CoFeB/MgO/CoFeB based magnetic tunnel junctions for memory applications: optimization of the fabrication process and magnetic properties
Spintronics has received a significant interest in the last decades, resulting in the development of magnetic nanostructured devices with important applications in the electronics industry, namely the much desired magnetic random access memory (MRAM) In the context of this thesis, the magnetic tunnel junction (MTJ) is the central nanostructure and, without any doubts, it is one of the most important nanostructured devices fabricated to date. The MTJ is defined as two ferromagnetic (FM) layers separated by a thin insulating barrier. To guarantee the electrical flowing by tunneling, the barrier is deposited with a few nanometers or even less (thickness below 10 A) The MTJ is so relevant because it presents the tunnel magnetoresistance (TMR) effect, which can be defined as a dependence of the MTJ resistance on its magnetic configuration. Although the TMR effect has been experimentally demonstrated in 1975, the major breakthrough only happened in 2004, when many scientific articles started to report TMR ratios above 200% at room temperature for MTJs with an MgO insulating barrier and CoFeB FM layers, thus having the largest potential for applications. Nowadays, for memory applications, the focus concerns the optimization of the MTJ properties, namely by decreasing its operation energy and improving its fabrication process. In general, this work provides an additional contribution towards the optimization of CoFeB/MgO/CoFeBbased MTJs with an ultra thin MgO barrier 1 nm) The work developed in this thesis resulted from a partnership between the Instituto de Fisica dos Materiais da Universidade do Porto (IFIMUP IN) and the International Iberian Nanotechnology Laboratory (INL) Both deposition and characterization techniques were performed at INL facilities, while some of the transport measurements were performed at IFIMUP IN. Specifically, a new optimization process is proposed to decrease the roughness of the buffer structure that is deposited below the fundamental CoFeB/MgO/CoFeB trilayer structure. The smoothing process was implemented in the buffer surface, before the deposition of the full MTJ stack, using the ion beam milling technique. The electrical and morphologic results show a direct influence of the roughness decrease on the improvement of the TMR ratio. In a second experimental study, the MTJ magnetic properties were studied by performing a thickness dependence study for both CoFeB FM layers. The objective was to know the thickness at which one obtains the transition for perpendicular magnetic anisotropy (PMA) in both FM layers. The magnetic results prove the achievement of PMA separately in both FM layers for different stack structures. Depending on the position of the FM layer, the transition occurred for different thicknesses (1.0 nm for the CoFeB layer below the MgO barrier and 1.4 nm for the CoFeB layer on top of the MgO barrier) thus proving the relevance of the FM layer position in the MTJ stack. Moreover, the comparison between annealed and as deposited samples translates an improvement of the magnetic properties of the FM layer after annealing at 300 *C, namely by decreasing the thickness of the dead layer and increasing the effective anisotropy constant. The third and final experimental study was implemented in already patterned CoFeB/MgO/CoFeB based MTJs. Electrical measurements were performed under different constant bias voltage values for both voltage polarities. In accordance with many scientific reported already published, the results prove a direct effect of the bias voltage in the MTJ magnetic properties.