Thin magnetic film magnetometer for providing independent responses from two orthogonal axes

Claim 2

A magnetometer system including a transducer comprised of ferromagnetic material capable of assuming opposed states of residual flux density along its easy axis of magnetization, said material being initially magnetized substantially in a predetermined one of said states and existing substantially as a single large domain of said predetermined state, said transducer further comprising winding means inductively coupled to said material, the inductance value of said transducer being dependent upon the response of the transducer to an external magnetic field environment which includes field components directed respectively along the easy and hard axes of magnetization of said material, means coupled to said transducer for cyclically applying chopper magnetic fields to said material directed along the hard axis thereof, said chopper fields being modulated by the hard axis field components present in said external environment, means operatively connected to said transducer for sensing said inductance value and for providing a transducer output signal containing easy axis field component information as well as modulated chopper field signals containing hard axis field component information, said means for sensing said inductance value being of such a nature that the magnetization of said material is disturbed but not permanently altered in state, signal separator means operatively connected to receive said transducer output signal for separating said easy axis information from said modulated chopper signals, and detector means coupled to said signal separator means for removing said hard axis information from said modulated chopper signals, said magnetometer system thereby providing separate and distinct information representative of the magnetic field components directed along two orthogonal axes.

Claim 3

A magnetometer system 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.

Claim 4

A magnetometer system as defined in claim 1 further characterized in that said winding means includes a winding having its coil axis oriented parallel with the hard axis of magnetization of said material.

Claim 5

A magnetometer system as defined in claim 1 wherein said means for cyclically applying a chopper field to said material includes second winding means inductively coupled to said material, and a source of chopper current having a preselected repetition frequency, said second winding means being adapted to be energized from said source of chopper current, thereby generating the chopper magnetic fields applied to said material.

Claim 6

A magnetometer system as defined in claim 4 wherein said chopper current exhibits a square or rectangular waveform.

Claim 7

A magnetometer system comprising in combination at least one ferromagnetic thin film element capable of assuming opposed states of residual flux density along an easy direction of magnetization, said element being initially magnetized in a predetermined one of said states, said element acting substantially as a single large domain of said predetermined state, an inductor winding disposed about said element in such a manner as to link the thin film element magnetic flux in the hard direction of magnetization, capacitive means connected in parallel with said thin film element inductor winding and forming therewith a parallel resonant tank circuit, a source of radio frequency current, said tank circuit being Adapted to be energized from said source of radio frequency current controlled in amplitude so as to limit the perturbation of the magnetization of said element to small angular rotations incapable of altering said single domain configuration, said film element and inductor winding comprising a transducer having a total inductance value dependent upon the external magnetic field environment to which said element is subjected, said external environment including magnetic field components directed respectively along the easy and hard directions of magnetization of said element, chopper winding means inductively coupled to said thin film element, a source of chopper current, said chopper winding means being adapted to be cyclically energized from said source of chopper current whereby hard direction chopper magnetic fields are applied to said element, the hard direction magnetic field components present in said external environment modulating said chopper fields, the relative amplitudes and phases of said modulated chopper fields being indicative of the magnitude and polarity of said hard direction magnetic field components, the radio frequency signals impressed across said tank circuit by said radio frequency current being modulated by the easy direction field components and the modulated hard direction chopper fields, radio frequency detector means coupled to said tank circuit for demodulating said radio frequency signals, the resulting output of said radio frequency detector containing easy direction field component information as well as modulated hard direction chopper field information, frequency-selective signal separator means operatively connected to receive the output of said radio frequency detector means for separating said easy direction information from said modulated chopper field information, and synchronous chopper detector means coupled to said signal separator means for demodulating the hard direction chopper field information.

Claim 8

A magnetometer system comprising in combination at least one ferromagnetic thin film element capable of assuming opposed states of residual flux density along an easy direction of magnetization, said element being initially magnetized in a predetermined one of said states, said element acting substantially as a single large domain of said predetermined state, an inductor winding disposed about said element in such a manner as to link the thin film element magnetic flux in the hard direction of magnetization, capacitive means connected in parallel with said thin film element inductor winding and forming therewith a parallel resonant tank circuit, a source of radio frequency current, said tank circuit being Adapted to be energized from said source of radio frequency current controlled in amplitude so as to limit the perturbation of the magnetization of said element to small angular rotations incapable of altering said single domain configuration, said film element and inductor winding comprising a transducer having a total inductance value dependent upon the external magnetic field environment to which said element is subjected, said external environment including magnetic field components directed respectively along the easy and hard directions of magnetization of said element, chopper winding means inductively coupled to said thin film element, a source of chopper current, said chopper winding means being adapted to be cyclically energized from said source of chopper current whereby hard direction chopper magnetic fields are applied to said element, the hard direction magnetic field components present in said external environment modulating said chopper fields, the relative amplitudes and phases of said modulated chopper fields being indicative of the magnitude and polarity of said hard direction magnetic field components, the radio frequency signals impressed across said tank circuit by said radio frequency current being modulated by the easy direction field components and the modulated hard direction chopper fields, radio frequency detector means coupled to said tank circuit for demodulating said radio frequency signals, the resulting output of said radio frequency detector containing easy direction field component information as well as modulated hard direction chopper field information, frequency-selective signal separator means operatively connected to receive the output of said radio frequency detector means for separating said easy direction information from said modulated chopper field information, and synchronous chopper detector means coupled to said signal separator means for demodulating the hard direction chopper field information.

Claim 9

A magnetometer system as defined in claim 6 further characterized in that said 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.

Claim 10

A magnetometer system as defined in claim 6 wherein said chopper winding means comprises a pair of windings inductively coupled to said film element and so disposed as to link the magnetic flux thereof in the hard direction of magnetization, said source of chopper current including a chopper signal generator and a chopper current driver, said chopper signal generator being coupled to said chopper current driver and providing a preselected repetition frequency for the operation thereof, said chopper driver being a bipolar current driver and being coupled to said pair of windings for causing chopper current flow therethrough at said repetition frequency whereby hard axis chopper magnetic fields are applied to said element.

Claim 11

A magnetometer system as defined in claim 8 wherein said chopper signal generator is an astable multivibrator having a square wave output, said preselected repetition frequency being of the order of 80 kHz.

Claim 12

A magnetometer system as defined in claim 6 further characterized in that said source of radio frequency current has a preselected fixed frequency, said parallel resonant tank circuit being detuned from resonance at said fixed frequency, the impedance magnitude of said tank circuit being a function of said total inductance value.

Claim 13

A magnetometer system as defined in claim 10 further characterized in that said source of radio frequency current comprises a transistor connected in a crystal controlled Pierce oscillator circuit having a fixed frequency of approximately 11 mHz.

Claim 14

A magnetometer system as defined in claim 6 further including means for applying a bias magnetic field to said thin film element directed along the easy direction of magnetization thereof.

Claim 15

A magnetometer system as defined in claim 6 wherein said synchronous chopper detector comprises an input high pass RC filter for passing the modulated chopper field information and rejecting the easy direction information, first and second transistors connected in a cascaded emitter follower configuration, a pair of field effect transistors each having source, drain and gate electrodes, means coupling the output signal of the cascaded emitter follower transistors in common to the respective source electrodes of said field effect transistors, third and fourth transistors connected in a differential switch configuration, means coupling the square wave output signal of said chopper generator to said differential switch transistors to control the operation thereof, while providing a reference signal of chopper generator frequency to said synchronous chopper detector, the output signals of said third and fourth transistors being applied respectively to the gate electrodes of said field effect transistors and causing each of said last mentioned transistors to alternately switch between conducting and nonconducting states, the demodulated hard direction chopper field information appearing on the drain electrodes of said field effect transistors.

Claim 16

A magnetometer system as defined in claim 6 further including separate easy axis and hard axis feedback networks, each of said networks including in series a feedback winding and a current sampling impedance, the easy axis and hard axis feedback windings being inductively coupled to said thin film element and being oriented to link the magnetic flux thereof respectively in the easy and hard directions of magnetization, easy and hard axis amplifier means, said easy axis amplifier means being operatively connected to said signal separator means for amplifying said easy direction information signals, the output of said easy axis amplifier means being applied to said easy axis feedback winding for causing current flow therethrough and generating a magnetic field directed along the easy direction of magnetization of said film element and opposed to the easy axis field component present in said external environment, the voltage appearing across the easy axis current sampling impedance in response to current flow through said easy axis feedback winding providing an indication of the magnitude and direction of said easy axis field component, said hard axis amplifier means being operatively connected to said synchronous chopper detector means for amplifying said hard direction information signals, the output of said hard axis amplifier means being applied to said hard axis feedback winding for causing current flow therethrough and generating a magnetic field directed along the hard direction of magnetization of said film element and opposed to the hard axis field component present in said external environment, the voltage appearing across the hard axis current sampling impedance in response to current flow through said hard axis feedback winding providing an indication of the magnitude and direction of said hard axis field component.

Claim 17

A magnetometer system as defined in claim 14 for providing a system output signal indicative of the easy axis component of the external magnetic field environment and in which the transducer response to the hard axis component of said environment has been effectively eliminated through the action of said hard axis feedback network, further including chopper filter means and an output amplifier, said chopper filter means being coupled to receive the easy axis information voltage appearing across said easy axis current sampling impedance and being comprised of low pass and high pass filter networks, said low pass filter network providing additional attenuation of residual signals at the chopper repetition frequency appearing in said easy axis information, said high pass filter network coupling the output of said low pass filter to said output amplifier, said output amplifier providing said system output signal.

Claim 18

A magNetometer system as defined in claim 14 further including a chopper frequency electrostatic and magnetic shield surrounding said hard axis feedback winding to minimize the coupling of electrical signal components of chopper frequency into the hard axis amplifier.