Characterization of magnetic mutual couplings in 2D arrays of magnetic tunnel junctions*

In array of magnetic tunnel junctions (MTJs) or magnetic sensor composing multiple MTJ elements, magnetic coupling between MTJ elements emerge from the stray field of the magnetic free and pinned layers. Understanding the coupling is important for applications in magnetic imaging and in spintronic devices.  Such mutual coupling, particularly for small element separation, undesirably increases the noise level in the MTJ elements and limits the MTJ element separation. MTJ with a vortex magnetization (vortex MTJ), with no stray field from the free layer, is one solution to reduce the magnetic couplings. In this work, we have performed a comprehensive characterization of the magnetic coupling between tightly spaced vortex MTJ elements, through measuring their cross correlation under various external magnetic field. No magnetic coupling between the free layers of MTJ elements have been found. Nevertheless, coupling between the magnetic pinned layer of one vortex MTJ and free layers of its neighbors has been observed. Such interlayer coupling not only sets a fundamental limit of MTJ element separation, but also implies additional noise source in MTJ sensors. Our work would provide important insights into improving noise level and spatial resolution of MTJ-based magnetic sensor arrays.