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Two hole SQUID behaviour in a bulk YBCO sample at 77K
Physica C 153-155 (1988) 1411-1412 North-Holland, Amsterdm'n
TWO HOLE SQUID BEHAVIOUR IN A BULK YBCO SAMPLE AT 77K
S. HARROP, C.M. MUIRHEAD, M.S. COLCLOUGH and C.E. G O U G H Department of Physics, University of Birmingham, Birmingham, B15 2TT, U.K.
We describe the development of a 2-hole radio frequency SQUID made from a single disc of YBa2CusOT_y. device displays single flux quantum sensitivity and an estimated noise level of 1 x 10 - a 9o/glq--~- above 50Hz.
1.
INTRODUCTION SQUIDs made from niobium are the most sensitive magnetometers yet devised and there is considerable incentive, therefore, to develop devices based on high-Tc superconductors. There are two basic forms of SQUID device; those based on the field dependent critical current of a superconducting ring containing pair of junctions (d.c. SQUID) and those based upon the radio frequency response of a ring containing a single link (r.f. SQUID). The former is the more sensitive but the latter has an element of simplicity which led to its early commercial development and continued widespread use in a large number of laboratories. We describe here the development of a 2-hole r.f. SQUID, made from a good quality 123-YBCO specimen, which displays single flux quantum sensitivity. 2.
SPECIMEN PREPARATION AND THE MEASUREMENT SYSTEM The specimen was made from a disc of YBCO ~10mm in diameter and 3mm thick. Although no measurements have yet been made of its resistance or susceptibility, it was prepared by a powder route which we have found to produce dense, high-quality specimens. Two l mm holes were drilled in the sample perpendicular to the large face to leave ,'-0.5mm of material between them. This connecting region was then filed away in stages until a bridge of cross section ~0.2mm x 0.2mm remained. Great care was taken with the filing process in order to reduce the chance of producing microcracks in the link. The r.f. coil was lmm O.D. and consisted of 49 turns of 36 swg enamelled copper wire. The system r~sonated at ~ 20 MHz. The r.f. response was studied with either commercial r.f. SQUID electronics or a separate r.f. generator and amplifier system for high r.f. level measurements. The field to be measured could be applied by current either in the r.f. coil or via a solenoid in the second hole. The SQUID was magnetically screened by three mu-metal shields all contained in a stainless steel liquid nitrogen dewar. RESULTS The response observed before the link was ground down was qualitatively very similar to that reported in bulk granular material (1,2). It displayed SQUID-like behaviour, but with a periodicity in applied magnetic field some three orders of magnitude less than that
The
corresponding to the transfer of single flux quanta between the holes and lacking the conventional phase reversal with increasing r.f. amplitude. We interpret this as being due to flux penetration some way into the bulk of the sample. The r.f. voltage versus current characteristic was close to that of the isolated tuned circuit. This behaviour persisted until the connecting link was less than 0.5mm x 0.5mm, at which point a pronounced knee appeared in the r.f. voltage - current characteristic corresponding to increased absorption above an r.f. flux 41800990 in the hole. Additional periods in the response to an applied flux were observed having periodicities ~100~0o in the hole. During a period of two weeks the strength of the link was reduced by a combination of mechanical erosion and thermal cycling and a corresponding movement of the knee to lower r.f. levels occured. When the knee reached a level corresponding to ~5~o in the hole,
3.
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1
Dependence of r.f. voltage across tank circuit on applied flux. Note that the space between successive traces has been reduced (see text).
1412
S. Harrop et al. / Two hole SQUID behaviour
behaviour corresponding to the movement of single flux quanta between the holes was observed starting at r.f. voltages immediately above the knee. The detailed behaviour of the device is now being investigated and will be reported later. Preliminary measurements, however, show a clear, "triangular pattern" displaying phase reversal of successive traces over a large range of r.f. amplitude, but with a separation in the r.f. voltage steps some ten times larger than the peak to peak height of the triangles. Fig. 1 is a composite picture showing the four lowest levels but with the region between successive traces considerably reduced. Application of d.c. flux to a level of 200~oo did not cause any dephasing of the response to within experimental accuracy. The electronics locks easily onto any of the triangles to provide magnetometer behaviour. Preliminary measurements of the flux noise in lock-on mode yield a value of 1 . 1 0 - a ~ o / , / ~ z " above 50Hz rising rapidly below 20Hz. DISCUSSION The behaviour of the device at large link areas and in particular the role of the knee in determining the flux sensitivity of the device is not yet clear. There seems little doubt, however, that with sufficient link degredation the device displays conventional r.f. SQUID behaviour, albeit with rather small triangles. From the position of the lowest triangle we estimate a value for ,~,= 21rLIchPo= l/..$ From the geometry we take L = 10-10H implying a value of Ic for the link ~1SpA. T h e noise level of 1 0 - a ~ o is a factor of 3 higher than that reported by Z i m m e r m a n et al for a break junction l - h o l e SQUID (3) but we have yet to optimise
the performance of the electronics It is clear that weak link behaviour in these materials is not a direct consquence of reduced link dimensions but that erosion of the link reduces the number of percolating paths in the link: subsequent degredation of the remaining paths serves to lower Ic to the required value. Further work is underway to fully characterise the existing device and to study the link degredation process with a view to producing a more controlled performance and to application of these techniques to thick and thin film devices. 4.
CONCLUSIONS 2-hole rf SQUID behaviour has been demonstrated in bulk YBCO with an estimated noise level of 1.10-3~Oo//Hz. There are a number of features of the behaviour, particularly during the link reduction process, which are not well understood and require further investigation.
3.
ACKNOWLEDGEMENTS We are grateful to J.S. Abell and F. Wellhofer for supplying the YBCO sample used in this experiment and to M.N. Keene for his valuable advice. REFERENCES
0) (2) f3)
M.S. Colclough et al, Nature 328 0 9 8 7 ) 47. C.M. Pegrum et al, Appl.Phys.Lett. 51 0 9 8 7 ) 1364. J.E. Z i m m e r m a n et al, AppI.Phys.Lett. 51 (1987) 617.
TWO HOLE SQUID BEHAVIOUR IN A BULK YBCO SAMPLE AT 77K
S. HARROP, C.M. MUIRHEAD, M.S. COLCLOUGH and C.E. G O U G H Department of Physics, University of Birmingham, Birmingham, B15 2TT, U.K.
We describe the development of a 2-hole radio frequency SQUID made from a single disc of YBa2CusOT_y. device displays single flux quantum sensitivity and an estimated noise level of 1 x 10 - a 9o/glq--~- above 50Hz.
1.
INTRODUCTION SQUIDs made from niobium are the most sensitive magnetometers yet devised and there is considerable incentive, therefore, to develop devices based on high-Tc superconductors. There are two basic forms of SQUID device; those based on the field dependent critical current of a superconducting ring containing pair of junctions (d.c. SQUID) and those based upon the radio frequency response of a ring containing a single link (r.f. SQUID). The former is the more sensitive but the latter has an element of simplicity which led to its early commercial development and continued widespread use in a large number of laboratories. We describe here the development of a 2-hole r.f. SQUID, made from a good quality 123-YBCO specimen, which displays single flux quantum sensitivity. 2.
SPECIMEN PREPARATION AND THE MEASUREMENT SYSTEM The specimen was made from a disc of YBCO ~10mm in diameter and 3mm thick. Although no measurements have yet been made of its resistance or susceptibility, it was prepared by a powder route which we have found to produce dense, high-quality specimens. Two l mm holes were drilled in the sample perpendicular to the large face to leave ,'-0.5mm of material between them. This connecting region was then filed away in stages until a bridge of cross section ~0.2mm x 0.2mm remained. Great care was taken with the filing process in order to reduce the chance of producing microcracks in the link. The r.f. coil was lmm O.D. and consisted of 49 turns of 36 swg enamelled copper wire. The system r~sonated at ~ 20 MHz. The r.f. response was studied with either commercial r.f. SQUID electronics or a separate r.f. generator and amplifier system for high r.f. level measurements. The field to be measured could be applied by current either in the r.f. coil or via a solenoid in the second hole. The SQUID was magnetically screened by three mu-metal shields all contained in a stainless steel liquid nitrogen dewar. RESULTS The response observed before the link was ground down was qualitatively very similar to that reported in bulk granular material (1,2). It displayed SQUID-like behaviour, but with a periodicity in applied magnetic field some three orders of magnitude less than that
The
corresponding to the transfer of single flux quanta between the holes and lacking the conventional phase reversal with increasing r.f. amplitude. We interpret this as being due to flux penetration some way into the bulk of the sample. The r.f. voltage versus current characteristic was close to that of the isolated tuned circuit. This behaviour persisted until the connecting link was less than 0.5mm x 0.5mm, at which point a pronounced knee appeared in the r.f. voltage - current characteristic corresponding to increased absorption above an r.f. flux 41800990 in the hole. Additional periods in the response to an applied flux were observed having periodicities ~100~0o in the hole. During a period of two weeks the strength of the link was reduced by a combination of mechanical erosion and thermal cycling and a corresponding movement of the knee to lower r.f. levels occured. When the knee reached a level corresponding to ~5~o in the hole,
3.
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1
Dependence of r.f. voltage across tank circuit on applied flux. Note that the space between successive traces has been reduced (see text).
1412
S. Harrop et al. / Two hole SQUID behaviour
behaviour corresponding to the movement of single flux quanta between the holes was observed starting at r.f. voltages immediately above the knee. The detailed behaviour of the device is now being investigated and will be reported later. Preliminary measurements, however, show a clear, "triangular pattern" displaying phase reversal of successive traces over a large range of r.f. amplitude, but with a separation in the r.f. voltage steps some ten times larger than the peak to peak height of the triangles. Fig. 1 is a composite picture showing the four lowest levels but with the region between successive traces considerably reduced. Application of d.c. flux to a level of 200~oo did not cause any dephasing of the response to within experimental accuracy. The electronics locks easily onto any of the triangles to provide magnetometer behaviour. Preliminary measurements of the flux noise in lock-on mode yield a value of 1 . 1 0 - a ~ o / , / ~ z " above 50Hz rising rapidly below 20Hz. DISCUSSION The behaviour of the device at large link areas and in particular the role of the knee in determining the flux sensitivity of the device is not yet clear. There seems little doubt, however, that with sufficient link degredation the device displays conventional r.f. SQUID behaviour, albeit with rather small triangles. From the position of the lowest triangle we estimate a value for ,~,= 21rLIchPo= l/..$ From the geometry we take L = 10-10H implying a value of Ic for the link ~1SpA. T h e noise level of 1 0 - a ~ o is a factor of 3 higher than that reported by Z i m m e r m a n et al for a break junction l - h o l e SQUID (3) but we have yet to optimise
the performance of the electronics It is clear that weak link behaviour in these materials is not a direct consquence of reduced link dimensions but that erosion of the link reduces the number of percolating paths in the link: subsequent degredation of the remaining paths serves to lower Ic to the required value. Further work is underway to fully characterise the existing device and to study the link degredation process with a view to producing a more controlled performance and to application of these techniques to thick and thin film devices. 4.
CONCLUSIONS 2-hole rf SQUID behaviour has been demonstrated in bulk YBCO with an estimated noise level of 1.10-3~Oo//Hz. There are a number of features of the behaviour, particularly during the link reduction process, which are not well understood and require further investigation.
3.
ACKNOWLEDGEMENTS We are grateful to J.S. Abell and F. Wellhofer for supplying the YBCO sample used in this experiment and to M.N. Keene for his valuable advice. REFERENCES
0) (2) f3)
M.S. Colclough et al, Nature 328 0 9 8 7 ) 47. C.M. Pegrum et al, Appl.Phys.Lett. 51 0 9 8 7 ) 1364. J.E. Z i m m e r m a n et al, AppI.Phys.Lett. 51 (1987) 617.