Abstract:
The invention relates to a SQUID arrangement for measuring a change in a magnetic field, the change in the magnetic field being caused by a specimen that is arranged in a magnetization field and the SQUID arrangement including a direct current SQUID. In order to provide a simple design in which the losses in the magnetic flux are also simultaneously minimized, it is provided that the SQUID itself is embodied for generating the magnetization field.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a SQUID arrangement for measuring a change in the magnetic field having a direct current SQUID, the change in the magnetic field being caused by a specimen that is arranged in a magnetization field. Moreover, the invention relates to a method for measuring such a change. Finally, the method relates to the use of such a SQUID arrangement or such a method for various applications. 
   If the change in the magnetization of a specimen is to be measured, this specimen must be disposed for this purpose in a constant magnetic field, the so-called exciter field or magnetization field. A change in the magnetization of the specimen can then be detected as a change in the magnetic field. A superconducting quantum interference detector (SQUID) can measure such a change in the magnetic field in the surroundings of the specimen. The SQUID is coupled to the specimen via a flux transformer. However, the flux transformer transfers only a portion of the magnetic flux and thus also only a portion of the signal energy to the SQUID. Or, in particular in the case of very small specimens, there is direct coupling in that the specimen is placed directly in the SQUID loop. This can reduce a loss in magnetic flux. Such an arrangement is known for instance in U.S. Pat. No. 4,588,947. In both cases, the magnetization field is produced by a separate superconducting coil into which a so-called exciter current or magnetizing current is injected. 
   One object of the present invention is to provide a simple design in which the losses in the magnetic flux are also simultaneously minimized. 
   SUMMARY OF THE INVENTION 
   This object is attained using a SQUID arrangement according to the invention. According thereto, it is provided that the SQUID arrangement for measuring a change in a magnetic field, the change in the magnetic field being caused by a specimen arranged in a magnetization field, has a direct current SQUID and the SQUID itself is generates the magnetization field. Furthermore, this object is attained using a method according to the invention. According to it, in a SQUID arrangement having a direct current SQUID for measuring a change in a magnetic field, the change in the magnetic field being caused by a specimen arranged in a magnetization field, it is provided that the magnetization field is generated by the SQUID itself. 
   In other words, a fundamental idea of the invention is comprised in a novel SQUID design such that the quantization loop of the SQUID is used on the one hand as a measuring circuit and on the other hand for generating the magnetization field. In accordance with the invention, a current that flows in a part of the quantization loop induces the magnetization field and the change in specimen properties generates a magnetic flux directly in the SQUID. 
   The invention creates a solution in which, first, the specimen is better coupled to the SQUID, and second, magnetic scatter fields outside of the specimen are minimized. 
   Moreover, it is advantageous that the arrangement is comparatively simply constructed and does not require any separate superconducting coil for generating the magnetization field. 
   The inventive SQUID arrangements can be used particularly advantageously for instance for the following purposes:
         in a susceptometer for measuring the magnetic susceptibility of a specimen, in particular in a micro-susceptometer for investigating extremely small particles,   for measuring the penetration depth of a magnetic field into a superconducting specimen,   in a magnetic (micro)-calorimeter for use in an x-ray detector, and,   in a bolometer, in particular for detecting weak radiation, as an alternative for superconducting or semiconducting thermistors of the bolometer.       

   In accordance with a first embodiment of the invention, the SQUID has at least two superconducting current loops such that the current loops have a common line interrupted with two Josephson junctions, whereby the two current loops are arranged such that one closed superconducting path is possible that does not run through the Josephson junctions. For this, for the measuring operation, two current loops are preferably switched parallel to the two Josephson junctions such that a parallel gradiometer arrangement occurs like a double gradiometer. For coupling the magnetization current in as ring current, the two current loops are then switched in series so that an (external) current path results that does not run through the Josephson junctions. 
   In accordance with another embodiment of the invention, the magnetization current is a temporal non-decaying (persistent) current that is injected into the current loops when a part of one current loop is in the resistive state. This is preferably achieved in that a thermal switch for interrupting the superconduction is provided in one of the two current loops. The thermal switch is preferably connected to a heating element that locally disconnects the superconduction. 
   According to another embodiment of the invention, one of the two current loops receives the specimen, while the other current loop acts as reference loop and for coupling in the feedback flux. The change in the magnetic properties of the specimen causes a change in the flux distribution between the two current loops, which generates a SQUID signal. 
   It is particularly advantageous to place the SQUID arrangement including the specimen on a chip. Because of this, and because of the direct coupling of the specimen to the SQUID, extremely small SQUID sensors can be provided. This is very advantageous for cost effective, rapid, and effective cooling for maintaining operation of the superconductor. 
   Briefly stated, the present invention provides SQUID arrangement for measuring a change in a magnetic field having a direct current SQUID, the change in the magnetic field being caused by a specimen that is arranged in a magnetization field, and wherein the SQUID itself is embodied so at to generate the magnetization field. 
   As a feature of the above SQUID arrangement the SQUID has at least two superconducting current loops such that the two current loops have a common line interrupted with two Josephson junctions and the current loops are arranged such that one closed superconducting path is possible that does not run through said Josephson junctions. 
   A further feature of the above present invention includes optionally providing a thermal switch in one of the current loops for in interrupting the superconduction for injecting a magnetization current (I) into the closed superconducting path. 
   Yet another feature of the above present invention includes the option of providing the sample arranged in one of the current loops while the other current loop acts as reference loop. Still further an option of the present invention includes using the reference for coupling in feedback flux from a coil coupled thereto. 
   The present invention further provides a method for measuring a change in a magnetization field using a SQUID arrangement, wherein SQUID arrangement has a direct current SQUID and configured such that the change in the magnetic fluid is caused by a specimen that is arranged in the magnetization field and the magnetization field is generated by the SQUID itself. 
   Further still the above method of present invention provides for the SQUID having at least two superconducting current loops arranged such that the current loops have a common line interrupted with two Josephson junctions and the current loops are arranged such that a closed superconducting path is possible that does not run through said Josephson junctions and wherein an interruption in the superconduction for injecting a magnetization current (I) into the closed superconducting path is provided for using a thermal switch in one of the two current loops. 
   Another feature of the arrangement and method of the present invention includes use of the above arrangements and methods in a susceptometer for measuring the magnetic susceptibility of a specimen, in particular in a micro-susceptometer for investigating extremely small particles wherein the specimen is arranged in the magnetization field. 
   Still another feature of the arrangement and method of the present invention includes use of the above arrangements and methods for measuring a penetration depth of a magnetic field into a superconducting specimen. 
   Yet another feature of the arrangement and method of the present invention includes use of the above arrangements and methods in a magnetic (micro-) calorimeter for use in an x-ray detector. 
   Still a further feature of the arrangement and method of the present invention includes use of the above arrangements and methods in a bolometer, in particular for detecting weak radiation, as an alternative for superconducting or semiconducting thermistors of the bolometer. 
   One exemplary embodiment of the invention is described in greater detail in the following with reference to the drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  provides a simplified schematic depiction of an inventive SQUID arrangement with its essential components. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The exemplary embodiment depicted illustrates a SQUID chip  100  and applied to its printed circuit board  101  is the entire SQUID arrangement with a direct current SQUID  1  and a specimen  2 . This application is by means of thin-film technology, especially by means of sputtering or laser ablation. 
   The direct current SQUID  1  essentially includes a superconducting loop that it is interrupted by two sites. Such weak sites are also called Josephson junctions. The manner in which such a SQUID  1  functions is known in general. It is based on the effect of flux quantization and superconducting loops and on the Josephson effect and will not be explained in greater detail herein. 
   The direct current SQUID  1  in the exemplary embodiment has a superconducting current loop  3  that is interrupted by Josephson junctions  5 ,  6  and that has a coil L 1 . Connected in parallel to this coil L 1  is another coil L 2  that forms the current loop  4 , which is also interrupted with the Josephson junctions  5 ,  6 . In other words, the loops  3  and  4  form the quantization loops for the SQUID  1 . 
   The current loops  3 ,  4  are arranged relative to one another such that a closed superconducting path is possible that does not run through the Josephson junctions  5 ,  6 . The arrangement occurs in particular such that the two current loops  3 ,  4  are folded to the Josephson junctions  5 ,  6 . This results in a line  19  that can be used jointly by both current circuits  3 ,  4  and on which the two Josephson junctions  5 ,  6  are arranged. The two current loops  3 ,  4  are switched together in series for coupling in a magnetization current I as ring current so that an external current path results that does not run through the Josephson junctions  5 ,  6 . 
   The quantization loop of the SQUID  1  that results from the two current loops  3 ,  4  is thus used on the one hand as a measuring circuit and on the other hand for generating the magnetization field. 
   The magnetization current I is a persistent current that is coupled into the current loops  3 ,  4  when part of the current loop  4  is in a resistive state. This interruption in the superconduction in the illustrated exemplary embodiment is attained in that a thermal switch  7  is provided in the current loop  4  for interrupting the superconduction. The thermal switch  7  is arranged between the coil L 2  and the common line  19  of the two current loops  3 ,  4 . The thermal switch  7  has a heating element  9  and is activatable by this heating element such that the superconduction is disconnected. The heating element  9  is connected to connecting lines  17  and joined to a control unit of the SQUID  1  (not shown) via connector contacts  18 . 
   The magnetization current I is coupled in via feed lines  13  that are connected to the current loop  4  on both sides of the thermal switch  7  and that are connected to a corresponding current source (not shown) via connector contacts  14 . Once coupled in, the heating element  9  is deactivated and superconduction is restored. The external current source is connected to the connector contacts  14  of the SQUID chip  100  via a corresponding connecting cable (not shown). The current loop  3  receives the specimen  2 , i.e. the specimen is disposed directly in the interior of this current loop  3 , while the other current loop  4  acts as reference loop and for coupling in the feedback flux and thus for operating the flux regulating loop of the SQUID. The feedback flux is coupled in using a feedback or compensation coil  8  that is also arranged on the SQUID chip  101 . The reaction coil  8  is connected to connecting lines  15  and connected via connector contacts  16  to a control unit (not shown) of the SQUID  1 . The control unit is externally arranged and connected via connecting lines (not shown) to connector contacts  16 . 
   A change in the magnetic properties of the specimen  2 , for instance due to a change in temperature as a result of the absorption of an x-ray quantum, causes a change in the flux distribution between the two current loops  3 ,  4 , which generates a SQUID signal. The pick-up of the SQUID signals occurs via a connecting line  10  to the second coil L 2  and via a connecting line  11  connected to the common line  19  between the two Josephson contacts  5 ,  6 . The evaluating unit (not shown) of the SQUID  1  is connected to the two connecting lines  10 ,  111  via the connector contacts  12 . The evaluating unit is arranged externally and is connected to the connector contacts  12  via connecting lines (not shown). 
   The embodiments just described in the foregoing can for instance be used with a paramagnetic specimen when the SQUID chip  100  is employed in a magnetic calorimeter for use in an x-ray detector. 
   All of the features described in the specification, subsequent claims, and drawing can be essential to the invention both individually and in any desired combination with one another. 
   LEGENDS 
   
       
         1  Direct current SQUID 
         2  Specimen 
         3  First current loop 
         4  Second current loop 
         5  First Josephson junction 
         6  Second Josephson junction 
         7  Thermal switch 
         8  Reaction coil 
         9  Heater 
         10  Connecting line 
         11  Connecting line 
         12  Connector contact 
         13  Feed line 
         14  Connector contact 
         15  Connecting line 
         16  Connector contact 
         17  Connecting line 
         18  Connector contact 
         19  Common line 
         100  SQUID chip 
         101  Printed circuit board