Wide-range TMR sensing with array-scaled low-frequency field resolution for strong near-field analog tracking

Strong near-field analog sensing requires magnetic sensors to accommodate large field excursions without saturation while retaining sufficient resolution for small low-frequency field variations. In tunneling magnetoresistance (TMR) sensors, extending the linear range through increased effective magnetic anisotropy reduces the field sensitivity and consequently penalizes the noise-equivalent field resolution. Here, we investigate this range–sensitivity–resolution coupling in perpendicular synthetic antiferromagnetic (p-SAF) TMR sensors. The CoFeB free-layer thickness is systematically varied from 1.6 to 2.2 nm to tune the effective anisotropy and establish the range–sensitivity relationship, yielding a hysteresis-free linear response over the experimentally accessible range of at least ±1000 Oe. We subsequently vary the number of series-connected magnetic tunnel junctions under otherwise comparable magnetic and electrical conditions to determine whether array scaling can recover the low-frequency detectivity penalty associated with wide-range operation. Increasing N from 200 to 2000 improves detectivity DB from 2.27 to 1.04μT/Hz at 10 Hz, while all arrays retain a verified linear-range lower bound of ±1000 Oe. The optimized array is finally evaluated in a magnetic-switch prototype as a representative strong near-field analog sensing platform, where continuous non-saturating response is maintained under a strongly varying magnetic field. Building on low-current sensor operation, a proposed ADC-free time-over-threshold readout architecture is further explored as a prospective system-level strategy for reducing data-conversion overhead. These results identify free-layer anisotropy and array size as complementary design variables for jointly controlling wide-range response and low-frequency field resolution, while highlighting their potential for low-power strong near-field analog sensing systems.