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

Abstract

The textured MgO(001) tunnel barrier grown on CoFeB is a fundamental building block for spintronic devices such as magnetic tunnel junctions. Although the insertion of an ultrathin Fe-Co layer between an MgO layer and a bottom CoFeB layer is a common technique for improving the magnetoresistance effect, the characteristics of this technique remain unclear. We systematically investigated the as-grown structure of Fe-Co by reflective high-energy electron diffraction and found that a highly textured MgO(001) is formed only on an amorphous Fe-Co surface. The diffusion of B atoms from underlying CoFeB into Fe-Co is what makes the as-grown Fe-Co layer amorphous.

CoFeB/MgO/CoFeB magnetic tunnel junctions prepared by layer-by-layer growth of naturally oxidized MgO

“…From similarity to the RHEED pattern reported in the previous systematic study, 10) we deduce that the observed diffraction pattern represents a bcc CoFe(110) structure. The observed thickness dependence is basically consistent with the findings of the previous work, 10) except that the polycrystalline structure was stabilized for the slightly thicker film (t CoFe = 2 nm).…”

“…Note that the ultra-thin CoFe is amorphous in the asdeposited state because of diffusion of B atoms into the CoFe. 10) Thus, the CoFe layer is actually CoFeB with a low B concentration, which promotes solid-phase epitaxial growth from the interface with MgO(001) and consequently improves the MR ratio.…”

“…The observed halo pattern without diffraction spots indicates that the CoFeB layer had an amorphous structure, which is consistent with previous observations. 7,10) After the CoFe layer (1 nm) was deposited, no appreciable change was observed in the RHEED pattern [Fig. 1(b)], indicating the CoFe grown on the amorphous CoFeB had an amorphous structure at t CoFe = 1 nm.…”

“…As mentioned in the introduction, it was found that a 1 nm thick CoFe layer deposited on a 3 nm thick CoFeB layer contained a certain number of B atoms that had diffused from the underlying CoFeB, which made the thin CoFe layer amorphous. 10) Similarly, in the present stack, B atoms are considered to have diffused into the amorphous Mg Next, let us discuss the oxidation process of the standard stack on the basis of the RHEED analysis. In particular, we will examine [Mg (0.78 nm)/oxidation] 2 on CoFe (1 nm) in comparison with the layer-by-layer growth stack discussed above (Fig.…”

Large magnetocapacitance of up to 456% at room temperature in FeCo/MgAl2O4/FeCo(001) magnetic tunnel junctions

“…[13][14][15][16] A large TMR ratio is attributed to the large tunnel transmittance of the Δ 1 states of MgO and the highly spin-polarized Δ 1 states of bcc Fe-based FMs. 3,4,[15][16][17][18] In addition to the large TMR, MgO-based MTJs exhibit a large tunnel magnetocapacitance (TMC) effect at room temperature. [19][20][21] The TMC effect is a phenomenon in which the capacitance changes with the magnetic field in tunneling systems.…”