Title Participants "Study of precessional switching speed control in voltage-controlled perpendicular magnetic tunnel junction" "Yueh Chang Wu, Jan Van Houdt, Guido Groeseneken" "Manufacturable 300mm platform solution for Field-Free Switching SOT-MRAM" "K Garello, F Yasin, H Hody, S Couet, L Souriau, SH Sharifi, J Swerts, R Carpenter, S Rao, W Kim, Yueh Chang Wu, KK Sethu, M Pak, N Jossart, D Crotti, A Furnemont, GS Kar" "Manufacturable 300mm platform solution for Field-Free Switching SOT-MRAM" "K Garello, F Yasin, H Hody, S Couet, L Souriau, SH Sharifi, J Swerts, R Carpenter, S Rao, W Kim, Yueh Chang Wu, KKV Sethu, M Pak, N Jossart, D Crotti, A Furnemont, GS Kar" "Synthetic-Ferromagnet Pinning Layers Enabling Top-Pinned Magnetic Tunnel Junctions for High-Density Embedded Magnetic Random-Access Memory" "Enlong Liu, Yueh Chang Wu, Jo De Boeck" "© 2018 American Physical Society. Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have been developed for decades for spin-transfer-torque magnetic random-access memory. Common stack designs use a hard layer (HL) with strong PMA to pin the reference layer (RL) by forming a synthetic antiferromagnet through a thin nonmagnetic coupling layer. Compared to bottom-pinned MTJs, very limited progress has been made to top-pinned MTJs, especially on the RL pinning due to its inferior thermal robustness. Herein, an alternative stack design is proposed for top-pinned MTJs, i.e., a synthetic ferromagnet (SFM). In the SFM, the RL is coupled with the HL ferromagnetically through a coupling layer. Micromagnetic simulations predict the advantage of the SFM design to stabilize the RL at scaled critical dimension (CD), which is experimentally proven by the observation of an increased RL pinning field on the device level. Because of the RL stray field acting on the free layer (FL), a compensation magnet (CM) is inserted below the FL to form a top-pinned MTJ stack without compromising magnetotransport properties. Devices with centered FL switching loops can be obtained after a two-step field setting. The stray field of CM has limited impact on the RL due to the large distance in between, thus keeping the RL's pinning field larger than 150 mT down to devices with 20-nm CD. Finally, current switching is realized in devices with SFM and CM, showing critical current density around 5-8 MA/cm2 and an averaged thermal stability as high as 50. Thus, the SFM pinning layer design shows great potential in stabilizing top-pinned devices and paves the way for multiple future spintronic applications requiring a top-pinned stack design." "Top-Pinned STT-MRAM Devices With High Thermal Stability Hybrid Free Layers for High-Density Memory Applications" "Enlong Liu, Yueh Chang Wu, Jo De Boeck" "© 1965-2012 IEEE. The hybrid free layer (HFL) design consisting of a single layer of CoFeB coupled via a spacer to [Co/Ni] multilayers was proposed previously to increase perpendicular magnetic anisotropy in the free layer of top-pinned magnetic tunnel junction stacks. It is thus expected that a high thermal stability factor (Δ) can be achieved in devices with small dimensions, which is required for data retention in high-density memory applications. Here, we report on the electrical and magnetic properties of the HFL in devices with various critical dimensions (CD), in which case the [Co/Ni] multilayers in the HFL have only one Co/Ni bilayer on Pt seed. It is shown first that the HFL can provide larger coercive field (H c) than dual-MgO FL counterparts. μ0H c around 125 mT is achieved in sub-20 nm devices with the HFL. To derive Δ, the field sweep method was applied. The fitting of the switching probability versus applied field was compared for a macrospin reversal and a domain wall mediated reversal (DWMR). The DWMR model is valid when the CD is larger than around 40 nm in both dual-MgO FL and HFL cases, where the switching is realized through domain wall motion as shown via micromagnetic simulations. In both regimes, the HFL provides a higher Δ than the dual-MgO FL." "Impact of operating temperature on the electrical and magnetic properties of the bottom-pinned perpendicular magnetic tunnel junctions" "Yueh Chang Wu, Simon Van Beek, Johanna Jochum, Margriet Van Bael, Jan Van Houdt, Guido Groeseneken" "© 2018 Author(s). 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 (M S ) of the [Co/Pt] x -based multilayers is much less affected by temperature [M S (400 K)/M S (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." "Thermal stability analysis and modelling of advanced perpendicular magnetic tunnel junctions" "Simon Van Beek, Koen Martens, Yueh Chang Wu, Guido Groeseneken" "© 2017 Author(s). 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." "Top-pinned STT-MRAM devices with high thermal stability hybrid free layers for high density memory applications" "Yueh Chang Wu, Jo De Boeck" "High Speed MRAM with Voltage Control of Magnetic Anisotropy (VCMA) Effect" "Yueh Chang Wu" "Magnetic random access memory (MRAM) is gaining intensive interest for embedded and stand-alone memory applications. Its inherent non-volatility is believed to address the large stand-by energy consumption issues in the present memory hierarchy. In recent years, the spin-transfer torque (STT)-MRAM has gradually matured and started to appear in the market. Typically, STT writing of perpendicular magnetic tunnel junction (pMTJ) is limited to a few nanoseconds. Because STT is collinear with intrinsic damping of the free-layer (FL), the FL needs to wait for the thermal fluctuation to tilt its magnetization to gain a finite torque. Further, it requires a large enough STT current to overcome the damping torque, which consumes a great amount of writing energy. One can overcome these issues by using different writing mechanisms that do not directly compete with the intrinsic damping. Among several proposals, the most attractive ones are the voltage control of magnetic anisotropy (VCMA) and the spin-orbit torques (SOT). In particular, VCMA promises low power operation of MRAM as it allows modification of the FL perpendicular magnetic anisotropy (PMA), a parameter directly linked to the writing current/energy. This PhD study will focus on the understanding of VCMA both as the writing mechanism and as the assisting mechanism to SOT."