Physical Unclonable Function (PUF) attracts enormous attention in recent years to securely preserve confidential information in computing systems. The conventional PUFs require many independent PUF components and/or are incapable of generating multiple response-bit per cycle, resulting in significant area overhead and power consumption. Recently, spin-transfer torque magnetic cell (STT-mCell) has emerged as a promising spintronic device to be used in Computing-In-Memory (CIM) system. However, it is challenging to guarantee the hardware security of STT-mCell based circuits. In this work, we propose a novel STT-mCell Delay based PUF design (SD-PUF) exploiting the unique manufacturing process variation (PV) of STT-mCell that can overcome these issues. A methodology is used to select appropriate logic gates in the all-spin chip to generate a unique identification key. A linear feedback shift register (LFSR) initiates SD-PUF and simultaneously generates a 64-bit signature at each clock cycle. Bit generation in SD-PUF is stabilized using an automatic write-back technique. For uniqueness enhancements, a masking scheme is applied for signature improvement. The uniqueness of the improved SD-PUF is 49.61%. With ±20% supply voltage, and 5 • C-105 • C temperature variations, SD-PUF shows a strong resiliency. In comparison with the state-of-the-art PUFs, our approach can reduce hardware overhead and energy consumption effectively. Finally, the robustness of SD-PUF against various attacks is verified as well.