A 2005 letter by Hayakawa, Ikeda, Matsukura, Takahashi and Ohno established that sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions — with amorphous CoFeB electrodes that crystallize upon post-deposition annealing — reach giant tunnel magnetoresistance (260% at room temperature, 403% at 5 K) and left open five follow-up issues: the microscopic link between electrode crystallization and the symmetry-filtered tunneling channels, the nucleation and kinetics of CoFeB crystallization, the origin of the slight bias-voltage asymmetry of the TMR, the limits of the barrier-thickness/annealing parameter window, and the connection of the effective barrier height (about 0.34-0.4 eV from resistance-area scaling and Simmons fits) to a microscopic potential profile. This task addresses the three interface-electronic-structure questions that remain open: a direct, channel-resolved demonstration of how interfacial crystallinity and boron/oxygen chemistry establish the Δ1\Delta_1-dominated tunneling channels in the sputtered (as opposed to epitaxial) system; an attribution of the residual TMR bias asymmetry in nominally symmetric junctions to specific electronic or chemical differences of the two MgO/CoFeB interfaces; and a reconciliation of the fitted effective barrier height with a microscopic potential profile of the MgO/CoFeB stack.

Background

A magnetic tunnel junction (MTJ) is a two-ferromagnet/insulator sandwich whose resistance depends on the relative orientation of the two magnetizations; the relative change between the parallel (P) and antiparallel (AP) magnetizations is the tunnel magnetoresistance (TMR) ratio, which sets the read signal of nonvolatile magnetic memory and read heads. The resistance-area product (RA) of a pinhole-free barrier grows exponentially with barrier thickness, and fitting that growth (for example with a Wentzel-Kramers-Brillouin, i.e. WKB, tunneling approximation, or fitting the nonlinear current-voltage curve with Simmons' standard tunneling formula) yields an 'effective' barrier height ϕ\phi, an empirical parameter that need not equal the insulator's band gap. First-principles calculations for crystalline Fe/MgO(001)/Fe (Butler, Zhang, Schulthess and MacLaren, 2001; Mathon and Umerski, 2001) predicted TMR ratios of hundreds to thousands of percent: MgO(001) preferentially transmits wavefunctions of Δ1\Delta_1 symmetry (Δ1\Delta_1 denotes the symmetry class of the states propagating perpendicular to the interface in the body-centered-cubic, i.e. bcc, electrode), and in Fe only one spin channel has a Δ1\Delta_1 band at the Fermi energy, so the barrier acts as a spin filter ('symmetry filtering'). Experiments on single-crystal and highly oriented junctions (Yuasa and co-workers, 2004) and on sputtered junctions with amorphous CoFeB electrodes (Djayaprawira and co-workers, 2005, with 230% TMR) confirmed giant TMR. Hayakawa, Ikeda, Matsukura, Takahashi and Ohno (2005) then mapped how TMR depends on MgO thickness (1.15-2.4 nm) and annealing temperature (270-375 C) in sputtered CoFeB/MgO/CoFeB on SiO2/Si: RA rose exponentially with MgO thickness (WKB ϕ≈0.34\phi\approx 0.34-0.38 eV0.38\ \mathrm{eV}, Simmons ϕ≈0.4 eV\phi\approx 0.4\ \mathrm{eV}), high-resolution transmission electron microscopy (HRTEM) showed the as-sputtered amorphous CoFeB electrodes fully crystallizing into the bcc structure after annealing at 375 C while the MgO stayed highly (001)-oriented, and TMR climbed to 260% at room temperature. The authors themselves hedged the mechanism: the data 'strongly suggest' that the crystalline electrode near the MgO is the origin of the giant TMR; they 'believe' crystallization nucleates at both MgO/CoFeB interfaces; and the slight asymmetry of the TMR's bias-voltage dependence 'may have been caused' by differences in the electronic states of the two MgO/CoFeB interfaces. The follow-up literature is now enormous, because sputtered CoFeB/MgO/CoFeB became the workhorse stack of spin-transfer-torque magnetic random-access memory (MRAM).

Problem Statement

For sputtered, post-deposition-annealed CoFeB/MgO(001)/CoFeB magnetic tunnel junctions of the type introduced by Hayakawa et al. (2005), establish the microscopic, interface-resolved account of the tunneling transport. Concretely: (1) demonstrate directly, for the sputtered (not epitaxial) system, how the annealing-induced bcc(001) CoFe(B) crystallization adjacent to MgO, together with the attendant boron migration and oxygen/transition-metal interfacial chemistry, establishes the symmetry-filtered, Δ1\Delta_1-dominated tunneling channels and their effective half-metallicity (a half-metal is metallic for one spin channel and insulating for the other; 'effective' here means transport that is nearly fully spin-polarized), i.e. the spatial extent and energy alignment of the Δ1\Delta_1 states at the two MgO/CoFeB interfaces; (2) attribute the residual bias-voltage asymmetry of the TMR in these nominally symmetric junctions to specific electronic states or chemical differences of the two MgO/CoFeB interfaces, disentangling that contribution from inelastic channels (magnon- and phonon-assisted tunneling, where a magnon is a quantized spin-wave excitation of the ferromagnetic electrode); and (3) reconcile the effective barrier height extracted from resistance-area (RA)-versus-thickness scaling (WKB, ϕ≈0.34\phi\approx 0.34-0.38 eV0.38\ \mathrm{eV}) and from Simmons fits to the antiparallel current-voltage curve (ϕ≈0.4 eV\phi\approx 0.4\ \mathrm{eV}) with a microscopic potential profile of the MgO/CoFeB stack, including image-force corrections (the electrostatic lowering of the barrier by induced charges at the electrode surfaces), atomic-scale barrier inhomogeneity, and the electrode band structure. Two further elements of the original 2005 bundle - the quantitative characterization of CoFeB crystallization nucleation and kinetics, and the mapping of the barrier-thickness/annealing optimization window for devices - have been substantially resolved by the follow-up literature cited in the references and are therefore no longer part of the open target. Parts (1)-(3) are jointly posed and share one scientific objective: an interface-resolved microscopic theory of transport in the sputtered CoFeB/MgO system, matched to experiment, that quantitatively accounts for the symmetry-filtered channel composition, the bias asymmetry, and the effective barrier height of the same junctions within one microscopic model.

References

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