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Temperature dependence of rf-SQUID effect due to natural grain boundary junctions in YNi2B2C
Physica C 316 Ž1999. 257–260
Temperature dependence of rf-SQUID effect due to natural grain boundary junctions in YNi 2 B 2 C Neeraj Khare a
a,)
, A.K. Gupta a , Z. Hossain b, R. Nagarajan b, L.C. Gupta b, R. Vijayaraghavan b
National Physical Laboratory, SuperconductiÕity Group, Dr. K.S. Krishnan Road, New Delhi 110012, India b Tata Institute of Fundamental Research, Bombay 400005, India Received 4 January 1999; accepted 19 February 1999
Abstract The rf-SQUID effect in YNi 2 B 2 C bulk superconductor due to natural grain boundary Josephson junctions has been observed up to 13.2 K. At 4.2 K, the peak to peak amplitude of the SQUID modulation was 1.5 mV. The bulk rf-SQUID operates in non-hysteretic mode. Flux noise spectrum of the SQUID at 4.2 K showed white noise down to 0.3 Hz and in this region flux noise density, SF was f 2 = 10y3 For 'Hz . Rapid increase in flux noise observed at T ) 10 K has been attributed to flux hopping. q 1999 Elsevier Science B.V. All rights reserved.
'
Keywords: rf-SQUID effect; YNi 2 B 2 C; Natural grain boundary junctions
1. Introduction Observation of superconductivity in Y–Ni–B–C w1x has stimulated interest in intermetallic superconductors. The layered tetragonal structure of the single phase quaternary borocarbides RNi 2 B 2 C ŽR s Rare earth atoms, Y. is highly anisotropic Ž cra f 3. and resembles that of high-Tc superconductors. The intriguing interplay of superconductivity and magnetism in several magnetic borocarbide materials RNi 2 B 2 C ŽR s Dy, Er, Ho and Tm. makes them rather unique among all the known magnetic superconductors w2–5x. The multiphase material Y–Pd– B–C w6x exhibits the highest Tc Žf 23 K. in bulk intermetallics. )
Corresponding author. Fax: q91-11-5764-189; E-mail: [email protected]
Detailed investigations have been reported on various superconducting properties of YNi 2 B 2 C ŽTc f 15.5 K. such as upper and lower critical fields Ž Hc1 and Hc2 ., coherence length, energy gap, Ginzburg– Landau parameter and penetration depth w7–12x. Temperature variation of Hc2 shows a concave upward feature near Tc which has been explained in terms of possible presence of intergranular weak links w9,10x. In our earlier studies, we reported w13x the rf-SQUID effect in bulk YNi 2 B 2 C at 4.2 K which confirmed that the natural grain boundary links in this superconductor behave as Josephson junctions. We have also observed white noise down to 0.2 Hz in the YNi 2 B 2 C rf-SQUID. This phenomenon is similar to that observed in polycrystalline high-Tc superconductors wherein natural grain boundary junctions have been found to behave as Josephson junctions w14,15x and similar types of
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 9 5 - 1
258
N. Khare et al.r Physica C 316 (1999) 257–260
rf-SQUID effects have been observed in bulk YBCO w16–19x. rf-SQUID based on natural grain boundary junction in cuprates exhibit large 1rf a noise particularly near Tc w18,19x. In order to further understand the bulk rf-SQUID behaviour in YNi 2 B 2 C, we have studied the SQUID effect as a function of temperature. This paper reports results of these studies.
2. Experimental Bulk samples of YNi 2 B 2 C were prepared by the arc melting technique. High purity elements of Y, Ni, B and C were taken in appropriate ratio and melted in an arc furnace under controlled argon gas atmosphere. The resultant samples were encapsulated in a quartz tube under vacuum and then annealed at 10508C for 7 days. Powder X-ray diffraction pattern showed that the sample was single phase YNi 2 B 2 C. ˚ The lattice parameters were found to be a s 3.526 A ˚ AC susceptibility Ž313 Hz; rms and c s 10.542 A. field 1.5 G. of the sample was measured as a function of temperature to monitor the diamagnetic response and, thereby, to determine its superconducting transition temperature. Fig. 1 shows the result of ac susceptibility Ž x . of YNi 2 B 2 C as a function of temperature. The transition temperature of the superconductor was found to be 15.5 K. A small piece of bulk YNi 2 B 2 C was taken for the rf-SQUID experiment. A 10-turn coil of copper wire, wound around the bulk sample, formed a part of the
tank circuit whose resonance frequency and Q were 19.4 MHz and 48, respectively, at 4.2 K. An rf oscillator was connected to the tank circuit to provide rf bias to the SQUID. The reflected rf signal from the tank circuit was amplified by a low-noise amplifier Žof 60 dB gain. and detected using a diode detector. For observing the voltage–flux characteristic of the rf-SQUID an ac signal was applied to the SQUID through a fifty turn solenoid Ž1.5 cm long; 1 cm diameter. using an audio frequency Žaf. oscillator. The field produced by the solenoid was 3.7 = 10y3 TrA. The rf detector output was connected to the Y-channel of a CRO while the output from the af oscillator was connected to the X-channel. Amplitude and frequency of the rf signal was adjusted for getting good voltage–flux SQUID characteristics. The noise spectrum of the SQUID was recorded using a lock-in amplifier and a dynamic signal analyser. A modulation signal of 500 Hz was applied to the SQUID through the solenoid and the amplitude of the modulation signal was adjusted so that the current through the coil was between I ŽFo .r4 and I ŽFo .r2 where I ŽFo . was the current in the solenoid required to change the flux in the SQUID loop by one flux quantum. The flux noise density, SF Ž f . was calculated from the measured voltage noise spectrum, S v Ž f . using the formula:
(
(
(S
F
2 Ž f . s S v Ž f . r Ž dVrd f .
1r2
,
where dVrdF was the transfer function of the SQUID.
3. Results and discussion
Fig. 1. Magnetic susceptibility Ž x . vs. temperature curve of YNi 2 B 2 C sample.
Fig. 2 shows voltage–flux modulations of the YNi 2 B 2 C bulk rf-SQUID at 4.2 and 9 K. The peak to peak amplitude of voltage–flux modulations was 1.5 mV. These voltage–flux modulations were observed when the rf-frequency was kept at 19.6 MHz. This value was slightly above the resonance frequency Ž f o s 19.4 MHz. of the tank circuit. These modulations were not observed at f s f o . Good voltage–flux characteristics were also observed when the operating rf-frequency was decreased to 19.2 MHz. Phase reversal in the V– F characteristics were observed when the rf-frequency was changed from 19.6 to 19.2 MHz. Such a phase reversal is a typical
N. Khare et al.r Physica C 316 (1999) 257–260
259
Fig. 2. Voltage–flux modulations of bulk YNi 2 B 2 C rf-SQUID at 4.2 and 9 K.
feature for a non-hysteretic rf-SQUID whose b parameter is less than one w20x. The b parameter is equal to 2p LIcrFo where L is the inductance of the SQUID loop, Ic is the critical current of the Josephson junction and Fo Žs hr2e. is the flux quantum. rf-SQUID loop area Ž A s ForBo . can be estimated by measuring the field Bo corresponding to a single oscillation in the voltage-field SQUID characteristics. In the present case Bo was 1.4 = 10y7 T. Assuming the loop to be circular, one obtains the radius of the loop to be 60 mm. Fig. 3 shows variation of the peak-to-peak amplitude of voltage–flux SQUID modulations with temperature. The peak-to-peak amplitude of SQUID modulations remains same Ž1.5 mV. up to 12.3 K and afterwards, it starts decreasing and finally disappears at 13.2 K.
Fig. 3. Variation of the peak-to-peak amplitude of the voltage–flux modulations of the bulk YNi 2 B 2 C rf-SQUID as a function of temperature.
Fig. 4. Flux noise spectrum of the YNi 2 B 2 C rf-SQUID at three operating temperatures.
We have also studied the flux noise spectrum of the bulk YNi 2 B 2 C rf-SQUID at different temperatures. Fig. 4 shows flux noise spectra at 4.2, 12.5 and 13 K. At 4.2 K, the rf-SQUID shows white noise region down to 0.3 Hz and in this region flux noise density, SF is 2 = 10y3 For 'Hz . However, at temperatures 12.5 and 13 K, large flux noise is observed. At high temperatures flux noise varies as 1rf a , where a s 0.75–0.9. Fig. 5 shows the temperature variation of the flux noise density of the SQUID at 1 Hz. The flux noise density remains constant up to 9.5 K and it increases rapidly at high temperatures. This increase in flux noise at higher temperature is due to flux hopping. Similar type of behaviour has been observed in high-Tc YBCO bulk rf-SQUID w16–19x.
(
Fig. 5. Variation of the flux noise density at 1 Hz of YNi 2 B 2 C bulk rf-SQUID as a function of temperature.
260
N. Khare et al.r Physica C 316 (1999) 257–260
4. Conclusion w6x
rf-SQUID effect in bulk YNi 2 B 2 C due to natural grain boundary junctions has been observed up to 13.2 K. The observation of the rf-SQUID effect in YNi 2 B 2 C bulk sample shows that the natural grain boundary junctions behave as Josephson junctions. The bulk rf-SQUID operates in non-hysteretic mode. The amplitude of the SQUID modulation was 1.5 mV at 4.2 K and it remained constant up to 12.3 K and above this temperature it started deceasing. At 4.2 K, the flux noise spectrum of the SQUID showed white noise region down to 0.3 Hz. Rapid increase in flux noise was observed for temperatures greater than 9.5 K and this rapid increase has been attributed to increase in flux hopping at higher temperatures.
w7x
w8x w9x
w10x
w11x w12x w13x
References w1x R. Nagarajan, C. Mazumdar, Z. Hossain, S.K. Dhar, K.V. Gopalkrishnan, L.C. Gupta, C. Godart, B.D. Padalia, R. Vijayaraghavan, Phys. Rev. Lett. 72 Ž1994. 274. w2x R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki, S. Uchida, Nature 367 Ž1994. 252. w3x T. Siegrist, H.W. Zandbergen, R.J. Cava, J.J. Krajewski, W.F. Peck, Nature 367 Ž1994. 254. w4x Z. Hossain, L.C. Gupta, R. Nagarajan, S.K. Dhar, C. Godart, R. Vijayaraghavan, Physica B 223–224 Ž1996. 99. w5x C. Godart, L.C. Gupta, R. Nagarajan, S.K. Dhar, H. Noel, M. Potel, C. Mazumdar, Z. Hossain, C. Levy-Clement, G.
w14x w15x w16x w17x w18x w19x w20x
Schriffmacher, B.D. Padalia, R. Vijayaraghavan, Phys. Rev. B, 51, 489. R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter, S. Uchida, Nature 367 Ž1994. 146. S.B. Roy, Z. Hossain, A.K. Pradhan, C. Mazumdar, P. Chaddah, R. Nagarajan, C. Godart, L.C. Gupta, Physica C 228 Ž1994. 319. X.Z. Zhou, H.P. Kunkel, P.A. Stampe, J.A. Cowen, G. Williams, Physica C 251 Ž1995. 183. K. Ghosh, S. Ramakrishnan, A.K. Grover, G. Chandra, T.V. Chandrasekhar Rao, P.K. Mishra, G. Ravikumar, V.C. Sahni, Phys. Rev. B 52 Ž1995. 68. K. Ghosh, S. Ramakrishnan, A.K. Grover, G. Chandra, T.V. Chandrasekhar Rao, P.K. Mishra, G. Ravikumar, V.C. Sahni, Physica B 223–224 Ž1996. 109. S.B. Roy, Z. Hossain, A.K. Pradhan, P. Chaddah, R. Nagarajan, L.C. Gupta, Physica C 256 Ž1996. 90. A. Andreone, F. Fontana, M. Iavarone, R. Vaglio, F. Canepa, P. Manfrinetti, A. Palenzona, Physica C 251 Ž1995. 379. N. Khare, A.K. Gupta, S. Khare, L.C. Gupta, R. Nagarajan, Z. Hossain, R. Vijayaraghavan, Appl. Phys. Lett. 69 Ž1996. 1483. A.K. Gupta, S.K. Agarwal, B. Jayaram, A. Gupta, A.V. Narlikar, Pramana 28 Ž1987. L705. J.T. Chen, L.E. Wegner, C.J. McEwan, E.M. Logothetis, Phys. Rev. Lett. 58 Ž1987. 1972. C.M. Pegrum, G.B. Donaldson, A. Carr, A. Hendry, Appl. Phys. Lett. 51 Ž1987. 1364. C.M. Pegrum, J.R. Buckley, M. Odehnal, IEEE Trans. Magn. 25 Ž1989. 872. N. Khare, A.K. Gupta, S.K. Arora, V.S. Tomar, V.N. Ojha, Pramana 35 Ž1990. L415. N. Khare, A.K. Gupta, S.K. Arora, V.S. Tomar, V.N. Ojha, Pramana 35 Ž1990. 243. K.K. Likharev, Introduction to the Dynamics of Josephson Junctions and Circuits, Chap. 14, Gordon & Breach, New York, 1986, p. 471.
Temperature dependence of rf-SQUID effect due to natural grain boundary junctions in YNi 2 B 2 C Neeraj Khare a
a,)
, A.K. Gupta a , Z. Hossain b, R. Nagarajan b, L.C. Gupta b, R. Vijayaraghavan b
National Physical Laboratory, SuperconductiÕity Group, Dr. K.S. Krishnan Road, New Delhi 110012, India b Tata Institute of Fundamental Research, Bombay 400005, India Received 4 January 1999; accepted 19 February 1999
Abstract The rf-SQUID effect in YNi 2 B 2 C bulk superconductor due to natural grain boundary Josephson junctions has been observed up to 13.2 K. At 4.2 K, the peak to peak amplitude of the SQUID modulation was 1.5 mV. The bulk rf-SQUID operates in non-hysteretic mode. Flux noise spectrum of the SQUID at 4.2 K showed white noise down to 0.3 Hz and in this region flux noise density, SF was f 2 = 10y3 For 'Hz . Rapid increase in flux noise observed at T ) 10 K has been attributed to flux hopping. q 1999 Elsevier Science B.V. All rights reserved.
'
Keywords: rf-SQUID effect; YNi 2 B 2 C; Natural grain boundary junctions
1. Introduction Observation of superconductivity in Y–Ni–B–C w1x has stimulated interest in intermetallic superconductors. The layered tetragonal structure of the single phase quaternary borocarbides RNi 2 B 2 C ŽR s Rare earth atoms, Y. is highly anisotropic Ž cra f 3. and resembles that of high-Tc superconductors. The intriguing interplay of superconductivity and magnetism in several magnetic borocarbide materials RNi 2 B 2 C ŽR s Dy, Er, Ho and Tm. makes them rather unique among all the known magnetic superconductors w2–5x. The multiphase material Y–Pd– B–C w6x exhibits the highest Tc Žf 23 K. in bulk intermetallics. )
Corresponding author. Fax: q91-11-5764-189; E-mail: [email protected]
Detailed investigations have been reported on various superconducting properties of YNi 2 B 2 C ŽTc f 15.5 K. such as upper and lower critical fields Ž Hc1 and Hc2 ., coherence length, energy gap, Ginzburg– Landau parameter and penetration depth w7–12x. Temperature variation of Hc2 shows a concave upward feature near Tc which has been explained in terms of possible presence of intergranular weak links w9,10x. In our earlier studies, we reported w13x the rf-SQUID effect in bulk YNi 2 B 2 C at 4.2 K which confirmed that the natural grain boundary links in this superconductor behave as Josephson junctions. We have also observed white noise down to 0.2 Hz in the YNi 2 B 2 C rf-SQUID. This phenomenon is similar to that observed in polycrystalline high-Tc superconductors wherein natural grain boundary junctions have been found to behave as Josephson junctions w14,15x and similar types of
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 9 5 - 1
258
N. Khare et al.r Physica C 316 (1999) 257–260
rf-SQUID effects have been observed in bulk YBCO w16–19x. rf-SQUID based on natural grain boundary junction in cuprates exhibit large 1rf a noise particularly near Tc w18,19x. In order to further understand the bulk rf-SQUID behaviour in YNi 2 B 2 C, we have studied the SQUID effect as a function of temperature. This paper reports results of these studies.
2. Experimental Bulk samples of YNi 2 B 2 C were prepared by the arc melting technique. High purity elements of Y, Ni, B and C were taken in appropriate ratio and melted in an arc furnace under controlled argon gas atmosphere. The resultant samples were encapsulated in a quartz tube under vacuum and then annealed at 10508C for 7 days. Powder X-ray diffraction pattern showed that the sample was single phase YNi 2 B 2 C. ˚ The lattice parameters were found to be a s 3.526 A ˚ AC susceptibility Ž313 Hz; rms and c s 10.542 A. field 1.5 G. of the sample was measured as a function of temperature to monitor the diamagnetic response and, thereby, to determine its superconducting transition temperature. Fig. 1 shows the result of ac susceptibility Ž x . of YNi 2 B 2 C as a function of temperature. The transition temperature of the superconductor was found to be 15.5 K. A small piece of bulk YNi 2 B 2 C was taken for the rf-SQUID experiment. A 10-turn coil of copper wire, wound around the bulk sample, formed a part of the
tank circuit whose resonance frequency and Q were 19.4 MHz and 48, respectively, at 4.2 K. An rf oscillator was connected to the tank circuit to provide rf bias to the SQUID. The reflected rf signal from the tank circuit was amplified by a low-noise amplifier Žof 60 dB gain. and detected using a diode detector. For observing the voltage–flux characteristic of the rf-SQUID an ac signal was applied to the SQUID through a fifty turn solenoid Ž1.5 cm long; 1 cm diameter. using an audio frequency Žaf. oscillator. The field produced by the solenoid was 3.7 = 10y3 TrA. The rf detector output was connected to the Y-channel of a CRO while the output from the af oscillator was connected to the X-channel. Amplitude and frequency of the rf signal was adjusted for getting good voltage–flux SQUID characteristics. The noise spectrum of the SQUID was recorded using a lock-in amplifier and a dynamic signal analyser. A modulation signal of 500 Hz was applied to the SQUID through the solenoid and the amplitude of the modulation signal was adjusted so that the current through the coil was between I ŽFo .r4 and I ŽFo .r2 where I ŽFo . was the current in the solenoid required to change the flux in the SQUID loop by one flux quantum. The flux noise density, SF Ž f . was calculated from the measured voltage noise spectrum, S v Ž f . using the formula:
(
(
(S
F
2 Ž f . s S v Ž f . r Ž dVrd f .
1r2
,
where dVrdF was the transfer function of the SQUID.
3. Results and discussion
Fig. 1. Magnetic susceptibility Ž x . vs. temperature curve of YNi 2 B 2 C sample.
Fig. 2 shows voltage–flux modulations of the YNi 2 B 2 C bulk rf-SQUID at 4.2 and 9 K. The peak to peak amplitude of voltage–flux modulations was 1.5 mV. These voltage–flux modulations were observed when the rf-frequency was kept at 19.6 MHz. This value was slightly above the resonance frequency Ž f o s 19.4 MHz. of the tank circuit. These modulations were not observed at f s f o . Good voltage–flux characteristics were also observed when the operating rf-frequency was decreased to 19.2 MHz. Phase reversal in the V– F characteristics were observed when the rf-frequency was changed from 19.6 to 19.2 MHz. Such a phase reversal is a typical
N. Khare et al.r Physica C 316 (1999) 257–260
259
Fig. 2. Voltage–flux modulations of bulk YNi 2 B 2 C rf-SQUID at 4.2 and 9 K.
feature for a non-hysteretic rf-SQUID whose b parameter is less than one w20x. The b parameter is equal to 2p LIcrFo where L is the inductance of the SQUID loop, Ic is the critical current of the Josephson junction and Fo Žs hr2e. is the flux quantum. rf-SQUID loop area Ž A s ForBo . can be estimated by measuring the field Bo corresponding to a single oscillation in the voltage-field SQUID characteristics. In the present case Bo was 1.4 = 10y7 T. Assuming the loop to be circular, one obtains the radius of the loop to be 60 mm. Fig. 3 shows variation of the peak-to-peak amplitude of voltage–flux SQUID modulations with temperature. The peak-to-peak amplitude of SQUID modulations remains same Ž1.5 mV. up to 12.3 K and afterwards, it starts decreasing and finally disappears at 13.2 K.
Fig. 3. Variation of the peak-to-peak amplitude of the voltage–flux modulations of the bulk YNi 2 B 2 C rf-SQUID as a function of temperature.
Fig. 4. Flux noise spectrum of the YNi 2 B 2 C rf-SQUID at three operating temperatures.
We have also studied the flux noise spectrum of the bulk YNi 2 B 2 C rf-SQUID at different temperatures. Fig. 4 shows flux noise spectra at 4.2, 12.5 and 13 K. At 4.2 K, the rf-SQUID shows white noise region down to 0.3 Hz and in this region flux noise density, SF is 2 = 10y3 For 'Hz . However, at temperatures 12.5 and 13 K, large flux noise is observed. At high temperatures flux noise varies as 1rf a , where a s 0.75–0.9. Fig. 5 shows the temperature variation of the flux noise density of the SQUID at 1 Hz. The flux noise density remains constant up to 9.5 K and it increases rapidly at high temperatures. This increase in flux noise at higher temperature is due to flux hopping. Similar type of behaviour has been observed in high-Tc YBCO bulk rf-SQUID w16–19x.
(
Fig. 5. Variation of the flux noise density at 1 Hz of YNi 2 B 2 C bulk rf-SQUID as a function of temperature.
260
N. Khare et al.r Physica C 316 (1999) 257–260
4. Conclusion w6x
rf-SQUID effect in bulk YNi 2 B 2 C due to natural grain boundary junctions has been observed up to 13.2 K. The observation of the rf-SQUID effect in YNi 2 B 2 C bulk sample shows that the natural grain boundary junctions behave as Josephson junctions. The bulk rf-SQUID operates in non-hysteretic mode. The amplitude of the SQUID modulation was 1.5 mV at 4.2 K and it remained constant up to 12.3 K and above this temperature it started deceasing. At 4.2 K, the flux noise spectrum of the SQUID showed white noise region down to 0.3 Hz. Rapid increase in flux noise was observed for temperatures greater than 9.5 K and this rapid increase has been attributed to increase in flux hopping at higher temperatures.
w7x
w8x w9x
w10x
w11x w12x w13x
References w1x R. Nagarajan, C. Mazumdar, Z. Hossain, S.K. Dhar, K.V. Gopalkrishnan, L.C. Gupta, C. Godart, B.D. Padalia, R. Vijayaraghavan, Phys. Rev. Lett. 72 Ž1994. 274. w2x R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki, S. Uchida, Nature 367 Ž1994. 252. w3x T. Siegrist, H.W. Zandbergen, R.J. Cava, J.J. Krajewski, W.F. Peck, Nature 367 Ž1994. 254. w4x Z. Hossain, L.C. Gupta, R. Nagarajan, S.K. Dhar, C. Godart, R. Vijayaraghavan, Physica B 223–224 Ž1996. 99. w5x C. Godart, L.C. Gupta, R. Nagarajan, S.K. Dhar, H. Noel, M. Potel, C. Mazumdar, Z. Hossain, C. Levy-Clement, G.
w14x w15x w16x w17x w18x w19x w20x
Schriffmacher, B.D. Padalia, R. Vijayaraghavan, Phys. Rev. B, 51, 489. R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter, S. Uchida, Nature 367 Ž1994. 146. S.B. Roy, Z. Hossain, A.K. Pradhan, C. Mazumdar, P. Chaddah, R. Nagarajan, C. Godart, L.C. Gupta, Physica C 228 Ž1994. 319. X.Z. Zhou, H.P. Kunkel, P.A. Stampe, J.A. Cowen, G. Williams, Physica C 251 Ž1995. 183. K. Ghosh, S. Ramakrishnan, A.K. Grover, G. Chandra, T.V. Chandrasekhar Rao, P.K. Mishra, G. Ravikumar, V.C. Sahni, Phys. Rev. B 52 Ž1995. 68. K. Ghosh, S. Ramakrishnan, A.K. Grover, G. Chandra, T.V. Chandrasekhar Rao, P.K. Mishra, G. Ravikumar, V.C. Sahni, Physica B 223–224 Ž1996. 109. S.B. Roy, Z. Hossain, A.K. Pradhan, P. Chaddah, R. Nagarajan, L.C. Gupta, Physica C 256 Ž1996. 90. A. Andreone, F. Fontana, M. Iavarone, R. Vaglio, F. Canepa, P. Manfrinetti, A. Palenzona, Physica C 251 Ž1995. 379. N. Khare, A.K. Gupta, S. Khare, L.C. Gupta, R. Nagarajan, Z. Hossain, R. Vijayaraghavan, Appl. Phys. Lett. 69 Ž1996. 1483. A.K. Gupta, S.K. Agarwal, B. Jayaram, A. Gupta, A.V. Narlikar, Pramana 28 Ž1987. L705. J.T. Chen, L.E. Wegner, C.J. McEwan, E.M. Logothetis, Phys. Rev. Lett. 58 Ž1987. 1972. C.M. Pegrum, G.B. Donaldson, A. Carr, A. Hendry, Appl. Phys. Lett. 51 Ž1987. 1364. C.M. Pegrum, J.R. Buckley, M. Odehnal, IEEE Trans. Magn. 25 Ž1989. 872. N. Khare, A.K. Gupta, S.K. Arora, V.S. Tomar, V.N. Ojha, Pramana 35 Ž1990. L415. N. Khare, A.K. Gupta, S.K. Arora, V.S. Tomar, V.N. Ojha, Pramana 35 Ž1990. 243. K.K. Likharev, Introduction to the Dynamics of Josephson Junctions and Circuits, Chap. 14, Gordon & Breach, New York, 1986, p. 471.