Abstract
Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. However, the major challenge of current-induced switching in the nanopillars with both a large tunnel magnetoresistance ratio and a low junction resistance is still to be met. Here, we report spin transfer torque switching in nano-scale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 Ω µm2, which consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient resonant tunnelling transmission induced by the atom-thick W layers could contribute to the larger magnetoresistance ratio than conventional structures with Ta layers, in addition to the robustness of W layers against high-temperature diffusion during annealing. The critical switching current density could be lower than 3.0 MA cm−2 for devices with a 45-nm radius.
Highlights
Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation
The Perpendicular anisotropy-based magnetic tunnel junctions (p-MTJs) stacks we studied here were composed of, from the substrate side, [Co (0.5)/Pt (0.2)]6/Co (0.6)/Ru (0.8)/Co (0.6)/[Pt (0.2)/Co (0.5)]3/W (0.25)/CoFeB (1.0)/ MgO (0.8)/CoFeB (1.3)/W (0.2)/CoFeB (0.5)/MgO (0.75)/Ta (3.0) (Fig. 1a, numbers in parenthesis denote layer thickness in nm), and were deposited on thermally oxidized Si substrate with a 75 nm Ta/CuN/Ta seed layer by a Singulus magnetron sputtering machine
We investigated the magnetic characteristics of blank samples using a physical properties measurement system-vibrating sample magnetometer (PPMS-VSM)
Summary
Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. We report spin transfer torque switching in nanoscale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 Ω μm[2], which consists of atom-thick W layers and double MgO/CoFeB interfaces. P-MTJs with a MgO/CoFeB/heavy metal (e.g., Ta, Hf) structure have attracted interest for their enhanced perpendicular anisotropy that originates from both MgO/CoFeB and CoFeB/heavy metal interfaces[14,15,16,17,18], bringing a reasonable magnetoresistance ratio (TMR) and STT switching critical current density (JC). There is a need to understand how to enable nano-fabrication compatibility, as well as to reach a compromise between low write energy and large sense margins To address those concerns, W was recently reported to replace Ta as spacer and bridging layers in top-pinned p-MTJ films[32,33,34,35,36,37]. STT switching behaviour and junction resistance were not shown in these studies
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