Reassessing magnetic tunnel junction detectability for ultrasensitive sensing using small-field sensitivity and Jiles-Atherton modeling.

Magnetic tunnel junction (MTJ) sensors are attractive for detecting extremely small magnetic fields, yet their performance is often quantified using sensitivities derived from major loops or larger-field minor loops which inherently include hysteretic contributions. As a result, these conventional metrics significantly overestimate detectability by including irreversible processes that vanish in the small-field limit. Here, we systematically measure the low-field response of uniaxial MTJs under both ac and dc magnetic excitation and demonstrate a distinct transition from hysteretic to fully reversible behavior as the field amplitude is reduced. By adapting the Jiles-Atherton model to analyze MTJ conductance, we establish a unified framework that captures magnetization processes down to nanotesla range, yielding quantitative parameters that resolve reversible and irreversible dynamics with excellent fidelity. In the reversible regime, the sensitivity converges to a constant value of 0.55%Oe ${}^{-1}$ , defining an intrinsic, history-independent response of the MTJ. Importantly, this is less than half of the 1.2%Oe ${}^{-1}$ obtained from higher-field minor loop estimates but is physically representative of the sensing response in low-field operation. The constant and intrinsic sensitivity corresponds to the true value of detectability, confirmed by noise spectral density measurements under low-field ac excitation. These results establish a quantitative framework for MTJ evaluation that emphasizes intrinsic, hysteresis-free performance, providing both a realistic basis for calculating field detectability and a general methodology for probing magnetization in micron- and nano-scale ferromagnets.