Abstract:
A superconductive quantum interference device (SQUID) system is provided for measuring the alternating current (AC) magnetic susceptibility of material at a single frequency or a mixing frequency from multiple frequencies, such as mf 1 +nf 2 , where f 1  and f 2  are two excitation frequencies of two primary coils. The system includes a magnetic-flux sourcing unit for producing an AC magnetic flux on a sample and a magnetic-flux reading unit for reading the induced magnetic flux from the sample via a magnetic flux transformer. The magnetic-flux reading unit includes a SQUID set for detecting the induced magnetic flux, so as to obtain the magnetic susceptibility of the sample in converting calculation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to measuring magnetic susceptibility of material. More particularly, the present invention relates to measuring magnetic susceptibility of material by a superconductive quantum interference device. 
   2. Description of Related Art 
   The superconductive quantum interference device (SQUID) has been known with the high capability for sensing magnetic flux, so that many kinds of magnetic detection systems using the SQUIDs have been developed. With its high sensitivity on magnetic flux, SQUID element has been applied to detect very small magnetic signal, such as the magnetic signals generated by bio-activity or nano-blocks, for example. 
     FIGS. 1A-1C  are drawings, schematically illustrating several types of the SQUID design. In  FIG. 1A , A SQUID  100  is usually fabricated on a substrate. The substrate has a boundary  101 . The boundary  101 , for example, forms two grain regions  102   a  and  102   b  with a grain boundary. Alternatively for example, the two regions  102   a  and  102   b  may have a step height to form a step boundary. The SQUID  100  has the superconductive film as shown in  FIG. 1A  by shading. The SQUID  100  includes two Josephson junctions  110  connected in parallel. The electrode lead  104   a  is deposited on the substrate at the region  102   a , usually having two lead terminals. One terminal I  108  is for applying a current through the Josephson junctions  110  and the other terminal V  106  is for detecting an induced voltage signal. The electrode lead  104   b  is grounded. 
   The basic characteristics of SQUID is introduced as follows. When a bias current a little higher than the critical current is injected at the terminal I  108  and flows through the Josephson junctions  110 , the Josephson junctions become resistively shunted junctions and a voltage across the Josephson junctions occurs. Due to the Meissener effect of superconductive material, when an external magnetic flux is shone onto a SQUID, a circulating current through these two junctions is induced to compensate the external magnetic flux within the area enclosed with the superconducting ring having these two Josephson junctions. Thus, an induced current by the external magnetic flux is generated to flow through the effectively shunted resistors of the Josephson junctions. As a result, the voltage detected at the terminal V  106  varies due to the application of an external magnetic flux. The voltage cross the junctions is a periodic function in response to the applied magnetic flux. 
   In  FIG. 1B , with the SQUID design in  FIG. 1A , two superconductive coils are connected to the SQUID element so as to have two detection regions  90   a  and  90   b . If the magnetic field is not uniform, then the detection regions  90   a  and  90   b  produces unbalance effect. As a result, a magnetic gradient can be detected by the gradiometer. The proceeding SQUID is operated in directly biased current (DC) mode. However, further example in  FIG. 1C , a radio-frequency (RF) SQUID magnetometer can be designed. The LC resonant circuit  94  can detect the magnetic field shone onto the superconductive coil  92  with a Josephson junction inserted in the ring. Further, if the superconductive coil  92  is replaced with two coils, which share a common Josephson junction, then a RF SQUID gradiometer can be obtained. The SQUID can be designed in various ways. 
   By utilizing the ability of SQUID to sense the magnetic flux, especially very weak magnetic flux, various SQUID-based systems have been developed in different aspect of applications, such as magnetocardography, magnetoencephalography, non-destructive detection, picovoltmeter, etc. However, the property of SQUID-based AC magnetic susceptibility χ AC  has not yet been considered. The present invention is more directed to the measurement of AC magnetic susceptibility χ AC . 
   SUMMARY OF THE INVENTION 
   The invention provides a SQUID system for measuring magnetic susceptibility of a material. Since the SQUID element is used, the sensitivity is significantly improved. 
   The invention provides a superconductive quantum interference device (SQUID) system for measuring magnetic susceptibility of a material. The SQUID system includes a magnetic-flux sourcing unit, which has an excitation coil set and a pick-up coil set. The excitation coil set supplies a varying magnetic flux on the material and the pick-up coil set senses an induced magnetic flux from the material. A magnetic-field transfer route is coupled to the pick-up coil set for transferring the induced magnetic flux to a location where the magnetic-flux reading blocks is seated. A magnetic-flux reading unit has at least a SQUID set at the location to sense the induced magnetic flux, and an electronic circuit block to read the induced magnetic flux from the SQUID set. 
   In other words, the proceeding embodiment of the SQUID system includes the sensing block and the reading blocks, which are coupled together by a magnetic-field transfer unit. As a result, the magnetic susceptibility property of a sample material can be detected. 
   Alternatively, the invention provides a SQUID system for measuring magnetic susceptibility, including a pick-up coil set at an environment with a varying magnetic field, so as to produce an induced current. A transforming element is coupled to the pick-up coil set for transferring the induced current to a location, wherein the induced current flows through a coil loop to produce an induced magnetic field. A SQUID unit is at the location to detect the magnetic field. 
   It should be understood that both the proceeding general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIGS. 1A-1C  are drawings, schematically illustrating several types of the SQUID design. 
       FIG. 2  is a drawing, schematically illustrating an operation mechanism of a SQUID system, according to an embodiment of the invention. 
       FIG. 3  is a drawing, schematically illustrating a structure of the pick-up coil, according to an embodiment of the invention. 
       FIG. 4  is drawing, schematically illustrating a structure of the SQUID system for measuring magnetic susceptibility of material, according to an embodiment of the invention. 
       FIG. 5  is experimental results in measuring the magnetic susceptibility of material, according to an embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the invention, a SQUID system for measuring varying magnetic field is provided. Because the magnetic field H(t) is varying with time, the varying magnetic field also produces a varying magnetic flux, which induces a current on a loop. 
   For a material being magnetized, the magnetic susceptibility χ is a usual parameter to describe the magnetic property of the material. Particularly, when an AC magnetic field H AC  with a desired frequency, which may be a single frequency or a mixing frequency, is applied on a sample material. The susceptibility χ AC  of the material is defined by χ AC =M AC /H AC , where M AC  is the AC magnetization of the detected material under an applied AC magnetic field H AC . AC magnetization is not only an important quantity to denote the magnetic property of the material, but also an indicator to characterize the amount of targeted material marked with magnetic markers, such as magnetically labeled immunoassay. The M AC  in conventional manner may be measured by an open-end Faraday coil. However, the M AC  can also be measured by the invention with improved sensitivity. Some embodiments are provided for describing the features of the invention but not for limiting the invention. 
     FIG. 2  is a drawing, schematically illustrating an operation mechanism of a SQUID system, according to an embodiment of the invention. In  FIG. 2 , the invention proposes a SQUID system, which can at least measure the M AC  of a material in high sensitivity. The SQUID system of the invention for measuring magnetic susceptibility includes, for example, a pick-up coil set  120  posited at an environment with a varying magnetic induction B 1 (t), so as to produce an induced current “i” on the pick-up coil set  120 . The pick-up coil set  120  can be, for example, a single loop coil or a solenoid coil, or even a pair of solenoids with opposite winding direction as to be described in  FIG. 3 . In further application, for example, a material with the pick-up coil set  120  is also magnetized by the varying magnetic induction B 1 (t), and produces a magnetization M. The varying magnetic induction B 1 (t), for example, is an AC magnetic field with a desired frequency. This magnetization M AC  is then produced, according to the electromagnetic phenomenon. The quantity of M AC  also accordingly changes the induced current “i”. In other words, the induced current carries the information of the M AC . Then, the susceptibility can also be indirectly measured. The measuring mechanism, as an example, will be further described later. 
   Further, a transforming element  122  is coupled to the pick-up coil set  120  for transferring the induced current “i” to a remote location, at which the induced current can be measured in high sensitivity. The induced current “i” flows through a coil loop  124  to produce an induced magnetic induction B 2 (t) under the physical phenomenon. In other words, it has a relation between B 1 (t) and B 2 (t), in which the information of susceptibility is carried by B 2 (t). Of course, the B 2 (t) also causes the changing of magnetic flux on the coil loop  124 . The coil loop  124  can be a single loop or a solenoid. Usually, the induced magnetic induction B 2 (t) is small. In this consideration a current amplifier  126  or a resonant circuit instead can be, for example, included to transfer the magnetic filed to the remote location. A SQUID unit  128  can be used at that location to detect the magnetic induction B 2 (t). Since the SQUID unit  128  is sensitivity to the magnetic flux, the B 2 (t) can be precisely measured, and the susceptibility M can be obtained by calculation, for example. In general, the SQUID unit  128  includes a SQUID element, which can be a usual design as previously described in  FIGS. 1A-1B  and so on, as the examples. 
   In  FIG. 2 , the coil loop  124  may also pick up the background effect from the magnetic filed B 1 (t), not just the effect of the M on the pick-up coil set  120 . However, the background effect from the magnetic filed B I (t) can be removed by a design as shown in  FIG. 3 , for example. In  FIG. 3 , the pick-up coil set can be formed by a pair of solenoids  130  and  132  coupled at in series, but with opposite wiring directions. For example, the solenoid  130  is winding clockwise from the terminal  136  to the terminal  138  wile the solenoids  132  is winding counterclockwise from the terminal  140  to the terminal  136 . When an external magnetic induction B(t)  134  is applied through the solenoids  130  and  132 , according to the electromagnetic phenomenon, the current i 1  and current i 2  are induced on the solenoid  130  and solenoid  132 . However, due to the opposite winding direction, the current i 1  is flowing from the terminal  136  to the terminal  138  while the current i 2  is flowing from the terminal  136  to the terminal  140 . As a result, the two currents i 1  and i 2  are opposite, too, and therefore cancel out each other. If the quantities of the currents i 1  and i 2  are equal, then it results in zero current between the terminals  138  and  140 . In this manner, the background current can be cancelled. In accordance with the intended applications, the pick-up coil set  120  in  FIG. 2  can be designed with the structure in  FIG. 3 . In this manner, when the magnetized material is posited in one of the solenoids, an output current can directly reflect the magnetization M(t) without including the background current. The design of  FIG. 3  is more helpful in removing background current. 
   Based on the proceeding design mechanism, the invention proposes a SQUID system.  FIG. 4  is drawing, schematically illustrating a structure of the SQUID system for measuring magnetic susceptibility of material. In  FIG. 4 , a SQUID-based AC magnetosusceptometry is provided as an embodiment. The design principle is based on the structure in  FIG. 2 . The SQUID system in general includes a magnetic-flux sourcing unit  400 , having an excitation coil set ( 156 + 160 ) and a pick-up coil set  142 . The excitation coil set in this example includes two coils  156  and  160 , which are for example wound on the circular barrels  154  and  158 . The coils  156 ,  160  can be for example a solenoid design, but the solenoid design is not the only option. In addition, the number of coils in the excitation coil set can be one or more, depending on actual operation frequency is desired. Basically, each coil is driven by a function generator to produce a magnetic induction B(t), proportional to magnetic field H(t), at a given frequency. When multiple coils are included together, then a mixed AC frequency, such as mf 1 +nf 2 , can be obtained, where m and n are integers, f 1  and f 2  are excitation frequencies. The excitation coil set is to produce a time-varying magnetic field to the sample material  162  so as to magnetize the material  162  with the magnetization M(t). The magnetization M(t) of the material  162  is to be measured, so as to obtain the AC susceptibility. 
   In order to sense the effect of the magnetization M(t) of the material  162 , a pick-up coil  142  on the barrel  140  can detect the material  162 . The magnetic induction (or magnetic field) produced from the excitation coil set can enter the pick-up coil  142  and the material  162 . The material  162  is then magnetized. As previously described in  FIG. 3 , the pick-up coil  142  can be designed with a pair of solenoids  142 ,  146  with opposite winding direction. The solenoids  142  and  146  can be, for example, wound on the barrels  140  and  144 . If the solenoids  142  and  146  are equal size and turns, then the background induced current can be cancelled. The net induced current is substantially just from the effect of magnetization of the material  162 . The element  150  and  152  are, for example, the supporting elements to support and cover the material inside the pick-up coil. Here, the magnetic-flux sourcing unit  400  further needs some mechanical support elements and so on. The detail for mechanical support elements is not described here. 
   The induced current from the pick-up coil  142  can be transfer to the remote end, which can be a coil loop  168  for example, through a magnetic-field transfer route  164 . When induced current “i” flows through the end coil loop  168 , a magnetic induction B(t) can be according produced. The end coil loop  168  can be a single loop or a solenoid, as a design option. The induced magnetic induction/field B(t)/H(t) also forms a magnetic-field flux. Here, the induced current “i” may be small, and can be amplified by a circuit  166 , such as a resonant circuit or a current amplifier. The location of the end coil  168  can be located away from the material  162 . 
   In addition, generally, a magnetic-flux reading unit  402  is included to read the induced magnetic field. Here, at least a SQUID set  170  near the end coil  168  is used to sense the induced magnetic flux. The electronic circuit block is used to read the induced magnetic flux from the SQUID set  170 . The electronic circuit block may, for example, include the SQUID electronic circuit  172  and readout electronic circuit  174 . According to the information to be detected, the SQUID set  170  can be, for example, radio-frequency SQUID magnetometer, radio-frequency SQUID gradiometer, direct-current SQUID magnetometer, direct-current SQUID gradiometer, or electronic gradiometer, without specific limit. The SQUID is more sensitivity to the magnetic flux compared to the convention flux detector, so that the measuring precision and sensitivity can be improved. 
   Further, since SQUID is operated at a low temperature in the range pertaining to high-T c  superconductive material, the SQUID is operated in a liquid nitrogen environment  184 , which can be formed by a house  182 . Further, the outer house  176  with magnetic and/or RF shielding function is used. Further, the sponge  178  or elastic supporting element can be used to avoid vibration. The detail for mechanical support elements is not described here. 
   The invention has proposed the SQUID system to, for example, measure the AC magnetic susceptibility from a material, which can be a bio-sample and so on. The applied frequency can be obtained by a mix from at least one coil respectively with a specific frequency.  FIG. 5  is experimental results in measuring the magnetic susceptibility of material, according to an embodiment of the invention. In  FIG. 5 , the χ AC  is measured with two target frequency f=mf 1 +nf 2 . The SQUID system is serving as the AC magnetosusceptometry, for example. The upper drawing is the spectrum under the situation without the material  162  while the lower drawing is the spectrum under the situation with the material  162 . It can be clearly seen that a significant χ AC  peak is detected out. In other words, the present invention can actually work well. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.