Structural optimization of CoFeB/MgO/CoFeB TMR elements

The Tunnel magnetoresistance (TMR) effect is being used in magnetic tunnel junctions (MTJs) consisting of two ferromagnetic electrodes separated by an insulator for a variety of present and future applications. Practical use requires a high junction quality mainly measured in terms of the ''TMR value''. MTJs are manufactured as a structure of nm scaled layers, a process which can be optimized in many ways to improve the TMR value. We fabricated MTJs consisting of MgO insulating tunnel barriers and CoFeB ferromagnetic electrodes on MgO substrates. These structures are not suited for practical use, however they show a visible TMR effect under laboratory conditions. Furthermore, they are dedicated to study crystallization processes at MgO/CoFeB interfaces while keeping the manufacturing process simple and easy to control. MgO was grown by Molecular Beam Epitaxy and the CoFeB layers were sputtered in the same chamber. The structures were then analysed by High Resolution Transmission Electron Microscopy for smooth MgO layer growth and crystallization. Various preparation parameters were changed in order to find optimal growth conditions for high TMR values.