Abstract
Abstract
Classical size dispersion and experimental artefacts have long obscured the discrete quantum energy levels of individual magnetic sites in disordered nanoparticle ensembles. We report a field-periodic, phase-coherent switching resonance—robust under instrumental and processing controls—in the first-order reversal curve (FORC) response of biogenic magnetoferritin at 2 and 5 K. The period, μ₀ΔH = 11.9 ± 1.6 mT, is invariant and phase-coherent (p ≈ 10⁻³), and the resonance disappears above the blocking temperature. Cross-correlation shows the ridges tilt in proportion to the core remanence slope (ρ ≈ 0.78): the structure tracks the internal hysteretic state of the core, not the applied field, excluding instrumental and processing origins. The spacing gives an axial zero-field splitting D = 0.016 ± 0.002 K, identifying the active unit as an individual surface Fe³⁺ ion (S = 5/2), whose relaxation is exponentially amplified by cores thermally poised at their switching threshold (Γt ≈ 1). The resulting size–temperature window (TB/9.0 ≤ T ≤ TB) accounts for the thirty-year absence of resolved resonances in conventional magnetometry and predicts the geometric signature recovered in independent data at 6.9σ. This establishes a paradigm where macroscopic metastability serves as a quantum amplifier, enabling benchtop spectroscopy of single-ion quantum states in inherently disordered nanomaterials.