MAGNETOMETER EMPLOYING MEANS RESPONSIVE TO VARIATIONS OF MAGNETIZATION VECTOR POSITION IN A THIN FILM SENSOR

• US 3,619,772 A
• Filed: 08/29/1969
• Issued: 11/09/1971
• Est. Priority Date: 08/29/1969
• Status: Expired due to Term

• Current Assignee: Unisys Corporation
• Original Assignee: David M. Ellis
• Inventors: Ellis, David M.
• Primary Examiner(s): Rolinec, Rudolph V.
• Assistant Examiner(s): Corcoran, R. J.
• Application Number:
  US04/854,197
• Time in Patent Office: 802 Days
• Field of Search: 324/43R,47,34R 340/174TF
• US Class Current: 324/249
• CPC Class Codes: B82Y 25/00 Nanomagnetism, e.g. magnetoimpedance, anisotropic magnetoresistance, giant magnetoresistance or tunneling magnetoresistance
• CPC Class Codes: G01R 33/02 Measuring direction or magnitude of magnetic fields or magnetic flux G01R33/20 takes precedence
• CPC Class Codes: G01R 33/028 Electrodynamic magnetometers
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

A thin magnetic film magnetometer is described in which the thin film sensor element is driven in both its easy and hard axes directions by oscillators having different frequencies. The nature of the signal appearing on a winding coupled to the thin film along its easy direction depends upon the angular position of the thin film magnetic moment or magnetization (M) vector and the film easy axis; which in turn is a function of an external field applied to the film along its hard axis. Both manual and automatic means are described for sensing the position of the M vector and restoring it to its rest position along the thin film easy axis- such action producing a measure of the magnitude and direction of the external applied field.

Claims

1. A magnetometer comprising ferromagnetic material capable of assuming opposed states of residual flux density along a preferred axis of magnetization, said material being magnetized substantially in a predetermined one of said states, and existing substantially as a single large domain of said predetermined state, alternating current source means, winding means disposed on said ferromagnetic material in fixed relationship to said preferred axis of magnetization, said winding means being adapted to be energized by said alternating current source means for applying dual frequency alternating magnetic fields concurrently to said material in directions respectively transverse to and parallel with said preferred axis, said dual magnetic fields being controlled in magnitude such that the magnetization of said material is disturbed but not permanently altered in state, the magnetic moment of said material being displaced from its position parallel with said preferred axis in response to the external magnetic field environment to which said material is subjected in a direction transverse to its preferred axis, means for sensing a preselected combinatorial frequency component of the magnetic flux generated by said dual frequency magnetic fields, the amplitude and phase of said frequency component being a function of the degree of displacement of the magnetic moment from the preferred axis of said material, said frequency component being zero when said magnetic moment lies parallel to the preferred axis of said material, correction winding means inductively coupled to said material and being so disposed therewith as to link the magnetic flux of said material in a direction transverse to said preferred axis, means operatively connected to said correction winding means for causing current flow therethrough, said last-mentioned current having an amplitude and polarity such that the correction magnetic field generated thereby and applied to said material is substantially equal and opposite to said magnetic field environment, said magnetic moment being restored by said correction field to its position parallel to the preferred axis of said material, the amplitude and polarity of the current flow in said correction winding means being indicative of the magnitude and direction of said external field environment.

2. A magnetometer as defined in claim 1 further characterized in that said ferromagnetic material is a thin film of nickel-iron alloy having a thickness of not more than 5,000 Angstrom units.

3. A magnetometer as defined in claim 1 wherein said winding means comprise first and second windings inductively coupled to said material and being so disposed therewith as to respectively link the magnetic flux of said material in directions transverse to and parallel with said preferred axis, said alternating current source means comprising first and second sources of respective different fixed frequencies of alternating current, said first and second windings being adapted to be energized respectively by said first and second source of alternating current.

4. A magnetometer as defined in claim 3 wherein said first and second sources of alternating current are individual radiofrequency oscillators.

5. A magnetometer as defined in claim 4 wherein said sensed combinatorial frequency component is the sum-frequency component of the flux generated by said radiofrequency oscillators.

6. A magnetometer as defined in claim 5 wherein said means for sensing said sum-Frequency component includes said second winding means, a band-pass filter coupled to said second winding means and adapted to pass said sum-frequency component, means coupled to the output of said filter for amplifying and detecting said sum-frequency component, and null indicator means for displaying the detected component.

7. A magnetometer as defined in claim 6 wherein said means operatively connected to said correction winding means for driving current therethrough comprises a DC current source.

8. A magnetometer as defined in claim 7 further including a nulling current indicator operatively connected to said correction winding means for indicating the amplitude and polarity of the current flowing therethrough.

9. A magnetometer comprising a ferromagnetic thin film element capable of assuming opposed states of residual flux density along an easy direction of magnetization, said element being magnetized substantially in a predetermined one of said states and acting substantially as a single large domain of said predetermined state, first and second windings disposed about said thin film element in fixed relationship to said easy directions of magnetization and being inductively coupled to said thin film element in a manner to link the magnetic flux respectively in the hard and easy directions of magnetization, first and second RF oscillators for providing respectively radiofrequency currents of different fixed frequencies, said first and second windings being adapted to be energized respectively by said first and second RF oscillators whereby dual radiofrequency fields are applied concurrently to said element, said radiofrequency fields being controlled in magnitude so as to limit the perturbation of the magnetization of said element to small angular rotations incapable of altering said single domain configuration, the angular displacement of the M magnetization vector of said element from the easy direction of magnetization being a function of the external magnetic field applied to the element along its hard direction of magnetization, means for applying a hard direction audiofrequency field to said element in concurrence with said dual radiofrequency fields, said audiofrequency field being controlled in magnitude such that the magnetization of said element is disturbed but not permanently altered in state, means including said second winding for sensing a preselected combinatorial frequency component of the magnetic flux generated by said radiofrequency magnetic fields and modulated by said audiofrequency magnetic field, the amplitude and phase characteristic of said modulated frequency component being a function of the degree and polarity of the angular displacement of the magnetization vector from the easy direction, correction winding means inductively coupled to said element in a manner to link the magnetic flux of said element in the hard direction of magnetization, current steering means for coupling said correction winding means to said means for sensing the modulated frequency component in a closed circuit path, said current steering means selectively providing current flow through said correction winding means as a function of said sensed frequency component, thereby generating a correction magnetic field which when applied to said element substantially cancels said external magnetic field, the nature of the current flow in said correction winding means being indicative of the magnitude and direction of said external field. 10 A magnetometer as defined in claim 9 further characterized in that said ferromagnetic thin film element is a nickel-iron alloy composed substantially of 83 percent nickel and 17 percent iron, and having a thickness of approximately 2,000 Angstrom units.

11. A magnetometer as defined in claim 9 wherein said sensed combinatorial frequency component is the sum-frequency component of the flux generated by said RF oscillators.

12. A magnetometer as defined in claiM 9 wherein said means for applying a hard direction audiofrequency field to said element includes an AF oscillator coupled to said first winding for causing audiofrequency current flow therethrough.

13. A magnetometer as defined in claim 9 wherein said correction winding means comprise a pair of correction windings wound in opposite phase and connected in parallel with respect to each other, the polarity and magnitude of said correction magnetic field applied to said element being a function of the respective amplitudes of the currents flowing through said correction windings at any given time.

14. A magnetometer as defined in claim 12 wherein said means for sensing said modulated frequency component which includes said second winding also comprises s band-pass filter coupled to said second winding for passing said component, a sense amplifier and detector coupled to the output of said filter for amplifying and detecting said component, and a phase discriminator operatively connected to receive the detector output signal, means coupling the output of said AF oscillator to said phase discriminator to furnish a reference signal therefor, said phase discriminator providing as its output a DC control signal which is related to the phase of the audiofrequency modulation of said frequency component, and means for applying said DC control signal to said current steering means.

15. A magnetometer as defined in claim 13 wherein said current steering means comprises two parallel current paths electrically connected between sources of DC current, each of said paths including in series a current amplifying device, one of said correction windings, and a resistor.

16. A magnetometer as defined in claim 15 further including a nulling current indicator connected across the resistors in the respective parallel current paths for indicating the difference voltages thereacross and providing a measure of the currents flowing respectively through said correction windings, the indicator reading providing information on the direction and strength of said external field.

17. A magnetometer comprising a plurality of thin magnetic film elements, each element being capable of assuming opposed states of residual flux density along an easy direction of magnetization, each element being magnetized substantially in a predetermined one of said states and acting substantially as a single large domain of said predetermined state, means for orienting said elements with their easy directions of magnetization in mutually orthogonal relationship, alternating current source means, winding means disposed on said elements in fixed relationship to the respective easy directions of magnetization thereof, said winding means being adapted to be energized by said alternating current source means for applying dual radiofrequency fields concurrently to each of said elements in respective hard and easy directions of magnetization, means for applying a different hard direction audiofrequency field to each of said elements, means for sensing in each of said elements a sum-frequency component of the magnetic flux generated by said dual radiofrequency fields and modulated by the particular audiofrequency field applied to the element, the amplitude and phase characteristic of said modulated sum-frequency component in each of said elements being a function of the angular displacement of the M-vector within the element in response to an external magnetic field component applied to the element along its hard direction of magnetization, correction winding means inductively coupled to each of said elements in a manner to link the magnetic flux of the element in the hard direction of magnetization, each said correction winding means being positioned with respect to the other of said plurality of elements such that mutual inductive coupling exists thereamong, a current steering circuit associated with each of said elements for coupling said correction winding means to said meaNs for sensing said modulated sum-frequency component in a closed circuit path, each said current steering circuit selectively providing current flow through its associated correction winding means as a function of said modulated sum-frequency component, thereby generating a correction magnetic field which when applied to a given element substantially cancels the external magnetic field component applied to said given element, the nature of the current flow in the correction winding means associated with said given element being indicative of the magnitude and direction of the field component applied thereto, and indicator means for providing a measure of said current flow in each said correction winding means.

18. A magnetometer as defined in claim 17 wherein said winding means comprise a first plurality of windings associated respectively with said plurality of elements, each of said last-mentioned windings being disposed to link the magnetic flux of one of said plurality of elements in the hard direction of magnetization, and a second winding common to said plurality of elements and being disposed to link the magnetic flux in the easy direction in each said element, said alternating current source means comprising first and second sources of respective different fixed radiofrequency currents, said first plurality of windings and said second winding being adapted to be energized respectively by said first and second sources of radiofrequency currents.

19. A magnetometer as defined in claim 18 wherein said means for applying a different hard direction audiofrequency field to each of said elements comprises a plurality of AF oscillators coupled respectively to said first plurality of windings for causing audiofrequency currents to flow therethrough in concurrence with said radiofrequency currents from said first source.

20. A magnetometer as defined in claim 19 wherein said means for sensing a modulated sum-frequency component in each said element comprises said second winding, a band-pass filter coupled to said second winding to pass the modulated sum-frequency components generated within said plurality of elements, a sense amplifier and detector coupled to the output of said filter for amplifying and detecting said last-mentioned components, a plurality of phase discriminators, said phase discriminators being operatively connected in common to receive the detector output signals, means coupling the outputs of said plurality of AF oscillators respectively to said phase discriminators for furnishing reference signals therefor, each of said discriminators being tuned to operate upon the sum-frequency component whose modulation is at the frequency of said reference signal and to provide as its output a DC control signal which is related to the phase of the audiofrequency modulation of the sum-frequency component, and means for applying the plurality of output DC control signals respectively to the current steering circuits.