Magnetic Field Sensor Based on a Spin-Wave Interferometer with a Positive Feedback

We demonstrate experimentally the operation of a spin-wave magnetometer integrated into a circuit with a positive feedback. The circuit consists of passive magnetic and active electric parts. The magnetic part include a sensing element, which is a magnetic cross junction made of Y₃Fe₂(FeO₄)₃. The electric part includes a non-linear amplifier and a phase shifter. The electric and magnetic parts are connected via micrometer size antennae. Spin waves are excited by two of these antennae while the output inductive voltage produced by the interfering spin waves is detected by the third antenna. Spin waves propagating in the orthogonal arms of the cross may accumulate significantly different phase shifts, depending on the direction and the strength of the external magnetic field. The output inductive voltage reaches its maximum in the case of constructive spin wave interference. The positive feedback provides further signal amplification. It appears possible to enhance the response function, compared to the passive circuits without a feedback, by a factor of ×100 without an increase in the noise level. The experimental data show a prominent response to the external magnetic field variation, exceeding 5×10³ V/T . At the same time, the intrinsic noise spectral density of the device can be as low as 10^-16 V²/Hz. The estimated sensitivity of the presented prototype is 2×10^-12 T/√Hz. at room temperature. We argue that spin-wave magnetometers can potentially be as sensitive as SQUIDs while operating at room temperature.