Magnetic coupling between high magnetization perpendicular electrodes in an epitaxial FePt/MgO/FePt magnetic tunnel junction

In this paper, we investigate the magnetic coupling between perpendicularly magnetized FePt electrodes in a fully epitaxial FePt/MgO/FePt magnetic tunnel junction grown by molecular beam epitaxy. The epitaxial growth is monitored in situ by reflection high energy electron diffraction and ex situ using x-ray diffraction and transmission electron microscopy. Despite the large magnetization of FePt (leading to high stray fields) and the thinness of the tunnel barrier, we do not observe magnetic coupling between the electrodes in major hysteresis loops or when reversing the soft electrode alone. However, the magnetization reversal of the soft layer (SL) strongly depends on the remanent state of the hard layer (HL), ending with full magnetic coupling between the electrodes and a single magnetization reversal for the zero-remanence state. We have closely related macroscopic magnetic measurements performed using the magneto-optical Kerr effect with microscopic observations of magnetic domains performed by magnetic force microscopy. In the coupling regime, the domain wall propagation in the soft layer is driven by the magnetic domain configuration of the hard layer. Indeed, the HL domain wall stray field becomes larger than the pinning field over structural defects in the SL.