In 2012, Kanai and co-workers demonstrated that nanosecond voltage pulses reversibly switch the free layer of a sputtered perpendicular-anisotropy CoFeB/MgO magnetic tunnel junction through the voltage-controlled magnetic anisotropy (VCMA) effect, by driving roughly half a precession period of the magnetization. The demonstration left four linked open issues, which together ask whether this electric-field write mechanism can become a practical memory technology: increasing the anisotropy modulation per unit electric field so that cell retention (thermal stability factor) and switching speed simultaneously meet memory targets; restoring coherent, deterministic switching whose reliability is not degraded by thermal fluctuations and nonuniform dipolar fields; eliminating the tunneling current that dominates the write energy, to approach the projected sub-femtojoule-per-bit regime; and reproducing all of this across device arrays with endurance and CMOS-compatible operation. Progress on each component is strong but partial, while the combined, field-free, array-level, memory-grade solution remains open.

Background

A magnetic tunnel junction (MTJ) is a two-terminal device consisting of two ferromagnetic electrodes separated by a thin MgO tunnel barrier, whose resistance differs between the parallel (P) and antiparallel (AP) magnetization alignments; this readout contrast is the tunnel magnetoresistance (TMR) ratio. In a perpendicular-anisotropy MTJ (pMTJ), the free (storage) layer is an ultrathin CoFeB film whose magnetization points out of plane because of interfacial perpendicular magnetic anisotropy (PMA), an energy contribution concentrated at the CoFeB/MgO interface. pMTJs are the basis of spin-transfer-torque MRAM (STT-MRAM), in which the bit is written by a large spin-polarized current; STT writing costs energies of order 100 fJ/bit and requires a large access transistor, leaving a large gap between write energy and the roughly 60 kBT60\,k_\mathrm{B}T needed for ten-year-scale retention. The voltage-controlled magnetic anisotropy (VCMA) effect offers an alternative: applying a voltage across the MgO barrier modifies the interfacial PMA by an amount linear in the electric field (slope conventionally called the VCMA coefficient, in fJ/Vm), with essentially no current required. If the PMA is transiently suppressed enough to turn the easy axis in-plane, the magnetization precesses about the remaining in-plane effective field, and switching results when the voltage pulse lasts about half a precession period. Precessional voltage switching was first shown in in-plane epitaxial FeCo layers (Shiota et al., Nat. Mater. 2012), and Kanai, Yamanouchi, Ikeda, Nakatani, Matsukura and Ohno (Appl. Phys. Lett. 101, 122403 (2012)) demonstrated it in a sputtered, device-relevant perpendicular CoFeB/MgO MTJ: a 70-nm junction with TMR of 150% switched back and forth under approximately 1.8 ns pulses of effective amplitude near 1.8 V in a static in-plane field, with a measured precession period near 3.5 ns. Quantitatively, they extracted a thermal stability factor Δ=30\Delta = 30 (the ratio of the PMA energy barrier to kBTk_\mathrm{B}T, which sets data retention; memory applications typically require Δ≳60\Delta \gtrsim 60), an electric-field-induced anisotropy-field change of about 52 mT, and a dissipated energy of about 0.2 pJ/bit, dominated not by the anisotropy change itself but by the current flowing through the tunnel barrier during the pulse; in the ideal, capacitor-only limit the projected energy is below 1 fJ/bit. They also observed that the oscillations of switching probability versus pulse length decay with pulse duration, attributed to thermal fluctuations and nonuniform dipolar fields, and they noted that combined spin-transfer-torque plus electric-field operation (Kanai et al., APL 2014) is one route to assisted switching. The field that grew from this demonstration is usually called voltage-torque, VCMA, or magnetoelectric MRAM, and its central figures of merit are the VCMA coefficient, the write error rate (WER, the fraction of failed write attempts, measurable by repeating the write many times and reading out via TMR), and the write energy per bit.

Problem Statement

Can electric-field-induced precessional switching of the free layer in sputtered perpendicular-anisotropy CoFeB/MgO magnetic tunnel junctions be developed into a practical, reliable and scalable write mechanism for nonvolatile memory, in the sense established by the 2012 demonstration of Kanai et al.? The single overarching objective decomposes into the four linked conditions the source paper left open, all of which must hold jointly: (i) the anisotropy modulation per unit electric field (VCMA coefficient) must be increased so that nanoscale cells can simultaneously retain a thermal stability factor Δ\Delta well above the demonstrated value of 30 and switch within a nanosecond-scale, low-voltage pulse, without sacrificing the TMR needed for readout; (ii) the coherence of precessional switching must be restored, so that the decay of the switching probability versus pulse length caused by the thermal fluctuations and nonuniform dipolar fields identified in the source is suppressed, and the write error rate reaches memory-grade values over a usable pulse-width and amplitude window; (iii) device structures must prevent or eliminate the tunneling current through the MgO barrier during the write pulse, so that the dissipated energy (0.2 pJ/bit in the source, dominated by current flow) approaches the projected sub-fJ/bit, capacitor-charging limit; and (iv) the effect must be reproduced across device arrays, with endurance under repeated pulsing and CMOS-compatible biasing and readout, including operation without the external static magnetic field used in the original demonstration. Because the four conditions are interdependent, the question is resolved only by a determination of whether they can hold simultaneously in a single device platform, not by progress on any one of them; a demonstrated or argued incompatibility on any single condition (for example, between the required VCMA coefficient and the TMR and thermal stability available at scaled cell size) resolves it negatively.

References

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