Improved Robustness against Magnetic Field in Spin-Orbit-Torque-Based Physical Unclonable Functions through Write-Back Operation

Physical unclonable functions (PUFs), which exploit uncontrollable and unpredictable randomness of materials or devices, have been investigated as a hardware-based security primitive owing to their robustness against adversarial attacks. Spin-orbit torque (SOT) switching is one of the promising techniques for PUF applications because it can provide randomness by the stochastic switching distribution of perpendicular magnetization. In this study, the improvement in the reliability of SOT-based PUFs against external magnetic fields with write-back operation (WBO) is demonstrated. A PUF consisting of 8 x 4 array Hall-bar devices with a Ta/CoFeB/MgO structure is fabricated, where the random distribution of the SOT switching current serves as an entropy source. However, the information stored in the PUF is easily modified by the application of an external magnetic field. To improve the robustness against magnetic fields, a WBO is introduced that applies an additional current to saturate the magnetization in either the upward or downward direction depending on the magnetic state. As a result, the SOT-based PUF maintains an entropy value close to unity under a magnetic field of up to the coercive field of the CoFeB layer. Furthermore, the WBO provides a digitalized output, which potentially reduces peripheral circuitry such as analog-to-digital converters.