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

Abstract

Despite superior compatibility to mass-production, magnetic tunnel junction (MTJ) with MgO barrier prepared by oxidation process (MgO x ) has shown unacceptable magnetotransport properties for proper operation of spintronics devices because poor crystalline MgO x cannot properly provide a template for crystallization of amorphous CoFeB layers, thus lack of pseudo-epitaxy in overall. We report novel stack structure for MgO x -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 MgO x and crystallization of CoFeB layers at higher temperature annealing. Microstructure analysis confirms highly crystalline MgO x 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 mm 2 and 253% at 5.9 mm 2 from novel MTJ stack successfully satisfy specifications for spintronics devices.

Characterization of Ultrathin Fe–Co Layer Grown on Amorphous Co–Fe–B by In situ Reflective High-Energy Electron Diffraction

“…The basic CoFeB/MgO/CoFeB structure is often modified, for example, by inserting an ultrathin Fe-Co layer between the MgO and CoFeB layers to further optimize the magneto-transport properties. [7][8][9][10] In our previous study, 8) we fabricated perpendicularly magnetized MTJs with the bottom electrode structure of CoPt/Co 60 Fe 20 B 20 (1 nm)/ Fe 30 Co 70 (0.3 nm). Without the 0.3-nm-thick insertion layer, we observed a significant reduction in the MR ratio.…”

“…This result contradicts the study by Choi et al, in which they report the formation of a textured bcc(001) Fe-Co layer on amorphous CoFeB in the as-grown state. 7) Because Choi et al used ex situ cross-sectional TEM observations for the structural analysis, the Fe-Co layer might have crystallized in the bcc(001) structure during the preparation processes for the TEM samples. Figures 4(a)-4(d) are the RHEED images for the 10-A-thick MgO layers grown on Fe-Co layers with various thicknesses and compositions.…”

“…This is considered to be the mechanism for the improvement of the MR ratio in previous studies. [7][8][9][10] In conclusion, we studied the growth of a CoFeB/Fe-Co/ MgO structure by in situ RHEED observation and obtained a phase diagram of the as-grown Fe-Co layer. Below the critical thickness of the Fe-Co layer, Fe-Co becomes amorphous in the as-grown state.…”

Enhanced magnetoresistance in naturally oxidized MgO-based magnetic tunnel junctions with ferromagnetic CoFe/CoFeB bilayers

“…9,14 It has recently been demonstrated that a large TMR effect can be realized in naturally oxidized MgO tunnel barriers by replacing CoFeB with bilayers of CoFe/CoFeB. 15,16 This is significant as high TMR values are generally observed only for rf-sputter deposited MgO, which has limited industrial applicability due to problems inherent to the deposition technique ͑particle generation, poor uniformity control, and low throughput͒. MgO growth using dc magnetron deposition of metallic Mg followed by oxidation permits higher yield and broader applicability.…”

“…Model 15 The cap ͓Co͑15͒/ Fe͑65͔͒ served as a sacrificial layer during APT sample preparation. The metallic layers were deposited by dc magnetron sputtering.…”