The use of embedded hardware devices and Internet of Things devices is increasing rapidly in the current decade. Due to increased demand, many IC design and manufacturing companies rely on third-party vendors. Malicious hardware modifications like hardware Trojan attacks on integrated circuit hardware pose major security concerns in the semiconductor industry. This work introduces a magnetic tunnel junction (MTJ) into an inverter chain as a hardware Trojan. The proposed Trojan works on an externally triggered activation mechanism and can degrade the IC performance or change functionality whenever the desired external magnetic field/temperature is applied. The MTJ is inserted between two inverters to evaluate its behavior as a Trojan, and its operation under particular conditions is validated. The designed hardware Trojan is sneaky to reduce the chances of detection in the testing and verification phase. It has good thermal stability in a specific range of temperatures and sufficient tolerance to the stray magnetic field for better-hidden operation.