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(a) Cross-sectional HRTEM lattice image of a CoFe-buffered Co2MnSi MTJ layer structure consisting of (from the lower side) Co50Fe50 (CoFe) buffer/lower Co2MnSi (3 nm)/MgO barrier (∼2.5 nm)/upper Co2MnSi (3 nm)/Ru (0.8 nm)/Co90Fe10 (2 nm)/Ir22Mn78 (10 nm)/Ru cap (5 nm) with Mn-rich Co2Mn1.29Si electrodes (MTJ-A); it was taken along the [1-10] direction of the Co2MnSi. (b) and (c) Nano-beam electron diffraction patterns for the lower (b) and upper (c) Co2MnSi electrodes; the beam diameter was 2 nm. (d) Two-beam bright-field TEM image of a CoFe-buffered Co2MnSi MTJ layer structure with Mn-rich Co2Mn1.29Si electrodes (the same sample as shown in Figs. 1(a)–1(c)) excited with reciprocal vectors parallel to the plane (g = MgO and Co2MnSi). The arrows provide a guide to show the misfit dislocation contrasts.
Typical TMR curves at 4.2 K and 290 K for a Co2MnSi MTJ consisting of (from the lower side) Co2MnSi lower electrode (3 nm)/MgO barrier (2.4 nm)/Co2MnSi upper electrode (3 nm) grown on a CoFe-buffered MgO(001) substrate with Mn-rich Co2Mn1.35Si0.88 electrodes (MTJ-B). The magnetoresistance was measured with a magnetic field applied along the [1-10] axis of the Co2MnSi film using a dc four-probe method. The bias voltages were 1 mV at 4.2 K and 5 mV at 290 K.
TMR ratios at (a) 4.2 K and (b) 290 K for fully epitaxial CoFe-buffered Co2Mn α Si0.96 (3 nm)/MgO/Co2Mn α Si0.96 (3 nm) MTJs of series C (blue solid and open circles are for 4.2 K and 290 K, respectively) as a function of Mn composition α ranging from 0.72 (Mn-deficient Co2MnSi) to 1.57 (Mn-rich Co2MnSi). The MgO tunnel barrier layer was deposited by electron beam evaporation using a linearly moving shutter with the nominal thickness (t MgO) ranging from 1.4 to 3.2 nm on each 20 × 20 mm2 substrate. The TMR ratio showed a slight increase with an increase in t MgO for a t MgO range from 1.8 to 3.0 nm. A typical TMR ratio in this t MgO range for each Mn composition is plotted. For comparison, those of MgO-buffered Co2Mn α Si (30 nm)/MgO/Co2Mn α Si (3 nm) MTJs (red solid and open circles are for 4.2 K and 290 K, respectively) as a function of Mn composition α ranging from 0.69 (Mn-deficient Co2MnSi) to 1.43 (Mn-rich Co2MnSi) reported in Ref. 26 are plotted. The bias voltage was 1 mV at 4.2 K and 5 mV at 290 K. The Si compositions γ in the expression of Co2Mn α Si γ are 0.96 for CoFe-buffered Co2MnSi MTJs of series C and 1.0 for MgO-buffered Co2MnSi MTJs.
Dependence of the TMR ratios at (a) 4.2 K and (b) 290 K on the average misfit dislocation spacing at the lower and upper interfaces with a MgO barrier for MTJs of group 1, including the CoFe-buffered Co2MnSi/MgO/Co2MnSi MTJ (blue solid circles) and MgO-buffered Co2MnSi/MgO/Co2MnSi MTJ (red solid circles). The TMR ratios and the misfit dislocation spacing for the latter are taken from Refs. 26 and 29, respectively. For comparison, similar dependences at 4.2 K for MTJs of group 2 (Ref. 30), including (from the lower side) MgO buffer/Co2MnSi/MgO/CoFe MTJs (black open triangle), CoFe buffer/Co2MnSi/MgO/CoFe MTJs (blue open square), and CoFe/MgO/Co2MnSi MTJs (red open circle) having Mn-rich Co2Mn1.29Si1.0, Co2Mn1.35Si0.88, and Co2Mn1.29Si1.0 electrodes, respectively, are also plotted in (a).
Typical TMR ratios at 4.2 K and 290 K for a MTJ from each MTJ series MTJ-A to MTJ-C, which had slightly different Si compositions γ with respect to Co2 in Co2Mn α Si γ electrodes, and the respective film compositions of Co2Mn α Si γ electrodes for each MTJ.
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