Torque magnetometer utilizing integrated piezoresistive levers

• US 5,798,641 A
• Filed: 03/17/1997
• Issued: 08/25/1998
• Est. Priority Date: 03/17/1997
• Status: Expired due to Term

• Current Assignee: Quantum Design International, Inc.
• Original Assignee: Quantum Design, Inc.
• Inventors: Spagna, Stefano, Simmonds, Michael Bancroft
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US08/819,376
• Time in Patent Office: 526 Days
• Field of Search: 324/201, 324/228, 324/235, 324/244, 324/249, 324/252, 324/259, 324/260, 324/261, 324/262, 324/226, 73/727, 73/514.21, 73/514.23, 73/514.29, 73/514.33, 73/514.34, 73/514.36, 73/862.338, 73/862.634
• US Class Current: 324/228
• CPC Class Codes: G01N 27/72 by investigating magnetic variables
• CPC Class Codes: G01R 33/1215 Measuring magnetisation; Particular magnetometers therefor G01R33/14 takes precedence; electrodynamic magnetometers G01R33/028

Abstract

A torque magnetometer employing integrated piezoresistive levers to measure magnetization of very small anisotropic samples. A sample holding platform is cantilevered from the substrate by parallel legs on which piezoresistors have been applied. A Wheatstone bridge, which includes the piezoresistors, is completely integrated on a magnetometer substrate. Specific output connections are made to the electrical conductors of the bridge in a manner to eliminate Hall effect error signals in an alternative embodiment.

Claims

1. A torque magnetometer for sensing the magnetic moment of a sample in the presence of an applied changing magnetic field in a test chamber, said magnetometer comprising:

an electrically non-conductive substrate formed with a cantilevered torque lever extending from one edge of said substrate, said substrate being configured to be removably mounted in the test chamber, said torque lever comprising;

a sample holding paddle distally projecting from said one edge; and

bridging means connecting said paddle to said one edge;

a pair of parallel spaced piezoresistors surface-mounted on said bridging means;

means on said substrate for detecting, comparing and processing signals from said piezoresistors indicative of resistance changes therein resulting from torsion on said paddle caused by magnetically induced torque on the sample to produce output signals representing the physical effect on the sample of the applied magnetic fields;

output terminals on said substrate for connecting said output signals to external signal processing circuitry; and

input terminals on said substrate for applying electrical power to said piezoresistors.

2. The magnetometer recited in claim 1, wherein said substrate surrounds said paddle, said paddle being formed in a pit formed through said substrate.

3. The magnetometer recited in claim 1, wherein said signal detecting, comparing and processing means comprises a Wheatstone bridge, wherein said piezoresistors comprise two of the legs of said bridge.

4. The magnetometer recited in claim 3, wherein said bridge is formed entirely on said substrate and on said torque lever.

5. The magnetometer recited in claim 3, wherein said bridge comprises:

bridge balancing resistance elements formed on said substrate; and

electrically conductive traces connecting said resistance elements and said piezoresistors so as to form four legs of said bridge, said traces having a predetermined width and, within the structure of said bridge, having inside edges and outside edges;

said output terminals being connected to said inside edges of said traces to thereby substantially eliminate Hall effect error signals.

6. The magnetometer recited in claim 1, wherein said piezoresistors are formed of constantan.

7. The magnetometer recited in claim 4, wherein all of the resistance elements of said Wheatstone bridge are formed of constantan.

8. The magnetometer recited in claim 1, wherein each said piezoresistor extends from said torque lever bridging means onto said substrate.

9. The magnetometer recited in claim 1, wherein said torque lever is coplaner with one surface of said substrate and is substantially thinner than said substrate.

10. The magnetometer recited in claim 9, wherein the thickness of said torque lever is at least an order of magnitude thinner than said substrate.

11. The magnetometer recited in claim 1, wherein said paddle is approximately square and is about 2 mm on a side, said paddle being spaced from said one edge of said substrate by about 1 mm, said torque lever being able to accommodate samples for test purposes having a mass equal to or less than about 0.01 gm and having dimensions as large as about 1.5×

1.5×

1.5 mm.

12. The magnetometer recited in claim 1, wherein said torque lever has a torque sensitivity of about 3×

10³¹ 10 Nm and a moment sensitivity of about 2×

10^-8 emu at 9 T.

13. The magnetometer recited in claim 1, wherein said bridging means comprises a pair of spaced parallel legs.

14. The magnetometer recited in claim 5, wherein said bridge comprises two balancing resistors in two additional legs of said bridge, and wherein all four of said resistor elements are mounted on said substrate in a physically parallel arrangement with each other, thereby making all Hall effect voltages that develop to be substantially equivalent in all said resistor elements.

15. The magnetometer recited in claim 1, and further comprising:

a modular carrier board to which is removably mounted said substrate, said board having connection means for removably mounting said board in the test chamber, and contact elements for making connection to said input and output terminals of said substrate.

16. A torque magnetometer for sensing the magnetic moment of a sample in the presence of an applied changing magnetic field in a test chamber, said magnetometer comprising:

an electrically non-conductive substrate formed with a cantilevered torque lever extending from one edge of said substrate, said torque lever comprising;

a sample holding paddle distally projecting from said one edge; and

bridging means connecting said paddle to said one edge;

a pair of parallel spaced piezoresistors surface-mounted on said bridging means;

an electrical bridge configuration normally balanced when in the quiescent condition, said piezoresistors comprising two legs of said bridge configuration, said electrical bridge configuration having balancing resistance elements, and output terminals connected in said electrical bridge configuration for detecting an unbalance resulting from torsion on said paddle caused by magnetically induced torque on the sample, said balancing resistance elements being made of the same material as are said piezoresistors;

output terminals on said substrate for connecting said output signals to external signal processing circuitry; and

input terminals on said substrate for applying electrical power to said piezoresistors.

17. The magnetometer recited in claim 16, wherein said bridge comprises:

electrically conductive traces connecting said bridge balance resistance elements and said piezoresistors so as to form four legs of said bridge, said traces having a predetermined width and, within the structure of said bridge, having inside edges and outside edges;

said output terminals being connected to said inside edges of said traces to thereby substantially eliminate Hall effect error signals.

18. The magnetometer recited in claim 16, wherein all of said piezoresistors and balancing resistor elements formed as traces of all-metallic material having a temperature coefficient of resistance substantially less than p-doped silicon piezoresistors.

19. The magnetometer recited in claim 16, wherein said output terminals are connected to the inside edges of said bridge traces to thereby substantially eliminate Hall effect error signals.

20. The magnetometer recited in claim 19, wherein said applied magnetic field in said test chamber is relatively large, being normally greater than about 1 T.

21. A torque magnetometer for sensing the magnetic moment of a sample in the presence of an applied changing magnetic field in a test chamber, said magnetometer comprising:

an electrically non-conductive substrate formed with a cantilevered torque lever extending from one edge of said substrate, said torque lever comprising;

a sample holding paddle distally projecting from said one edge; and

bridging means connecting said paddle to said one edge;

a pair of parallel spaced piezoresistors surface-mounted on said bridging means;

an electrical bridge configuration normally balanced when in the quiescent condition, said piezoresistors comprising two legs of said bridge configuration, said electrical bridge configuration having balancing resistance elements and output terminals connected in said electrical bridge configuration for detecting an unbalance resulting from torsion on said paddle caused by magnetically induced torque on the sample, said balancing resistance elements being made of the same material as are said piezoresistors;

output terminals on said substrate for connecting said output signals to external signal processing circuitry; and

input terminals on said substrate for applying electrical power to said piezoresistors;

said bridge configuration further comprising electrically conductive traces connecting said balancing resistance elements and said piezoresistors on said substrate so as to form four legs of said bridge, said traces having a predetermined width and, within the structure of said bridge, having inside edges and outside edges;

said output terminals being connected to said inside edges of said traces to thereby substantially eliminate Hall effect error signals that occur at magnetic fields greater than about 1 T.

22. The magnetometer recited in claim 21, wherein said piezoresistors and said balancing resistor elements are formed of the same all-metallic material.

23. The magnetometer recited in claim 22, wherein said resistance elements are formed of constantan.

24. The magnetometer recited in claim 23, wherein said output terminals are connected to voltage sensing points on the inside edges of said traces which are at equal potential.

25. A method for sensing the magnetic moment of a sample in the presence of an applied changing magnetic field in a test chamber, said method comprising the steps of:

forming an electrically non-conductive substrate with a cantilevered torque lever extending from one edge of the substrate and configured with a paddle on which the sample to be tested is retained, the paddle being connected to the substrate by bridging means;

forming a pair of parallel spaced piezoresistors on the surface of the bridging means and extending onto the substrate;

forming an electrical balancing bridge configuration on the substrate, the piezoresistors being two legs of the bridge, there being two other balancing resistance element legs of the bridge;

forming output terminals on the substrate which are connected in the bridge to sense unbalance conditions, the output terminals being configured to connect signals indicative of the state of balance and unbalance of the bridge to external signal processing circuitry;

forming input terminals on the substrate configured to apply electrical power to the piezoresistors;

mounting the substrate with a rotation platform within the test chamber;

applying a magnetic field to the sample on the paddle in the test chamber;

varying the strength of the magnetic field; and

rotating the platform while applying the magnetic field to the sample in the test chamber.

26. The method recited in claim 25, wherein the piezoresistors, the balancing resistance elements and the output terminals are formed on the substrate.

27. The method recited in claim 25, wherein the output terminals are connected to the inside of the traces forming the bridge to thereby substantially eliminate Hall effect error signals.