CoFeB/MgO-based magnetic tunnel junctions for film-type strain gauge

We show that a CoFeB/MgO-based magnetic tunnel junction (MTJ) formed directly on a flexible substrate has considerable potential as a high-sensitivity strain gauge. A gauge factor of ∼1000, which represents the highest sensitivity reported, thus far, for a film-type strain gauge and is ∼500 times larger than that of the most prevalent metal-foil strain gauge, is realized in the flexible MTJ with a pseudo-spin valve structure under the condition of external magnetic-field assistance. Additionally, using a flexible MTJ with strain-insensitive exchange-biased pinned and strain-sensitive free layers, we demonstrate that a large resistance change due to the strain application can be achieved under magnetic field-free conditions. According to simulations based on the coherent magnetization rotation model, conditions are suggested for improving the gauge factor, as well as for using the flexible MTJ as a strain gauge in practical applications.