Strain-engineered flexible magnetic tunnel junctions for wearable spintronic applications

Abstract Flexible magnetoresistive tunnel junctions (MTJs) have emerged as a promising foundation for next-generation wearable and stretchable spintronic systems. This review presents a comprehensive overview of recent progress in the fabrication, strain engineering, and functional integration of flexible MTJs, particularly those based on CoFeB/MgO stacks, owing to their high tunneling magnetoresistance (TMR), energy efficiency, and compatibility with low-power operation. We discuss advanced fabrication approaches, including transfer printing and direct sputtering onto polymer substrates, as well as strategies to modulate magnetic anisotropy and TMR via strain engineering, both passively through mechanical deformation and actively through piezoelectric coupling. Emphasis is placed on achieving mechanical robustness and dynamic tunability of TMR under bending, stretching, and repeated cycling. Emerging applications in health monitoring, movement detection, magnetoelectronic skin and soft robotics are highlighted, where flexible MTJs offer clear advantages over conventional anisotropic magnetoresistance and Hall sensors in terms of sensitivity, spatial resolution, and integrability. Finally, we identify critical challenges, such as thermal budget limitations, large-area scalability, mechanical reliability, environmental stability, and seamless multifunctional integration, that must be addressed to unlock the full potential of flexible spintronic technologies in real-world applications.