The voltage-gate-assisted spin-orbit-torque (VGSOT) writing scheme combines the advantages from voltage control of magnetic anisotropy (VCMA) and spin-orbit-torque (SOT) effects, enabling multiple benefits for magnetic random-access-memory (MRAM) applications. In this work, we give a complete description of the VGSOT writing properties on perpendicular magnetic tunnel junction (PMTJ) devices, and we propose a detailed methodology for their electrical characterization. The impact of gate assistance on the SOT switching characteristics is investigated using electrical pulses down to 400 ps. The VCMA coefficient (ξ ) extracted from the current-switching scheme is found to be the same as that from the magnetic-field-switch method, which is in the order of 15 fJ/Vm for 80-150-nm devices. Moreover, as expected from the pure electronic VCMA effect, ξ is revealed to be independent of the writing speed and gate length. We observe that SOT switching current characteristics are modified linearly with gate voltage (V g ), similar to that for the magnetic properties. We interpret this linear behavior as the direct modification of perpendicular magnetic anisotropy induced by VCMA. At V g = 1 V, the SOT write current is decreased by 25%, corresponding to a 45% reduction in total energy down to 30 fJ/bit at 400 ps speed for the 80-nm devices used in this study. To test the operation reliability, we investigate the gate-SOT pulse configurations and overlays, and we find that an extended gate duration is able to preserve maximized gate benefit and selectivity. Furthermore, the device-scaling criteria are proposed, and we reveal that the VGSOT scheme is of great interest, as it can mitigate the complex material requirements of achieving high SOT and VCMA parameters for scaled MTJs. Finally, we perform design-to-technology co-optimization analysis to show that VGSOT MRAM can enable high-density arrays close to two-terminal geometries, with high-speed performance and low-power operation, showing great potential for embedded memories as well as in memory computing applications at advanced technology nodes.
STT-MRAM is a promising non-volatile memory for high speed applications. The thermal stability factor (Δ = Eb/kT) is a measure for the information retention time, and an accurate determination of the thermal stability is crucial. Recent studies show that a significant error is made using the conventional methods for Δ extraction. We investigate the origin of the low accuracy. To reduce the error down to 5%, 1000 cycles or multiple ramp rates are necessary. Furthermore, the thermal stabilities extracted from current switching and magnetic field switching appear to be uncorrelated and this cannot be explained by a macrospin model. Measurements at different temperatures show that self-heating together with a domain wall model can explain these uncorrelated Δ. Characterizing self-heating properties is therefore crucial to correctly determine the thermal stability.
Analogous device parameters in both the parallel (P) and anti-parallel (AP) states ensure a symmetric spin-transfer-torque magnetic random-access memory operation scheme. In this study, however, we observe an increasing asymmetry in the performance metrics with operating temperature of the bottom-pinned perpendicular magnetic tunnel junction (p-MTJ) devices. A temperature-dependent increase in the contribution of the stray field is observed in the tunneling magnetoresistance loop analysis. The switching current for P-to-AP decreases by 30% in the thermally activated switching regime by increasing the temperature from 300 K to 400 K, while it remains similar for AP-to-P. In addition, with the same temperature range, the thermal stability factor for the P state decreases 20% more than that for the AP state. We attribute those observations to the increase in the overcompensation of the stray field from the synthetic anti-ferromagnet structure. Saturation magnetization (MS) of the [Co/Pt]x-based multilayers is much less affected by temperature [MS(400 K)/MS(300 K) = 97%] compared to that of the CoFeB-based multilayers (88%). Such an impact can be more severe during the electrical switching process due to the Joule heating effect. These results suggest that, to understand and to evaluate the performance in a wide range of temperatures, it is crucial to consider the contribution of the entire magnetic components in the p-MTJ stack.
We study the characteristics of the precessional switching induced by voltage control of magnetic anisotropy (VCMA) in back-end-of-line (BEOL)-compatible perpendicular magnetic tunnel junction devices. Using micromagnetic simulation, we find three operation regimes differentiated by zero excess energy, lower boundary, zero energy barrier, and upper boundary. Experimentally, the switching speed (fs) is characterized by two phases: non-precession and acceleration. Non-precession is a thermal mediated phase, where fs cannot be deduced, while in acceleration, both the higher electric field (EF) and in-plane field (Bx) increase fs progressively. We find that the intrinsic thresholds can be retrieved by linear extrapolation of fs as a function of EF. Those thresholds and experimental results are in good agreement with the simulation. In addition, we numerically calculate the characteristic switching speed of 2γ*mz*Bx and verify it experimentally. This work provides insights into the VCMA-induced precessional switching, including detailed understandings of the switching mechanism and modeling of switching speed for reliable write duration control for practical applications.
Voltage control of the magnetic anisotropy (VCMA) effect enables a voltage-mediated magnetization switching mechanism for lower-power applications. In this work, we experimentally investigate the characteristics of VCMA-induced switching and we observe a clear decrease in the critical switching voltage (Vc) at elevated temperatures. A 50% reduction in Vc is quantified when increasing the ambient temperature (T) from 300 K to 360 K. Such a T-dependence of Vc is well explained with the variations of saturation magnetization (MS), interfacial anisotropy (Ki), and VCMA coefficient (ξ). In addition, the dependences of these properties on temperature are well fitted and explained with the power law of MS(T). Our findings on the T-dependent magnetic and switching characteristics of VCMA are of technological importance for implementing VCMA in magnetic random access memory (MRAM) applications.
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