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

Abstract

Three-dimensional elemental distributions in magnetic tunnel junctions containing naturally oxidized MgO tunnel barriers are characterized using atom-probe tomography. Replacing the CoFeB free layer (reference layer) with a CoFe/CoFeB (CoFeB/CoFe) bilayer increases the magnetoresistance from 105% to 192% and decreases the resistance-area product from 14.5 to 3.4 Ω μm2. The CoFe/CoFeB bilayer improves the compositional uniformity within the free layer by nucleating CoFeB crystals across the entire layer, resulting in a homogeneous barrier/free layer interface. In contrast, the simple CoFeB free layer partially crystallizes with composition differences from grain to grain (5–30 nm), degrading the tunnel junction performance.

Magnetization dynamics and interface studies in ion-beam sputtered Si/CoFeB(8)/MgO(4)/CoFeB(8)/Ta(5) structures

“…7 The magnetic and electric response of the device depends critically on the interfacial microstructure between CoFeB and MgO and elemental distribution across the FM, barrier layers 8,9 and is sensitive to the growth process of MgO. [10][11][12][13] In the present paper, we report our studies on effect of growth process of MgO on magnetization dynamics of the top CoFeB free FM layer of MTJ using ferromagnetic resonance (FMR). The observed changes in the Gilbert damping constant in ion-beam sputtered Si/CoFeB(8)/MgO(4)/CoFeB(8)/Ta(5) MTJ stacks have been correlated with the Boron distribution across the layers and the interface quality between CoFeB and MgO using secondary ion mass spectroscopy (SIMS), specular X-ray reflectivity (XRR), and cross-sectional high resolution transmission electron microscopy (HRTEM).…”

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

“…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.…”

Resonance Measurement of Nonlocal Spin Torque in a Three-Terminal Magnetic Device

“…When comparing results for different fields, we take into account that the nonlocal spin torque should be proportional to the component of the spin current perpendicular to the free-layer magnetization, so that jj ¼ ð@=2eÞ jj I SV sin SV [or @ jj =@ SV j I SV ¼ ð@=2eÞ jj I SV cos SV ], where jj is a dimensionless efficiency. We estimate SV by assuming that the magnetization of F2 aligns with the applied field and calculating the magnetization angle of F1 by assuming that it responds as a macrospin to the combined action of H and the exchange field H ex ¼ 1:1 AE 0:2 kOe [19]. Figure 2(c) shows separate measurements of the spin-torque efficiency jj for a range of field magnitudes (0.6-1.3 kOe at an angle of 75 ) that correspond to resonance frequencies of 8-12 GHz and offset angles SV between 49 and 35 .…”