Ferromagnetic Resonance Assisted Optomechanical Magnetometer

M. F. Colombano, G. Arregui, F. Bonell, N. E. Capuj, E. Chavez-Angel, A. Pitanti, S. O. Valenzuela, C. M. Sotomayor-Torres, D. Navarro-Urrios, and M. V. Costache

Phys. Rev. Lett. 125, 147201 – Published 1 October 2020

Abstract

The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this Letter, we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity of $850 pT {Hz}^{- 1 / 2}$ at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field, we demonstrate sensitivity values at resonance around a few $nT {Hz}^{- 1 / 2}$ up to the gigahertz range. Our results show that our hybrid system can be used to build a high-speed sensor of oscillating magnetic fields.

Received 4 March 2020
Revised 3 July 2020
Accepted 2 September 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.147201

Physics Subject Headings (PhySH)

Ferromagnetism Magnetoacoustic effect Magnons Mechanical & acoustical properties Optomechanics

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalNonlinear Dynamics

Authors & Affiliations

M. F. Colombano ^1,2, G. Arregui ^1,2, F. Bonell¹, N. E. Capuj^3,4, E. Chavez-Angel ¹, A. Pitanti⁵, S. O. Valenzuela^1,6, C. M. Sotomayor-Torres^1,6, D. Navarro-Urrios ^7,*, and M. V. Costache¹

¹Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain
²Departamento de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
³Departamento Física, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Spain
⁴Instituto Universitario de Materiales y Nanotecnología, Universidad de La Laguna, 38071 Santa Cruz de Tenerife, Spain
⁵NEST, CNR-Istituto Nanoscienze and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
⁶ICREA-Instituciò Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
⁷MIND-IN2UB, Departament d’Enginyerìa Electrònica i Biomèdica, Facultat de Física, Universitat de Barcelona, Martì i Franquès 1, 08028 Barcelona, Spain

^*Corresponding author. dnavarro@ub.edu

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Issue

Vol. 125, Iss. 14 — 2 October 2020

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Images

Figure 1
(a) Schematic representation of the magnetometer, including a microstrip waveguide, ferromagnetic YIG film, and barium-titanium-silicate microsphere. (b) Conceptual schematic of the device. The optomechanical system is modeled as a Fabry-Perot cavity with a moving mirror. $F_{th}$ and $F_{mag}$ denote the thermal force and the magnetostrictive forces, respectively. (c) A simplified schematic of the full experimental setup. A tapered fiber is used to probe the optical modes of the microsphere. TE polarization is set with a fiber polarization controller (FPC) and the transmitted signal is sent to a fast photodetector. Two electromagnetic coils are used to generate static magnetic fields that tune ferromagnetic resonance modes on the YIG film. (d) Optical whispering gallery modes spectrum measured on the microsphere. The inset shows a schematic of the coupling scheme between the tapered and the sphere and a simulation of the optical WGMs. (e) Mechanical mode spectrum of the microsphere. Only thermally driven motion is observed corresponding to radial breathing modes. The inset shows the displacement profile of the first three modes. (f) FMR resonances of the YIG film applying different static magnetic fields measured by detecting the reflected signal $Re (S_{11})$ .
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Figure 2
(a) Spectral response of the magnetometer $S (ω)$ measured with a spectrum analyzer. (b) Mechanical spectrum of the microsphere excited by applying a rf magnetic field of $2.3 μ T$ at 206 MHz, in red. The black curve corresponds to the mechanical mode without excitation. (c) Square root of SNR of the system as a function of the applied rf magnetic field. (d) System response $N (ω)$ as a function of the frequency of the rf field. The FMR is shown (inverted) on the back of the graph to illustrate the mechanical modes affected by the resonant effect. (e) Magnetic-field sensitivity $δ B_{\min} (ω)$ as a function of frequency defined where there is overlapped with mechanical modes. Five excited mechanical modes (labeled from I to V) allow us to calculate the sensitivity in a frequency window of $\sim 10 MHz$ around the frequencies of the mechanical resonances. A peak sensitivity of $δ B_{\min} (ω) \sim 850 pT {Hz}^{- 1 / 2}$ is achieved.
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Figure 3
(a) System response for different FMR modes excited on the YIG film. Gray curves show magnon resonances for different values of static magnetic field. (b) Peak magnetic-field sensitivity obtained by moving the FMR mode.
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