Open microscopic mechanisms in the MgO/CoFeB/Ta/CoFeB/MgO double-interface recording structure: damping-controlled switching current, boron/spacer-layer chemistry, and Ta-thickness interlayer coupling

A 2012 study of perpendicular-anisotropy CoFeB-MgO magnetic tunnel junctions with the double-interface recording structure MgO/CoFeB(1.6)/Ta(0.4)/CoFeB(1.0)/MgO showed that the thermal stability factor $\Delta$ of 70-nm junctions increases by a factor of 1.9 over single-interface junctions while the intrinsic spin-torque switching current $I_{C0}$ stays essentially unchanged ($123\,\mu$A vs $125\,\mu$A), and left several issues explicitly unresolved: the mechanism behind the $\Delta$-$I_{C0}$ decoupling (the authors speculated a reduced effective Gilbert damping constant $\alpha$ (the phenomenological damping parameter of the Landau-Lifshitz-Gilbert equation) with thicker CoFeB but made no damping measurement), the causal quantitative role of boron redistribution and Ta spacer chemistry in setting the interface anisotropy $K_i$ and the tunnel magnetoresistance (TMR), the Ta-thickness-dependent interlayer exchange coupling map (ferromagnetic vs antiferromagnetic regimes), and the scaling and operating-regime behavior below 70 nm. Later literature has settled part of this: device-level scaling is now established down to 11 nm, and direct ferromagnetic-resonance (FMR) damping measurements support the reduced-$\alpha$ route to the switching-current decoupling, while the microscopic origin of the interfacial damping, the atomically resolved boron-chemistry-to-anisotropy relation in the double-interface stack, and a controlled Ta-thickness coupling phase map in device-scale junctions remain open.

Background

A magnetic tunnel junction (MTJ) consists of two ferromagnetic electrodes separated by an MgO tunnel barrier; its resistance depends on the relative magnetization orientation of the two electrodes through the tunnel magnetoresistance (TMR) effect. In spin-transfer-torque MRAM (STT-MRAM), a spin-polarized current writes the state of the soft ("recording") layer by transferring angular momentum to it. For high-density memory the recording layer must simultaneously have a large thermal stability factor $\Delta=E/k_{\mathrm{B}}T$ (the energy barrier $E$ between the two magnetization states divided by the thermal energy, which sets retention) and a small intrinsic critical current $I_{C0}$ (the zero-temperature current below which spin-torque switching cannot occur), since the write current scales with $I_{C0}$. CoFeB layers thinner than about 1.5 nm in contact with MgO develop interfacial perpendicular magnetic anisotropy with areal interface constant $K_i$, arising from orbital hybridization at the CoFeB-MgO boundary, but a single CoFeB-MgO interface supports only a thin recording layer, capping $\Delta$ at a given junction size. In the macrospin approximation (treating the layer magnetization as a single uniformly rotating spin vector) $I_{C0}$ is proportional to the damping $\alpha$ times the barrier, so the switching efficiency $\Delta/I_{C0}$ scales roughly as $1/(\alpha\,\eta)$ with $\eta$ the spin-torque efficiency; the Gilbert damping constant $\alpha$ (the phenomenological damping parameter of the Landau-Lifshitz-Gilbert equation) is known to be enhanced in ultrathin layers by spin pumping, the emission of spin angular momentum from a precessing magnetization into an adjacent metal that appears as an additional damping-like loss, into adjacent heavy-metal layers. Ikeda et al. (Nat. Mater. 2010) established the single-interface perpendicular CoFeB-MgO MTJ with $\Delta\approx43$ at 40 nm and noted that $\alpha$ increases from roughly 0.01 to roughly 0.025 as the CoFeB thickness falls below about 2 nm. Sato et al. (Appl. Phys. Lett. 2012, DOI 10.1063/1.4736727) then introduced the double-interface recording structure MgO/CoFeB(1.6 nm)/Ta(0.4 nm)/CoFeB(1.0 nm)/MgO: the two CoFeB-MgO interfaces jointly hold perpendicular easy axis at 2.6 nm total CoFeB (after annealing, with the Ta spacer acting as a getter, a layer that absorbs boron by diffusion and chemical reaction, boron being widely believed to impede the interfacial crystallization that produces $K_i$), giving $\Delta=95$ (average 90 over seven devices) versus $\Delta=51$ for an otherwise identical single-interface junction at 70 nm — a factor of 1.9 — while $I_{C0}$ stayed comparable ($123\,\mu$A vs $125\,\mu$A averaged over the parallel and antiparallel switching branches). The same paper observed that the Ta spacer thickness controls the interlayer exchange coupling between the two CoFeB layers — 0.4 nm giving strong ferromagnetic coupling (the two layers switch as one recording layer) and 1.0 nm giving independent switching — in line with the oscillatory interlayer exchange coupling through transition-metal spacers established by Parkin (Phys. Rev. Lett. 1991); the Co/Ta/Co and Fe/Ta/Fe multilayer studies built on that work place the first antiferromagnetic coupling peak near 0.7 nm of Ta. The paper itself offered no damping measurement, no boron or interface-chemistry measurement, only two spacer thicknesses, and no data below 70 nm.

Problem Statement

For the double-interface perpendicular CoFeB-MgO recording structure MgO/CoFeB/Ta/CoFeB/MgO introduced by Sato et al. (Applied Physics Letters 2012, DOI 10.1063/1.4736727), establish the microscopic, quantitative mechanisms that were left unresolved by the original work: (1) identify and measure the mechanism by which the intrinsic spin-torque switching current $I_{C0}$ (the zero-temperature critical current below which spin-torque switching cannot occur) of a double-interface junction remains comparable to that of a single-interface junction of the same diameter even though the thermal stability factor $\Delta$ (the energy barrier between the two magnetization states divided by the thermal energy, which sets data retention) nearly doubles — in particular, quantify the effective Gilbert damping constant $\alpha$ (the phenomenological damping parameter of the Landau-Lifshitz-Gilbert equation) of the double-interface stacks and determine the physical origin of its thickness and interface dependence (interfacial spin pumping into the Ta spacer, i.e. the emission of spin angular momentum from the precessing magnetization into the adjacent metal; interfacial spin-orbit scattering at the CoFeB-MgO boundaries; or bulk-like contributions); (2) determine the causal, quantitative relation between boron redistribution and Ta spacer-layer chemistry/structure and the resulting interface anisotropy $K_i$ (the areal interfacial anisotropy constant) and tunnel magnetoresistance (TMR) of the two CoFeB-MgO interfaces after annealing; (3) map the interlayer exchange coupling (the magnetic interaction transmitted through the nonmagnetic Ta spacer, which aligns the two CoFeB magnetizations parallel or antiparallel) between the two CoFeB layers as a function of Ta spacer thickness, including the ferromagnetic and antiferromagnetic regimes and the spacer-thickness window that yields either strongly coupled single-layer switching or independently switching layers; and (4) determine how the $\Delta$ enhancement, coupling, and switching current scale to junction sizes below the 70 nm originally studied and under device-relevant switching-speed and temperature conditions. The common scientific objective is a predictive, mechanism-level understanding of this recording structure — the damping, spacer chemistry, and coupling parameters that jointly set the retention-versus-write-current trade-off — so that double-interface stacks can be designed rather than empirically tuned. Later literature has already settled the device-level size-scaling part of item (4) down to 11 nm (reference 5) and has directly measured the damping reduction invoked in item (1) (reference 6), so the open core is: within item (1), the microscopic physical origin of the thickness- and interface-dependent damping; within item (2), the atomically resolved causal link between boron content, interfacial crystallization, $K_i$ and TMR in the double-interface recording stack; and within item (3), a controlled Ta-thickness interlayer-coupling phase map in device-scale junctions of the recording structure.

References

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