Ferrimagnetic Oscillator Magnetometer

Patent metadata

Field	Value
Publication number	US-2022011383-A1
Application number	US-202117317983-A
Country	US
Kind code	A1
Filing date	May 12, 2021
Priority date	Jul 12, 2020
Publication date	Jan 13, 2022
Grant date	—

Abstract

Ferrimagnetic oscillator magnetometers do not use lasers to stimulate fluorescence emission from defect centers in solid-state hosts (e.g., nitrogen vacancies in diamonds). Instead, in a ferrimagnetic oscillator magnetometer, the applied magnetic field shifts the resonance of entangled electronic spins in a ferrimagnetic crystal. These spins are entangled and can have an ensemble resonance linewidth of approximately 370 kHz to 10 MHz. The resonance shift produces microwave sidebands with amplitudes proportional to the magnetic field strength at frequencies proportional to the magnetic field oscillation frequency. These sidebands can be coherently averaged, digitized, and coherently processed, yielding magnetic field measurements with sensitivities possibly approaching the spin projection limit of 1 attotesla/√{square root over (Hz)}. The encoding of magnetic signals in frequency rather than amplitude relaxes or removes otherwise stringent requires on the digitizer.

First claim

1 . A magnetometer comprising:
a ferrimagnetic crystal comprising an ensemble of entangled electronic spins, the ensemble of entangled electronic spins having a resonance that shifts in response to an external magnetic field; a sustaining amplifier, in electrical communication with the ferrimagnetic crystal, to amplify a microwave signal modulated by a shift in the resonance of the ensemble of entangled electronic spins; and a digitizer, in electrical communication with the sustaining amplifier and/or the ferrimagnetic crystal, to digitize the microwave signal.

2 . The magnetometer of claim 1 , wherein the ferrimagnetic crystal and the sustaining amplifier are connected in a transmission geometry.

3 . The magnetometer of claim 1 , wherein the ferrimagnetic crystal and the sustaining amplifier are connected in a reflection geometry.

4 . The magnetometer of claim 1 , wherein the sustaining amplifier comprises a bipolar junction transistor.

5 . The magnetometer of claim 1 , further comprising:
a bandpass filter, in electromagnetic communication with an input of the sustaining amplifier, to filter the microwave signal.

6 . The magnetometer of claim 1 , further comprising:
a bias magnet, in electromagnetic communication with the ferrimagnetic crystal, to apply a bias magnetic field to the ensemble of entangled electronic spins.

7 . The magnetometer of claim 1 , wherein the shift in the resonance varies linearly with an amplitude of the external magnetic field.

8 . The magnetometer of claim 1 , wherein the shift in the resonance modulates sidebands onto the microwave signal with amplitudes proportional to an amplitude of the external magnetic field at offset frequencies proportional to an oscillation frequency of the external magnetic field.

9 . The magnetometer of claim 8 , wherein the magnetometer has a sensitivity versus the oscillation frequency of the external magnetic field that is substantially constant for f c <f m <f L , where f m is the oscillation frequency of the external magnetic field, f c is an observed noise corner of the sustaining amplifier, and f L is the Leeson frequency of the magnetometer.

10 . The magnetometer of claim 1 , further comprising:
an input coupling loop, inductively coupled to the ferrimagnetic crystal, to couple the microwave signal into the ferrimagnetic crystal; and an output coupling loop, inductively coupled to the ferrimagnetic crystal, to couple the microwave signal out of the ferrimagnetic crystal.

11 . The magnetometer of claim 10 , further comprising:
a directional coupler having an input port coupled to the output coupling loop, a through port coupled to an input of the sustaining amplifier, and a tap port coupled to the digitizer.

12 . The magnetometer of claim 1 , further comprising:
a feedback loop, in electromagnetic communication with the ferrimagnetic crystal and the sustaining amplifier, to generate and apply an error signal correcting an error between a frequency of the microwave signal and a center frequency of the resonance.

13 . A method of sensing an alternating current (AC) magnetic field with a self-sustaining oscillator comprising a ferrimagnetic material exhibiting a ferrimagnetic resonance and operably coupled to a sustaining amplifier, the method comprising:
applying the AC magnetic field to the ferrimagnetic material, the AC magnetic field shifting a center frequency of the ferrimagnetic resonance exhibited by the ferrimagnetic material and modulating a microwave oscillation supported by the self-sustaining oscillator; while applying the AC magnetic field to the ferrimagnetic material, amplifying, by the sustaining amplifier, the microwave oscillation transmitted by the ferrimagnetic resonance; and determining an amplitude and/or a frequency of the AC magnetic field based on the modulation of the microwave oscillation.

14 . The method of claim 13 , wherein modulating the microwave oscillation produces sidebands with amplitudes proportional to an amplitude of the external magnetic field at frequencies proportional to an oscillation frequency of the external magnetic field.

15 . The method of claim 14 , wherein determining the amplitude and/or the frequency of the AC magnitude field magnetometer has a sensitivity versus the oscillation frequency of the external magnetic field that is substantially constant for f c <f m <f L , where f m is the oscillation frequency of the external magnetic field, f c is an observed noise corner of the sustaining amplifier, and f L is the Leeson frequency of the magnetometer.

16 . The method of claim 13 , wherein determining the amplitude and/or the frequency of the AC magnetic field comprises:
measuring a real component of the microwave oscillation; reconstructing a complex representation of the microwave oscillation from the real component of the microwave oscillation; determining a phase angle of the microwave oscillation as a function of time based on the complex representation; and determining the AC magnetic field based on the phase angle.

17 . The method of claim 13 , wherein determining the amplitude and/or the frequency of the AC magnetic field comprises coherently averaging a digital representation of the microwave oscillation.

18 . The method of claim 13 , further comprising:
generating an error signal correcting an error between a frequency of the microwave oscillation and a center frequency of the resonance; and correcting the error based on the error signal.

19 . The method of claim 13 , further comprising:
applying a bias magnetic field to the ferrimagnetic material.

20 . A gradiometer comprising:
a first ferrimagnetic oscillator magnetometer to generate a first signal representing an amplitude of an external magnetic field at a first location; a second ferrimagnetic oscillator magnetometer to generate a second signal representing an amplitude of the external magnetic field at a second location; a mixer, operably coupled to the first ferrimagnetic oscillator magnetometer and the second ferrimagnetic oscillator magnetometer, to mix the first signal with the second signal, thereby producing a beat signal representing a gradient of the external magnetic field.

Assignees

• Massachusetts Inst Technology

Inventors

• Barry John F
• Irion Reed Anderson
• Kedziora Jessica
• Steinecker Matthew
• Freeman Daniel K
• Braje Danielle A

Classifications

• H03B15/006

  using spin transfer effects or giant magnetoresistance · CPC title

• G01R33/24

  for measuring direction or magnitude of magnetic fields or magnetic flux · CPC title

• G01R33/1284

  Spin resolved measurements; Influencing spins during measurements, e.g. in spintronics devices · CPC title

• G01R33/02 Primary

  Measuring direction or magnitude of magnetic fields or magnetic flux (G01R33/20 takes precedence) · CPC title

• G01R33/022 Primary

  Measuring gradient · CPC title