Transmission electron microscopy study on the effect of various capping layers on CoFeB/MgO/CoFeB pseudo spin valves annealed at different temperatures

Abstract

The microstructure of pseudo spin-valve magnetic tunnel junctions (MTJs) comprising a stacking structure of Ta/Ru/Ta/CoFeB/MgO/CoFeB/ with and without X = Pd, Ti, Ta fabricated on thermally oxidized Si wafer with different annealing temperatures, Ta = 250 °C, 300 °C, 400 °C, and 500 °C, has been investigated. The as-deposited MTJs exhibit an amorphous CoFeB structure that crystallizes into bcc Fe-Co (001) from the MgO (001) interface upon annealing at Ta ≥ 250 °C. A bcc Fe-Co (110) crystallizes from the fcc Pd (111) interface. The Fe-Co layer is alloyed with Pd layer at Ta = 500 °C to form an (Fe, Co)-Pd alloy layer, which causes a drastic reduction in the tunneling magnetoresistance (TMR) from 171% to −2.7%. In the Ti capped MTJs, bcc Fe-Co (001) crystallizes from the hcp (001) Ti interface at Ta = 300 °C. Upon further annealing to Ta ≥ 400 °C, the Ti oxidizes to form amorphous Ti-Ox. The rejected B diffuses back to the CoFe layer at Ta = 500 °C that degrades the TMR. On the other hand, the Ta capped MTJs annealed at 300 ≤ Ta ≤ 500 °C show a perfect grain-to-grain epitaxy with an orientation relationship of (001)[110]MgO//(001)[100]CoFe without interdiffusion or oxidation, resulting in the highest TMR value among all the MTJs with various capping layers.

Optimization of Tunneling Magnetoresistance in Perpendicular Magnetic Tunnel Junctions With Co|Pd Reference Layers

“…Similar results were obtained in inverted stacks, where the CoFeB free layer was grown on top of the MgO tunnel barrier and capped with Ru or Pd layers. As discussed earlier, a CoFeB layer can crystallize into a different structure/texture depending on the adjacent seed-or cap-layer [Yuasa 2007, Miyajima 2009, Karthik 2012]. Based on published results, CoFeB crystallizes into bcc (0 0 1), fcc (1 1 1), or bcc (1 1 0) when it is directly in contact with a Ru layer, a NiFe layer, or a Pd layer, respectively [Yuasa 2007, Miyajima 2009, Karthik 2012.…”

“…In MgO tunnel junctions, high TMR requires good bcc (0 0 1) texture of the CoFeB layer at the (0 0 1) MgO tunnel barrier interface. It has been reported extensively that CoFeB crystallization is sensitive to the structure of its adjacent layers [Yuasa 2007, Miyajima 2009, Karthik 2012. For example, when CoFeB is capped with a Ta or Ru layer, the CoFeB layer crystallizes into the desired bcc (0 0 1) texture, giving high TMR.…”

Influence of spin-orbit interaction within the insulating barrier on the electron transport in magnetic tunnel junctions

“…It is interesting to note that such dependence of the magnetoresistance amplitude on the direction of the current with respect to the magnetization has already been observed with the giant magnetoresistance in spin-valves [16]. In MTJs, from a general experimental point of view, the TMR of MgO based in-plane magnetized MTJs has reached values above 600% [17,18]. In contrast, the largest TMR of MgO based out-of-plane magnetized MTJs is in the range (200-350% [19]), significantly lower than the values obtained in their in-plane magnetized counterparts.…”

“…In theory, the TMR amplitude predicted by ab-initio calculations neglecting spinorbit [14] is much larger than the experimentally obtained values. In experiments, the TMR amplitude is limited by defects which can have several origins: interdiffusion of metallic species in the composite magnetic electrodes [17,18,20], structural defects associated with fcc/bcc in-stack structural competition [21], presence of dislocations in MgO associated with Fe(Co)-MgO crystallographic mismatch, adsorbed water molecules etc.…”

Dependency of anti-ferro-magnetic coupling strength on Ru spacer thickness of [Co/Pd]n-synthetic-anti-ferro-magnetic layer in perpendicular magnetic-tunnel-junctions fabricated on 12-inch TiN electrode wafer

“…2 Co 2 Fe 6 B 2 (CFB)/MgO based p-MTJ spin-valve has great potential in p-STT MRAM with a high TMR ratio and low resistance-area (RA) to realize p-STT MRAM devices. [5][6][7][8][9] Recent studies have provided evidence that CFB/MgO based p-MTJ spin-valves with a synthetic anti-ferro-magnetic (SyAF) layer based on [Co/Pd] n -multilayers (MLs), which enhance the thermal and magnetic stabilities of the sensors, have a high TMR ratio and low RA for realizing p-STT MRAM devices. [10][11][12][13][14][15][16][17] In addition, anti-ferro-magnetic coupling strength (J ex ) in a SyAF layer is a critical device parameter to attain a terra-bit-level nonvolatile memory since a smaller J ex leads to a high occurrence of cross-talk in p-MTJ-cell array.…”