Fisher-information limits and fundamental bounds of magnetoplasmonic refractive-index sensing

Magnetization reversal is widely regarded as the element that adds signal to magnetoplasmonic sensors. We test this by deriving statistical, material, and geometric bounds on surface plasmon resonance (SPR) and magneto-optic SPR of Au, Au/Co/Au, and Co/Au films at 785 nm with unknown source power, background, tilt, and incidence angle. The Fisher information separates exactly into common-mode and ratio channels, and reversal improves the single-angle bound by only 0.02%. With the power unknown, only the ratio channel survives, and its Cramér–Rao bound reduces to the conventional magneto-optical resolution, 6.9 × 10⁻⁸ refractive-index units (RIU) for the Au/Co/Au trilayer. An angular profile rejects the power at about 1% cost; only an independent angular reference separates index from angle drift.