with all the atoms in the target, namely Y, Ba, Cu and 0, during its passage, the probability of 0 ions getting displaced is highest because 0 is the lightest atom amongst the others and, also, they are loosely bound in the lattice compared to the other atoms. Since it is believed that an ordered arrangement of oxygen vacancy is crucial for even normal state metallic conductivity’. then from the previous argument the decrease in resistance strongly suggests an improvement of the ordered arrangement of oxygen vacancy as the major effect of the oxygen irradiation in our YBaQ,O, ~, bulk sample. Also, the possible incorporation of oxygen in the YBaQ,O, lattice at the end of the projectile’s range might be a factor in reducing the resistance of the samples from its initial value. At this point, it is worthwhile to compare the nature of the two variations in R,, vs fluence at two temperatures. The larger value of AR/R,,,,, for low temperature irradiation shows that there is a favorable situation for improving the vacancy ordering of the oxygen sites due to ion irradiation and at the same time freezing this situation, thus retaining the ordered vacancy state ; this is not the case at room temperature irradiation, since a part of the ordered vacancy arrangement due to irradiation again gets disordered due to a larger amplitude of lattice vibration. Secondly, the change in resistance due to irradiation at low temperature shows a very marginal increase of R,, at higher Auences. leading to the creation of other types of defects (point defects and cascades etc.). which have a deteriordtory effect on conductivity. From the nature of the two curves. it seems that up to a fluence of about IO” cm-’ the vacancy ordering phenomenon prevails over the other types of effects. The result of the present experiment may be compared with that of the work on a thin film irradiated by 25 MeV ‘“0 ions at 100 K (ref 6). In the case of the thin film. the resistance of the irradiated sample is always more than the unirradiated value, while in the present case of the bulk sample it is always less. This anomaly seems to be due to the fact that bulk samples used in the present study may have more intrinsic imperfections compared to the thin film. Acknowledgements
0
0920
I
-
10'2
I
IO'3
IO
I
I4
lOI3
FLUENCE (/cm.cm)
Figure 2. Variation of normalised oxygen at low temperature.
(9
for
low
temperature
resistance
irradiation
with fluence
the
resistance
of 75 MeV
shows
a
room temperature irradiation the nature of the resistance wrt fluence shows an initial flat region at lower doses, followed by a gradual decrease at higher doses ; (ii) the normalised change in resistance (AR/R,,,,,) is much more in the case of low temperature irradiation than the case at room temperature irradiation (at 1 x 10” cm-’ fluence, for low temperature irradiation AR/R,,,,, z -0.077 and at room temperature irradiation AR/ R “lllrrz -0.002). slight
trend
of increase
at higher
doses,
while
References
for
In order to explain the decrease in resistance from its initial value as a result of irradiation, one must remember the fact that
228
The authors are grateful to the Director. National Physical Laboratory and the Director. Nuclear Science Center, New Delhi for their continuous encouragement during the course of the work. One of us, SKD, is grateful to the Director, National Physical Laboratory, for the award of a senior research fellowship. The co-operation of the Pelletron accelerator crew is gratefully acknowledged.
‘J G Bednorz and K A Mull&r, Z Ph~,sik, B64, 189 (1988). ‘T Terai, T Masegi. T Takahashi, Y Enomoto and S Kubo, Jpn. .I Appl Ph,vs, 29, L2053 (1990). ‘A D Marwick. G J Clark, Nucl Instrum Meth, B37/38,910 (1989). 4D Bourgoult, S Boufford, M Toulmonde. D Groult, J Provost, F Studert. N Ngyuen and B Raveau, Phys REV, B39,6549 (I 989). ‘M Toulmonde, Nucl. Ins/rum Me/h, B19/20, 120 (1987). ‘B Hensel, B Roas, S Henke, R Hopfengartner, M Lippert. J P Strobel. M Vildic, G Saemann Ischenko and S Klaumunzer, Phy.s RN, 42, 4135 (1990). ‘D Kanjilal, S Chopra, M M Narayanan. I S Iyer. V Jha. R Joshi and S K Dutta. ,%‘:uclIns/rum Me/h, A328, 97 (1993). ‘G J Clark, A D Marwick. F Legoues. R B Laibowitz. R Koch and P Madakson, Nucl Instrum Melh, B32, 405 (1988).