Magnetic coupling in epitaxial Fe/MgO/Fe microtunnel junction arrays

The magnetic properties of planar 10 nm Fe/x nm MgO/10 nm Fe (x = 1, 2, 5 and 7 nm) epitaxial square-shaped elements have been measured for different edge sizes in the micrometre range. When the top and bottom electrodes magnetizations are exchange uncoupled, they orient in an antiparallel fashion in the zero-field state due to the magnetostatic energy reduction. On the other hand, the two electrodes magnetizations orient in a parallel way when they are are effectively exchange coupled through the barrier; most probably due to pinholes. This happens for a 1 nm thick MgO barrier (which is just about two MgO unit cells) for junctions that are ~4μ m in lateral size. The field ranges where magnetic switching occurs agree reasonably well with predictions from a simple analytical model, in which we solve the energetic balance between magnetostatic energy reduction and potential energy gain for our Fe/MgO/Fe sandwich geometry. In addition, we obtain evidence that when the top and bottom electrodes are exchange uncoupled, the junctions can be placed closer before they interact laterally. This is due to a preferred magnetic flux closure along the growth direction instead of in the plane, reducing the field distribution at the neighbour's positions. The same argument implies that the junction electrodes retain their single domain character for smaller sizes than single layer Fe tiles (Costa-Krämer J L, Martín J I, Menéndez J L, Cebollada A, Anguita J V and Briones F and Vicent J L 2000 Appl. Phys. Lett. 76 3091).