In order to enhance the tunnel magnetoresistance (TMR) of magnetic tunnel junctions with perpendicular anisotropy the thicknesses of the electrodes on both sides of the MgO barrier have to be optimized. By carefully adjusting the thicknesses of the magnetic layers, taking into account a non-magnetic “dead layer” formed at the interfaces, it is possible to obtain higher TMR signals which allow for faster reading of magnetic memory dots.
Magnetic tunnel junctions with perpendicular anisotropy are used as building blocks to create Magnetic Random Access Memories (MRAM) have significant advantages compared to in-plane anisotropy junctions, achieving higher storage densities, better thermal stability and lower currents for spin transfer torque switching. In order to obtain perpendicular anisotropy, materials parameters have to be adapted, in particular the magnetic electrodes thickness. We studied the case of junctions consisting of Co/Pt multilayers coupled through a Ta spacer to a CoFeB layer as the bottom reference electrode. The storage layer is a single CoFeB layer deposited on top of the MgO barrier. The thicknesses of both bottom and top CoFeB were varied independently so as to evaluate the impact on the anisotropy and TMR signal.
By measuring the evolution of magnetization with thicknesses, we were able to estimate the magnetic “dead layer” of both electrodes, i.e. the thickness under which the material becomes non-magnetic. The “dead layer” forms at the interface with the seed or capping material, Ta in our case. It was then possible to measure the TMR signal as a function of effective thickness. The same trend is then observed for both electrodes; TMR increases with thickness and gets to its maximum value (90%) as soon as the effective thickness is above 0.6 nm.
This work benefited from financial support of the ANR (French National Research Agency) under project ANR-NANO PATHOS and of the European Union under ERC HYMAGINE project n°246942. CIPT measurements were carried out at Crocus Technology.
L. Cuchet, B. Rodmacq, S. Auffret, R. C. Sousa, and B. Dieny, Appl. Phys. Lett. 105, 052408, 2014
Last update : 10/02 2014 (1071)