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

Abstract

Atomic-scale electron spectroscopic imaging on sputtered magnetic tunnel junctions ͑MTJs͒ with a thin, Ͻ2 nm, MgO layer and B-alloyed electrodes reveals B diffusion into the MgO, resulting in a Mg-B-O tunnel barrier. This ϳ2 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 ͑TMR͒ of ϳ200% in IrMn/CoFeB/Mg-B-O/CoFeB MTJs after annealing, demonstrating that thin Mg-B-O barriers can produce relatively high TMR.

Depth analysis of boron diffusion in MgO/CoFeB bilayer by x-ray photoelectron spectroscopy

“…[6][7][8][9][10][11][12][13][14][15][16] However, different conclusions came out from the experiments. Some groups reported that a considerable quantity of B diffuse into the MgO barrier after annealing, [6][7][8][9][10][11] while some others observed that boron tends to segregate at the MgO/CoFeB interface and form BO x after the crystallization of CoFeB layer. [12][13][14][15] Moreover, recently, the group of Miyajima et al 16 found that the crystallization of CoFeB layers was strongly dependent on the capping materials.…”

“…The large concentration of B distributed in our MgO layer after annealing agrees both with the claims of Read et al 7 that an intermediate oxide Mg x B y O could form after the annealing procedure and with the recent study of boron distribution by Cha et al by using EELS. 11 However, to end up this discussion, further experiments are required. In any case, the large concentration of B species in MgO should introduce a high density of defect states in the MgO band gap 10 and influence the transport property of the CoFeB/MgO/CoFeB MTJs as recently evidenced in our work.…”

“…It leads to two possibilities: ͑i͒ B could be in a substitutional position, occupying the Mg vacancies to form a ͑MgO͒ x ͑B 2 O 3 ͒ y compound 11 or ͑ii͒ it could be in the amorphous form of B 2 O 3 localized in the MgO grain boundaries. The large concentration of B distributed in our MgO layer after annealing agrees both with the claims of Read et al 7 that an intermediate oxide Mg x B y O could form after the annealing procedure and with the recent study of boron distribution by Cha et al by using EELS.…”

Asymmetric angular dependence of spin-transfer torques in CoFe/Mg-B-O/CoFe magnetic tunnel junctions

“…[15][16][17][18] Meanwhile, the B atoms are pushed out of the CoFeB electrodes and then diffused into the MgO layers forming Mg-B-O spacer. [19][20][21][22][23] around (001) axis and compressing lattice slightly.…”

Conductance anomalies of CoFeB/MgO/CoFeB magnetic tunnel junctions

“…CoFeB grows amorphous when sputtered, while MgO grows polycrystalline beyond five atomic layers with a pronounced (001) out-of-plane texture [12]. During annealing, boron from the CoFeB layer is assumed to diffuse away [13,14], and layers of FeCo(100) crystallize at the MgO(100) interface, with the MgO serving as a template for FeCo(100) grains. Theoretical studies have indicated that a single atomic layer of crystalline Fe at the interfaces of a crystalline MgO barrier should be sufficient to achieve a giant TMR ratio [15].…”