Impact of gate voltage on switching field of perpendicular magnetic tunnel junctions with a synthetic antiferromagnetic free layer

Abstract: We present micromagnetic simulations and experiments on voltage-assisted field switching in perpendicular magnetic tunnel junctions (MTJs) with a synthetic antiferromagnetic (SAF) free layer, where the magnetic state of one sublayer is detected via tunneling magnetoresistance (TMR). Simulations reveal that local modulation of perpendicular magnetic anisotropy (PMA) in one SAF sublayer leads to distinct switching characteristics. The switching field varies linearly with the anisotropy field, indicating voltage-controlled magnetic anisotropy (VCMA)-dominated dynamics similar to single free-layer devices. We then experimentally study the magnetic switching field of MTJ devices with SAF free layers under applied gate voltage. By varying the MgO tunnel barrier thickness to systematically modulate the resistance-area (RA) product, we enable quantitative separation of spin-transfer torque (STT), VCMA, and Joule heating contributions. Our findings indicate that VCMA dominates in devices with a high RA product, while low-RA devices exhibit nonlinear switching behavior due to enhanced contributions from STT and Joule heating. Furthermore, the effective fields derived from STT, VCMA, and Joule heating contributions under various gate voltages show minimal dependence on device critical dimensions, indicating favorable scaling behavior. This work presents a unified framework analyzing the roles of STT, VCMA, and Joule heating in SAF-based voltage-gated spin-orbit torque (SOT) MRAM, offering key insights for the optimization of performance, energy efficiency, and scalability in SOT-MRAM technologies.

• The paper presents a discovery that voltage-controlled magnetic anisotropy dominates switching field modulation in high-resistance MTJs.
• It employs micromagnetic simulations and systematic experiments to decouple VCMA, STT, and Joule heating effects across varied RA products.
• Results indicate a linear, device-size–independent VCMA-induced switching field, guiding scalable and energy-efficient VG-SOT MRAM design.

Impact of Gate Voltage on Switching Field in Perpendicular MTJs with Synthetic Antiferromagnetic Free Layers

Introduction

This paper presents a comprehensive study of voltage-assisted switching in perpendicular magnetic tunnel junctions (p-MTJs) employing a synthetic antiferromagnetic (SAF) free layer. Through a combination of micromagnetic simulations and systematic experimental investigations, the authors establish the voltage-controlled magnetic anisotropy (VCMA) effect as the dominant mechanism for switching field modulation in devices with high resistance-area (RA) products, while elucidating the nonlinear influence of spin-transfer torque (STT) and Joule heating in low-RA devices. The work further develops and validates an analytical model that quantitatively separates and characterizes the contributions of VCMA, STT, and Joule heating to overall switching dynamics. These findings directly inform the design and scaling of voltage-gated spin-orbit torque (VG-SOT) MRAM technologies, addressing critical issues of power consumption, device retention, and integration density.

Physical Mechanisms in SAF-HFL Magnetic Tunnel Junctions

The study focuses on three-terminal SOT-MTJ stacks incorporating an SAF-HFL structure. The SAF-HFL comprises a composite M1 magnet (CoFeB/Co) ferromagnetically coupled and subsequently antiferromagnetically coupled via a Ru spacer to a second Co layer (M2), leveraging Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange. This configuration allows decoupled tuning of the anisotropy and effective moment, thereby enhancing retention, enabling deterministic SOT switching, and supporting integration into BEOL-compatible stacks.

Micromagnetic MuMax3 simulations capture distinct switching phenomena arising from local PMA modulation, induced by varying the gate voltage across the MgO barrier. The switching field of the detected sublayer (M1) exhibits a strictly linear dependence on its anisotropy field $B_k$, analogous to single free-layer behavior observed in VCMA studies. However, the SAF structure introduces nontrivial cascade switching modes (two- or three-step reversal processes) determined by the interlayer coupling and PMA asymmetry, underlying the necessity for a tailored analysis in SAF-HFL systems.

Experimental Separation of VCMA, STT, and Joule Heating Contributions

The authors systematically vary MgO thickness to modulate the RA product (110, 600, and 1500 $\Omega\cdot\mu\mathrm{m}^2$), thereby controlling the relative strengths of tunneling current-induced effects. Devices are characterized for their TMR, coercivity, and RA-dependence under a range of gate voltages.

• High-RA Regime: Parasitic STT and Joule heating are suppressed, isolating VCMA as the primary switching field modulation mechanism. The switching field varies linearly with applied $V_g$, in concordance with simulation results.
• Low-RA Regime: The increased tunneling current in thin MgO barriers amplifies STT and Joule heating, as evidenced by a parabolic voltage dependence of the switching field. These effects are quantified using a global analytical fit that incorporates the intrinsic coercivity, offset fields, and effective STT, VCMA, and Joule heating fields. The fitted coefficients provide direct numerical estimates of each contribution.

Notably, the VCMA effect is quantified to yield an effective field slope of 16 mT/V, superior to the 7 mT/V associated with STT, while Joule heating peaks around 25 mT for $|V_g| = 1$ V and is strictly quadratic in voltage. These dependencies are robust across device sizes and designs, indicating that the model and decomposition hold for a variety of SAF-HFL configurations.

Device Scaling and Switching Performance

A comparative study across critical device dimensions (50–200 nm) and RA products illustrates that, with increasing RA:

• STT and Joule heating effects decrease precipitously, attributable to reduced tunneling current and power dissipation.
• The VCMA-induced effective field remains invariant at $\sim 20$ mT per volt, demonstrating that voltage-gated switching efficiency is decoupled from device area and power loss in the high-RA regime.

Furthermore, SOT-driven magnetization switching measurements under pulse current and in-plane assist fields confirm deterministic reversibility and tight switching threshold modulation via gate voltage, validating the practical applicability of VCMA tuning for ultrafast, energy-efficient switching in SOT-MRAM architectures.

Implications and Future Prospects

The delineation of VCMA, STT, and Joule heating contributions as a function of RA and device geometry provides crucial design criteria for the next generations of SOT-MRAM. The evidence that VCMA-driven switching field modulation exhibits minimal dependence on device dimensions and retains linear scaling establishes SAF-HFL-based voltage-gated switching as a scalable route for high-density, ultra-low-energy MRAM solutions. Practically, these findings support the integration of such devices into standard CMOS back-end processes and position VG-SOT MRAM as a viable solution for SRAM replacement and compute-in-memory applications.

The analytical framework enables performance optimization through targeted engineering of MgO barrier thickness and free layer composition, balancing energy efficiency against reliability and write selectivity. The insight that VCMA dominates in suitably engineered devices, with STT and Joule heating only emerging in low-RA extremes, allows for deliberate trade-offs between switching speed, retention, and power dissipation.

Conclusion

This paper provides a rigorous, quantitative dissection of voltage-driven switching field modulation in SAF-HFL MTJs, with clear experimental demonstration of the dominant role of VCMA in high-RA regimes. The validated analytical model for effective field decomposition establishes design guidelines for scaling and energy efficiency of VG-SOT-MRAM devices. The robust, linear voltage control of the switching field across device geometries highlights the technological readiness of SAF-based voltage-gated spintronic memories for high-performance, dense memory and logic applications.