Characterizaion of Micro-Magnets on Mechanical Oscillators, and Other Sensors Using Fiber-Optic Interferometry*

We report the design, construction, and use of a fiber-optic interferometer system with variable applied dc and ac magnetic fields and magnetic field gradients for the characterization of micro-magnets on oscillators. The system has measured calibrated displacements to determine resonant frequencies (~1–800 kHz), quality factors (~100–8000), amplitudes, and spring constants (~0.01 N/m) of resonances. Thermal-noise-driven data determined spring constants. The driven response to ac magnetic field gradients (amplitudes ~0.4–40 x10^–4 T/m) provided direct measurement of magnetic moments. E.g., for a ~45±5-µm-diameter permalloy sphere, we measure a magnetic moment of 2.8±0.1 x10^–8 J/T, in agreement with the expected saturation moment of 3.1±0.6 x10^–8 J/T. Characterization of these micro-magnets supports our nuclear magnetic resonance force microscopy (NMRFM) studies. In addition to NMRFM, other ongoing work involving fiber-optic interferometry includes its use in integrated, narrow (~0.2–2 mm Φ) oscillator-fiber sensors for placement in thin channels (e.g., for fast, local quench detection) and its use in a fast-response pressure sensor for operation under extreme (jet/rocket engine) conditions.