Young-Suk Choi

Effect of Ta getter on the quality of MgO tunnel barrier in the polycrystalline CoFeB∕MgO∕CoFeB magnetic tunnel junction

X-ray photoelectron spectroscopy and high-resolution Rutherford backscattering reveal that Ta getter presputtering enhances the stoichiometry and lowers the interstitial defect density of MgO barrier. This results in higher magnetoresistance ratio, 205%, of magnetic tunnel junction, compared to 46% for no Ta getter, at 1.2nm MgO thickness. Fitting yields the corresponding barrier height of the MgO of 3.0eV, which is higher compared to 2.3eV for without Ta getter. However, the tunnel junction prepared with Ta getter shows lower resistance-area product by an order of magnitude. Microstructure of MgO barrier and oxidation of bottom electrode can be attributed to the contradictory results.

Crystallization of amorphous CoFeB ferromagnetic layers in CoFeB/MgO/CoFeB magnetic tunnel junctions

We demonstrate that the crystallization of ferromagnetic CoFeB layers originates at the interface with a MgO layer in CoFeB/MgO/CoFeB magnetic tunnel junctions by annealing using cross-sectional transmission electron microscopy and electron diffraction. The CoFeB layers, which are amorphous in the as-deposited state, crystallize with a (001) out-of-plane texture by annealing at 360 °C. Crystal grains of 15–20 nm in the CoFeB layers are observed at the interface with the MgO layer, but not at the interface with a Ta or Ru layer. Much smaller crystal grains with random crystal orientations are formed in a region away from the MgO interface in the CoFeB layers. The depth profiles obtained by X-ray photoelectron spectroscopy show that boron diffuses from the crystallized region at the interface into the MgO layer and the rest of the region in the CoFeB layers during crystallization, indicating that crystal grains have much lower B contents than the original composition.

Novel Stack Structure of Magnetic Tunnel Junction with MgO Tunnel Barrier Prepared by Oxidation Methods: Preferred Grain Growth Promotion Seed Layers and Bi-layered Pinned Layer

Despite superior compatibility to mass-production, magnetic tunnel junction (MTJ) with MgO barrier prepared by oxidation process (MgOx) has shown unacceptable magnetotransport properties for proper operation of spintronics devices because poor crystalline MgOx cannot properly provide a template for crystallization of amorphous CoFeB layers, thus lack of pseudo-epitaxy in overall. We report novel stack structure for MgOx-based MTJ to assure acceptable magnetotransport properties: insertion of preferred-grain-growth-promotion (PGGP) seed layer and bi-layered ferromagnetic pinned layer (bi-PL) to induce preferred grain growth in MgOx and crystallization of CoFeB layers at higher temperature annealing. Microstructure analysis confirms highly crystalline MgOx in pseudo-epitaxy with fully crystallized CoFeB via PGGP by high temperature annealing, attributed to enhanced thermal stability of bi-PL. Tunneling magnetoresistance (TMR) 132.6% at resistance-area product (RA) 1.2 Ω µm2 and 253% at 5.9 Ω µm2 from novel MTJ stack successfully satisfy specifications for spintronics devices.

Influence of Chemical Composition of CoFeB on Tunneling Magnetoresistance and Microstructure in Polycrystalline CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

We report, for the first time, the correlation between tunneling magnetoresistance (TMR) and the microstructure of polycrystalline CoFeB/MgO/CoFeB magnetic tunnel junctions with various Co/Fe ratios in the (CoFe)81B19 reference and free layers. It is found that the Co/Fe ratio in the (CoFe)81B19 reference layer strongly affects the (001) out-of-plane texture of the MgO tunnel barrier, resulting in the variation in TMR ratio. Further microstructure characterization of the magnetic tunnel junction with a higher TMR ratio and a stronger (001) out-of-plane texture in the MgO tunnel barrier reveals a grain-to-grain lattice match between the crystallized bcc CoFeB reference layer and MgO with a 45° rotational epitaxial relationship, that is, CoFeB(001)[110]∥MgO(001)[100].

Significant reduction of coercivity without reduction of tunneling magnetoresistance ratio of CoFeB∕MgO∕CoFeB-based magnetic tunnel junction using sandwich-structured free layer

We report the significant reduction of coercivity (Hc) without huge drop of tunneling magnetoresistance ratio (TMR) in the CoFeB∕MgO∕CoFeB-based magnetic tunnel junction by employing sandwich-structured free layer of CoFeB/metallic spacer/NiFe. Strong magnetostatic coupling without physical contact induces magnetic softness from NiFe to CoFeB and avoids considerable reduction of TMR. This achievement satisfies the requirements given to the magnetic read head in hard disk drive and the spin torque transfer magnetic random access memory for the application to the higher areal density and the reduction of critical current density for current-induced magnetization switching, respectively.