Atomistic simulation of AlOx magnetic tunnel junction growth

The tunneling magnetoresistance sTMRd of thin dielectric tunnel barriers that are sandwiched between pairs of ferromagnetic metal thin films is highly sensitive to the barrier layer atomic scale thickness, the uniformity in thickness, and the composition. Widely used AlOx barriers are formed by the oxidation of 1–2 nm thick aluminum layers vapor deposited onto one of the ferromagnetic metal electrodes. The device is completed by vapor depositing the second ferromagnetic layer upon the oxide. Efforts to increase the TMR and tunneling conductance by reducing the thickness of the barrier have been successful until the aluminum layer thickness is decreased below ,1 nm whereupon the TMR disappears. The TMR loss is thought to occur because the oxide layer becomes discontinuous leading to regions of metal contact across the barrier layer in the completed device. Using a molecular dynamics simulation technique combined with a recently developed charge transfer potential for metal alloy oxides, we have investigated the atomistic scale phenomena responsible for the disruption of the oxide film’s continuity. We show that discontinuous oxides always form during the oxidation of ,0.6 nm thick crystalline aluminum films on s111d Ni65Co20Fe15 single-crystal layers even when the precursor aluminum layer is continuous and of uniform thickness. The discontinuous mechanism of oxidation is shown to result from a surface-tension-driven dewetting as aluminum is converted to an amorphous oxide. The phenomenon establishes a lower limit of about 1 nm for the thickness of an aluminum oxide tunnel barrier fabricated by oxidation on s111d single-crystal Ni65Co20Fe15 surfaces.