Superconducting magnetic sensor

Abstract

A magnetometer for measuring a magnetic flux and also the absolute magnetic flux, the magnetometer comprising a plurality of superconducting quantum devices (SQUIDs) connected in series, each SQUID including: a superconducting loop containing two Josephson junctions connected to each other in parallel; and a flux-focussing region, the flux-focussing region configured to generate a screening current in response to the magnetic flux, the screening current modulating the corresponding voltage response for each SQUID which is in-phase with the voltage response of each other SQUID in the array.

Claims

1. A magnetometer for measuring an applied magnetic flux, the magnetometer comprising: an array of superconducting quantum devices (SQUIDs) connected in series, each SQUID in the array including a superconducting loop containing two Josephson junctions connected to each other in parallel; and a set of flux-focussing regions, each of the set of flux-focussing regions being associated with a corresponding SQUID in the array, each of the set of flux-focussing regions being configured to generate a screening current in response to the applied magnetic flux, the screening current modulating the corresponding voltage response of the corresponding SQUID such that the voltage response of the corresponding SQUID is in-phase with the voltage response of each other SQUID in the array.

2. A magnetometer according to claim 1, wherein the array is linear, and defines a long axis of the array.

3. A magnetometer according to claim 2, wherein the flux-focussing region is an additional portion of superconducting material connected to a portion of the superconducting loop of each SQUID.

4. A magnetometer according to claim 3, wherein the additional portion of superconducting material is integrally formed with the superconducting loop of the SQUID.

5. A magnetometer according to claim 4, wherein the flux-focussing regions which are connected to a portion of the superconducting loop of the SQUID extend outwards from the array in a direction which is substantially perpendicular to the long axis of the array.

6. A magnetometer according to claim 5, wherein each flux-focussing region has a length, measured in a direction perpendicular to the long axis of the array, and a width, measured in a direction parallel to the long axis of the array, the length being longer than the width.

7. A magnetometer according to claim 1, wherein: when the voltage response of an i.sup.th SQUID in the array is defined as V.sub.i=V.sub.i,max cos(.sub.i/.sub.0), the voltage response of an (i+1).sup.th SQUID in the array is defined as V.sub.i+1=V.sub.i+1,max cos(.sub.i+1/.sub.0), the phase difference between the i.sup.th and (i+1).sup.th SQUID is defined as (.sub.i.sub.i+1)/.sub.0, and there are N SQUIDs in the array the phase difference between the i.sup.th and (i+1).sup.th SQUID deviates by no more than /N from 2k, where k is an integer.

8. A magnetometer according to claim 1, wherein each flux-focussing region in the array is connected to at least two SQUIDs in the array.

9. A magnetometer according to claim 1, wherein each SQUID in the array is connected to at least two flux-focussing regions in the array.

10. A magnetometer according to claim 7, wherein each superconducting loop contains two half-loops of superconducting material, the two half-loops being separated from each other by a gap which forms the two Josephson junctions, and wherein when one side of the long axis of the array defines a first-side, and the other side of the long axis of the array defines a second-side, the array is formed of a series of repeating units, wherein each repeating unit includes: a first-side flux-focussing region on a first side of the array connected to: a first-side half-loop of a first superconducting loop, and a first-side half-loop of a second superconducting loop; and a second-side flux-focussing region on a second side of the array connected to: a second-side half-loop of the second superconducting loop, and a second-side half-loop of a third superconducting loop.

11. A magnetometer according to claim 8, wherein the array has reflectional symmetry about the long axis of the array.

12. A magnetometer according to claim 1, wherein the array of SQUIDs is formed, at least in part, of a high temperature crystalline superconducting material, capable of superconducting at temperatures above 30K.

13. A magnetometer according to claim 10, wherein the high temperature superconducting material is yttrium barium copper oxide.

14. A magnetometer according to claim 1, wherein the array of SQUIDs is formed from a bicrystal of the high temperature superconducting material, such that the lattices of the two crystals making up the bicrystal are oriented such that the angle between them is non-zero at the boundary between them, and wherein the Josephson junctions are formed at the boundary.

15. A magnetometer according to claim 1, wherein the array of SQUIDs includes a plurality of sub-arrays of SQUIDs, each sub-array itself being made up of a plurality of serially connected SQUIDs, and each sub-array being serially connected to at least one other sub-array.

16. A magnetometer according to claim 12 wherein, within each sub-array, the area of the superconducting loops decreases from the first SQUID in the sub-array to the last SQUID in the sub-array.

17. A method of fabricating a serially connected array of superconducting quantum interference devices (SQUIDs) of claim 1, including the steps of: depositing epitaxially a thin film of superconducting material onto a bicrystal substrate; depositing a layer of electrically conductive material on top of the thin film of superconducting material; patterning the electrically conductive layer and the thin superconducting film using optical lithography, to form the layout of the serially connected array; etching the pattern using an argon ion beam.

18. A method according to claim 17, wherein the superconducting material is yttrium barium copper oxide, and the substrate is strontium titanate.

19. A serially connected array of superconducting quantum interference devices (SQUIDs), each SQUID in the array including a superconducting loop having two Josephson junctions connected in parallel relative to each other; wherein each SQUID includes a flux-focussing region of a set of flux-focussing regions, such that, when a magnetic flux is incident on the flux-focussing region, a screening current is generated that modulates the voltage response, and wherein the simultaneous coupling of each SQUID to the incident magnetic flux causes the voltage responses of each SQUID to be in-phase with the responses of each of the other SQUIDs in the array.

20. A serially connected array of SQUIDs according to claim 19, wherein when the voltage response of an i.sup.th SQUID in the array is defined as V.sub.i=V.sub.i,max cos(.sub.i/.sub.0), the voltage response of an (i+1).sup.th SQUID in the arrays defined as V.sub.i+1=V.sub.i+1,max cos(.sub.i+1/.sub.0), the phase difference between the i.sup.th and (i+1).sup.th SQUID is defined as (.sub.i.sub.i+1)/.sub.0, and there are N SQUIDs in the array the phase difference between the i.sup.th and (i+1).sup.th SQUID deviates by no more than (/N) from 2k, where k is an integer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1a shows a schematic circuit diagram of part of an array of SQUIDs connected in series according to and embodiment of the present invention;

(3) FIG. 1b shows a representation of a Josephson junction in the RCSJ model;

(4) FIG. 1c shows schematically the realization of a Josephson junction in the bicrystal technology.

(5) FIG. 2a is a diagram showing the asymmetric layout of a magnetometer according to an embodiment of the present invention.

(6) FIG. 2b is a diagram showing a symmetric layout of a magnetometer according to an embodiment of the present invention. The asymmetry and symmetry are with respect to an imaginary line separating the square design into two identical parts: the upper half and the bottom half.

(7) FIG. 3 shows a micro-photo of the fabricated magnetometer of FIG. 2a;

(8) FIG. 4 shows a detailed layout of part of the array of SQUIDs according to a first embodiment of the present invention;

(9) FIG. 5 shows a micro-photo of part of the fabricated array of SQUIDs shown in FIG. 4.

(10) FIG. 6 shows a high-resolution micro-photo of part of the SQUID array shown in FIG. 4;

(11) FIG. 7 shows part of the design of the array of SQUIDs according to a second embodiment of the present invention; in this case the narrow flux-focusing regions are larger in size.

(12) FIG. 8 shows the total voltage response versus an externally applied flux to be measured (produced by a dc current I.sub.ctrl in a magnetic coil) recorded at a temperature T=40K obtained using a magnetometer according to an embodiment of the present invention. This particular magnetometer consists of 4411=484 SQUIDs connected in series.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

(13) FIG. 1a shows a schematic circuit diagram of a serial array of SQUIDs suitable for use in a magnetometer of the present invention.

(14) Each SQUID includes a superconducting loop containing two Josephson junctions and a flux-focussing region, the flux-focussing region configured to generate a screening current in response to the magnetic flux.

(15) The Josephson junctions JJ are represented by crosses. Each SQUID i consists of two Josephson junctions JJ.sub.i1 and JJ.sub.i2 connected in parallel via a SQUID inductance L.sub.i. Each pair of consecutive SQUIDs are connected in series via a mutual inductance M.sub.i,i+1. The SQUID array is biased with a DC current I provided by a current source (not shown). The total voltage response V (i.e. the sum of the voltage across each individual SQUID) can be measured by a nanovoltmeter.

(16) FIG. 1b shows the equivalent circuit of a Josephson junction i within the RCSJ (Resistively and Capacitively Shunted Junction) model. It consists of a parallel connection of a capacitance C.sub.i, a resistance R.sub.i and the non-linear element Josephson current I.sub.Ci sin(.sub.i).

(17) FIG. 1c shows schematically the realization of a Josephson junction in the bicrystal technology. In this technology a high temperature superconducting thin-film is epitaxially deposited on a SrTiO.sub.3 or an MgO bicrystal substrate. To produce such bicrystal substrates, two c-axis oriented single crystals are joined together so that along the bicrystal line there is a misorientation angle between the a and b axis of the two single crystals. The misorientation angle takes specific values, such as 24, 30, 36.8 or 45 degrees. The bicrystal line, i.e., the boundary between the two single crystals, acts as a weak link between the two superconductors and therefore the Josephson junctions can be formed as superconducting bridges crossing the bicrystal line using standard photolithography technology.

(18) FIG. 2a shows an example of an asymmetric layout of a magnetometer 110 according to the present invention. FIG. 2b shows an example of a symmetric layout of a magnetometer 110 according to the present invention. To achieve superior flux-coherent operation a symmetrical layout is preferable.

(19) The SQUID array 110 is located at the centre of the magnetometer 100. In this embodiment, the SQUID array 110 is a straight, linear array. Details of a part of the array itself are shown in FIG. 4. At each end of the array 110 is a voltage terminal 120a, 120b which can be connected to a voltmeter (not shown) in order to measure the summed voltage across the whole array 110. In the embodiment of FIG. 2a, the length of the array is 5808 m. In the embodiment of FIG. 2b, the length of the array is 9240 m.

(20) A micro photo of a series array of SQUIDs fabricated according to the embodiment shown on FIG. 2a of the present invention is shown in FIG. 3. The array is made up of 11 identical sub-arrays 125, each containing 411 individual SQUIDs (not visible in FIG. 3, but see micro-photo in FIG. 6 in which the SQUIDs are visible). In the array 110 shown in FIGS. 2a and 3, there are 484 SQUIDs, containing a total of 968 Josephson junctions. Details of the junctions themselves are described below, in relation to FIG. 4. Between each sub-array 125 there is a pick-off connection 130, each pick-off connection leading to a voltage terminal 140.

(21) Instead, for example, of connecting the voltmeter across the voltage terminals 120a and 120b, and in doing so taking a reading across all 484 SQUIDs in the array 110, it is possible to take a reading over, for example voltage terminal 120a and voltage terminal 140a, in order only to take a reading over 5411=220 SQUIDs. The voltage terminal 140 which is chosen determines the sensitivity of the measurement made. The multiple rectangular shaped superconducting coils 150, 150a located on both sides of the design are the pick-up coils of the SQUID serial array for enhancing the magnetic flux sensitivity of the magnetometer.

(22) FIG. 4 shows a zoomed in view of the SQUID array 110 in the magnetometer 100 layout of FIG. 2a.

(23) The structure of the array 110 at this point is best described in relation to a series of repeating units 116 (made of 116a and 116b) which are single DC SQUIDs.

(24) The magnetometer 100 shown in both FIGS. 2a and 3, is fabricated using bicrystal technology. In this technology schematically shown in FIG. 1c two c-axis oriented SrTiO.sub.3 single crystals (grains) are joined together to form a bicrystal substrate. There is a misorientation angle of either 24, 30, 36.8 or 45 between the a and b axis of the single crystals. An YBCO superconducting thin film is deposited on top of the SrTiO.sub.3 substrate. The bicrystal line, i.e., the boundary between the two single crystals, acts as a weak link between the two superconductors and therefore the Josephson junctions can be formed as superconducting bridges crossing the bicrystal line using standard photolithography technology. The Josephson junctions are seen as bridges crossing the bicrystal line shown with dotted line running across the centre of the array 110 in FIG. 4. The bicrystal line divides the array 110 into a first side A and a second side B.

(25) The structure will be described with reference to the left-most repeating unit 111 (highlighted). This repeating unit 111 is made up of two half loops and a central full superconducting loop (full loop) 112 in between them. Repeating unit 111 is connected to two flux-focussing regions 116A and 116B on sides A and B respectively.

(26) Flux-focussing region 116A is connected also to a second half-loop 113A which, assuming that current flows (i.e. meanders along the bicrystal line) from left to right in the array, is a half-loop on side A of the array, which is part of the full-loop through which current flows before full-loop 112. Flux-focussing region 116B is also connected to a second half-loop 113B, on side B of the array, the half-loop 113B being part of the full-loop which comes after full-loop 112 in the array.

(27) In this embodiment, flux-focussing regions 116A and 116B are rectangular sections of the same superconducting material as the loops 112, 113A, 113B, which are attached to the edges of these loops. In this micro-photo shown in FIG. 3, the superconducting material is YBCO. The total width of the array is 88 m (see FIGS. 4, 5 and 6), with each flux-focussing region, e.g. 116A being approximately 40 m long (see L.sub.FFR).

(28) In full loop 112, and all other full loops in the array, the half-loop 113A and half-loop 113B are each composed of one part of the two crystals making up the bicrystal. The full loop 112 contains two identical Josephson junctions J.sub.1 and J.sub.2 which can be seen as two parallel bridges across the bicrystal line represented with dotted line. The two half-loops of the full loop 112 meet at the bicrystal line, which acts as a weak link between the two superconducting single crystals due to the misalignment of the two crystals. This arrangement allows loop 112 to act as a single DC SQUID.

(29) In this embodiment, when moving along the array 110, from left to right, as in FIG. 4, the holes 115 of each successive loop 112 increase in height. In each set of 11 SQUIDs, the size of the hole 115 increases monotonically from 83 m to 123 m, then returning back to 83 m at the beginning of the next set of 11. The space between each flux-focussing region and the adjacent flux-focussing region on the same side is also 3 m. This size is limited by the photolithography process used to etch the array 110 from a single bicrystal.

(30) In this embodiment, pick-off connection 125 is integrally formed with a flux-focussing region, and leads to a pick-off terminal 140 (see FIG. 2a).

(31) FIGS. 5 and 6 show micro-photos of a magnetometer fabricated in accordance with the design of FIG. 4.

(32) FIG. 7 shows a series array of SQUIDs according to a second embodiment of the present invention. This is identical to the array of the first embodiment, differing only in the dimensions of the holes in the centre of the loops 115 and the flux-focussing regions 116.

(33) In this embodiment, the holes 115 are all the same size, which is 103 m. The flux-focussing regions are also larger. The entire width of the array 110 in this embodiment is 400 m.

(34) Experimental Results

(35) FIG. 8 shows the voltage response as a function of an externally applied flux to be measured of a magnetometer according to an embodiment of the present invention. The series SQUID arrays were fabricated by depositing high quality epitaxial, 100-150 nm thick c-axis oriented YBa.sub.2Cu.sub.3O.sub.7x (YBCO with x=0.15) films on 1010 mm.sup.2, symmetric [001] tilt SrTiO.sub.3 bicrystals by pulsed laser deposition (see FIG. 1c and FIG. 3). Angle can take the following values: 24, 30, 36.8 or 45. A 200 nm thick Au layer was deposited in situ on top of the YBCO film to facilitate fabrication of high quality electrical contacts for electric transport measurements. The films, with a critical temperature of T.sub.c of 92K, were subsequently patterned by optical lithography and etched by an Ar ion beam to form a serial array of 4411 SQUIDs consisting of 44 identical sub-arrays of 11 SQUIDs. All 484 SQUIDs have 2 identical 3 m wide Josephson junctions. Within each sub-array of 11 SQUIDs the SQUIDs have superconducting loops of identical width of 3 m while the length of these loops varies logarithmically from 12 m to 8 m.

(36) The value on the x-axis is the current I.sub.ctrl used to drive the applied magnetic field, and is directly proportional to the magnetic flux. Clearly, the results display the expected quasi-periodic nature: 7 oscillations are clearly visible which are symmetrically distributed around the largest oscillation which has its maximum at about I.sub.ctrl 1.2 mA.

(37) The results were taken at 40K, and using a range of current biases from 60 A to 60 A. It is known that the array can be biased at any bias current (as long as it is greater than the critical current of the Josephson junctions within the array) and display signal modulation with the magnetic flux. The response is not linear, however, and the maximum peak-to-peak voltage V.sub.max in this case was achieved at a current biases of 50 A and 50 A. Here, V.sub.max=17 mV, which is of the order of a thousand times greater than what can typically be achieved from single DC SQUIDs at comparable temperatures. It is noted that for a specific design layout the maximum peak-to-peak voltage V.sub.max is reached at a unique certain temperature where the condition .sub.L=1 is fulfilled. Since .sub.L is proportional to the Josephson junction's critical current .sub.L increases monotonically with decreasing temperature. .sub.L is also proportional to the SQUID inductance whose value can be controlled while designing the SQUID array layout. Therefore, for alternative array layouts, the maximum peak-to-peak voltage V.sub.max can be reached at any desirable temperature, in particular, at any temperature in the range 4.2-90 K.

(38) These results demonstrate the effects of the in-phase condition in a serial SQUID array consisting of 484 SQUIDs. The maximum output signal measured for this particular array was 17 mV which is 484 times larger than about 35 V which is about twice larger than that of a typical output voltage of the best single DC SQUIDs at the same temperature of 40 K.

(39) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(40) All references referred to above are hereby incorporated by reference.