J. C. Read

X-ray photoemission study of CoFeB∕MgO thin film bilayers

The authors present results from an x-ray photoemission spectroscopy study of CoFeB∕MgO bilayers where they observe process-dependent formation of B, Fe, and Co oxides at the CoFeB∕MgO interface due to oxidation of CoFeB during MgO deposition. Vacuum annealing reduces the Co and Fe oxides but further incorporates B into the MgO forming a composite MgBxOy layer. Inserting an Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control over B content in the barrier.

Sidewall oxide effects on spin-torque- and magnetic-field-induced reversal characteristics of thin-film nanomagnets

The successful operation of spin-based data storage devices depends on thermally stable magnetic bits. At the same time, the data-processing speeds required by todays technology necessitate ultrafast switching in storage devices. Achieving both thermal stability and fast switching requires controlling the effective damping in magnetic nanoparticles. By carrying out a surface chemical analysis, we show that through exposure to ambient oxygen during processing, a nanomagnet can develop an antiferromagnetic sidewall oxide layer that has detrimental effects, which include a reduction in the thermal stability at room temperature and anomalously high magnetic damping at low temperatures. The in situ deposition of a thin Al metal layer, oxidized to completion in air, greatly reduces or eliminates these problems. This implies that the effective damping and the thermal stability of a nanomagnet can be tuned, leading to a variety of potential applications in spintronic devices such as spin-torque oscillators and patterned media.

Atomic-scale spectroscopic imaging of CoFeB/Mg–B–O/CoFeB magnetic tunnel junctions

Atomic-scale electron spectroscopic imaging on sputtered magnetic tunnel junctions (MTJs) with a thin, <2 nm, MgO layer and B-alloyed electrodes reveals B diffusion into the MgO, resulting in a Mg–B–O tunnel barrier. This ∼2 nm thick interfacial layer forms due to oxidation of CoFeB during radio frequency sputtering of MgO and subsequent B diffusion into MgO during annealing. We measure a room-temperature tunneling magnetoresistance (TMR) of ∼200% in IrMn/CoFeB/Mg–B–O/CoFeB MTJs after annealing, demonstrating that thin Mg–B–O barriers can produce relatively high TMR.

Spatially resolved electron energy-loss spectroscopy of electron-beam grown and sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions

Electron energy-loss spectroscopy at subnanometer resolution is used to investigate CoFeB∕MgO∕CoFeB magnetic tunnel junctions grown by electron-beam evaporation and radio frequency (rf) sputtering before and after annealing. Gap states were observed in the MgO layer for both growth methods although the rf-sputtered MgO layer showed significantly more gap states. Asymmetry in oxygen bonding between the top and bottom CoFeB∕MgO interfaces was also observed. Moreover, significant amounts of diffused B as BOx were observed in the rf-sputtered MgO layer. A Mg underlayer between the MgO layer and the bottom electrode greatly reduced BOx formation in the barrier upon annealing.

Oxygen stoichiometry and instability in aluminum oxide tunnel barrier layers

We present x-ray photoelectron spectroscopy data that show that the chemisorbed oxygen previously observed to be on the surface of thin

High magnetoresistance tunnel junctions with Mg-B-O barriers and Ni-Fe-B free electrodes

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Tunneling spectroscopy studies of treated aluminum oxide tunnel barrier layers

layers formed by room temperature thermal oxidation is bound by oxygen vacancies in the oxide. Increasing the electric field across the oxide, either by overcoating with a metallic electrode, or by electron bombardment, drives this surface chemisorbed oxygen into the vacancy sites. Due to the low bonding energies of these oxygen sites, subsequent oxygen exposures draw these

Disorder, defects, and band gaps in ultrathin (001) MgO tunnel barrier layers

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Tunneling Magnetoresistance and B Diffusion in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions Characterized by STEM-EELS

ions back to the surface, reforming chemisorbed

Spatially Resolved Core-loss EELS Study of E-beam Grown and RF-sputtered CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

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