Usually, magnetic tunnel junctions consist of electrodes made of an alloy of cobalt, iron and boron (CoFeB) and a tunnel barrier in magnesium oxide (MgO). To bring perpendicular anisotropy to such structures, multilayers of cobalt and platinum (Co/Pt) may be used. However, it is necessary to separate the growth of the CoFeB layer from the underlying multilayer. Adjusting the thickness of the tantalum (Ta) spacer will have a significant impact on both the magnetic and transport properties, allowing us to find an optimal value.
Recently, a lot of efforts have been done towards achieving perpendicular anisotropy in magnetic tunnel junctions, as they present higher storage densities, high thermal stabilities and lower critical current densities in the case of Spin Transfer Torque switching. This anisotropy can be obtained using Co/Pt multilayers, combined with standard CoFeB/MgO structures. However, high Tunnel Magnetoresistance (TMR) signals can only appear if CoFeB gets the right bcc (100) texture from the MgO. This is not the case if it grows directly on Co or Pt. Thus a Ta spacer is introduced to structurally decouple both layers. Nevertheless, the thickness of this spacer has to be carefully controlled, in order to avoid magnetic decoupling between the layers.
In our structures, we were able to correlate the magnetic and transport properties of the junctions while varying the Ta thickness. As shown in the graph, when the spacer thickness increases, there is a first improvement on the CoFeB texture characterized by an increase in TMR (up to about 70%), followed by a decoupling of the magnetic layers. This leads the CoFeB magnetization to fall in-plane, resulting in zero-magnetoresistance. It was possible to find the best compromise for a thickness of 0.3-0.4nm of Ta. This thickness allows improving the CoFeB texture while keeping perpendicular anisotropy in the whole structure.
We acknowledge the financial support from the ANR (French National Research Agency) under project ANR-NANO PATHOS and from the European Union under the ERC HYMAGINE project n° 246942.
Last update : 10/10 2013 (861)