From Hysteresis to Linearity: Intrinsic Low-Field Sensitivity of Magnetic Tunnel Junctions

Magnetic tunnel junction (MTJ) sensors are widely explored for detecting ultra-low magnetic fields, yet their reported sensitivities are often inflated by hysteretic effects rather than genuine linear response. In this work, we identify the conditions under which hysteresis vanishes and the MTJ enters a fully reversible regime, revealing a stable, intrinsic sensitivity that remains constant across specific drive conditions. Using carefully constrained low-field ac excitation and validated analytical modeling, we show that this reversible response is governed by the intrinsic susceptibility of the free layer, independent of domain switching or irreversible magnetization processes. The resulting figure of merit provides a rigorous basis for comparing MTJ performance without ambiguity from hysteretic gain. This framework redefines how sensitivity should be evaluated in magnetic sensors and establishes the reversible regime as the true benchmark for low-field detection, with implications for biomagnetic and precision field-sensing applications.