In the Electrodes input section of the MTJ Builder (the upper frame to the left of the 3D-View in Figure 37), you specify parameters that refers to the electrode. For instance, here you can change the electrode material from Fe to Co by selecting Co from the Material drop-down list. After you have changed the electrode material, you should also change the value of the lattice constant to describe the electrodes correctly. For this particular example, the Lattice constant of Co should be equal to 2.51.

Similar to regular two-probe systems built with the Atomic Manipulator, for magnetic tunnel junctions it is also important to include layers of surface atoms between the bulk electrode and the insulator oxide. These atoms are necessary to screen out the perturbation from the insulator region on the electrode surface, in order for the remaining electrode to be treated as a bulk without compromising the physical description. The number of surface layers are changed in the Left surface and Right surface spin boxes in the Surface layers input section. As with regular two-probe systems, the recommended approach is to choose a moderate value and increase it systematically until your result converges.

In the Barrier input section, you specify the insulating material and atomic positions of the central region. Similar to the electrode material, you can change the barrier-region material from MgO to ZrO by selecting ZrO from the Material drop-down list. You can also vary the thickness of the barrier region by choosing the number of layers between the electrodes. If you left-click the Layer separations, you are presented with a window where you can specify the individual separations between each insulator layer.

For the Fe-MgO-Fe system, the geometry “deformation” of the electrode and insulator surfaces due to the misfit between these regions is comparatively small; the distance between the last and second-last electrode surface layers is compressed by 2%, and the first layer of Mg atoms extends 0.05 Å towards the electrode. In order to adjust for that, you need to change the Inner layer separation from 1.433 to 1.40, and the interface Buckling from 0.00 to 0.05. Finally, you can also adjust the gap between the electrode surface and the barrier region by changing the Electrode/Oxygen distance to make space for atoms of various sizes.

Together, the Inner layer separation and the Electrode/Oxygen distance can also be used to set the position and size of the outermost surface layer. This kind of control can be very useful if you wish to study systems where the electrode surfaces, for instance, have been coated with a layer of Ag atoms. In this case, however, you need to change the element to Ag either by hand or import the configuration into the Atomic Manipulator.

When you are satisfied with your magnetic tunnel junction setup you can either

If you on the other hand are unsatisfied with your magnetic tunnel junction setup, you can left-click the Restore Defaults button to start over from the default setup.