arXiv subjects

G. S. Kar

Publications and source records attributed to G. S. Kar.

9 recordsLinked to original sources

arXiv2026

Band Tail State Broadening in IGZO TFTs After pBTI-Induced Negative VT Shift Revealed via DC and 1/f Noise Measurements

We investigate the origin of negative threshold voltage shifts in back-gated amorphous IGZO TFTs under positive bias and high temperature stress. Combined DC and 1/f noise measurements reveal that the stress does not generate new dielectric traps but instead broadens the IGZO conduction band tail states. A recovery experiment confirms that the associated threshold voltage, subthreshold swing, and noise degradation are reversible. Simulations using an in-house Poisson solver confirm the experimental observations that high-temperature stress increases hydrogen doping and the density of sub-gap states.

cond-mat.mtrl-sci↗

arXiv2026

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

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.

cond-mat.mes-hall↗

arXiv2021

Voltage-Gate Assisted Spin-Orbit Torque Magnetic Random Access Memory for High-Density and Low-Power Embedded Application

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 VGSOT writing properties on perpendicular magnetic tunnel junction (pMTJ) devices, and we propose a detailed methodology for its electrical characterization. The impact of gate assistance on the SOT switching characteristics are investigated using electrical pulses down to 400ps. The VCMA coefficient (ξ) extracted from current switching scheme is found to be the same as that from the magnetic field switch method, which is in the order of 15fJ/Vm for the 80nm to 150nm 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 as for the magnetic properties. We interpret this linear behavior as the direct modification of perpendicular magnetic anisotropy (PMA) and nucleation energy induced by VCMA. At V_g = 1V, the SOT write current is decreased by 25%, corresponding to a 45% reduction in total energy down to 30fJ/bit at 400ps speed for the 80nm devices used in this study. Further, the device-scaling criteria are proposed, and we reveal that 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, how 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.

cond-mat.mes-hall↗

arXiv2020

Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.

cond-mat.mtrl-sci↗

arXiv2019

Effect of Tantalum spacer thickness and deposition conditions on the properties of MgO/CoFeB/Ta/CoFeB/MgO free layers

To get stable perpendicularly magnetized tunnel junctions at small device dimensions, composite free layers that comprise two MgO/FeCoB interfaces as sources of interface anisotropy are generally used. Proper cristallisation and annealing robustness is typically ensured by the insertion of a spacer layer of the early transition metal series within the FeCoB layer. We study the influence of the spacer thickness and growth condition on the switching metrics of tunnel junctions thermally annealed at 400$^\circ$C for the case of 1-4 Å Ta spacers. Thick Ta spacer results in a large anisotropies indicative of a better defined top FeCoB/MgO interface, but this is achieved at the systematic expense of a stronger damping. For the best anisotropy-damping compromise, junctions of diameter 22 nm can still be stable and spin-torque switched. Coercivity and inhomogeneous linewidth broadening, likely arising from roughness at the FeCoB/Ta interface, can be reduced if a sacrificial Mg layer is inserted before the Ta spacer deposition.

cond-mat.mtrl-sci↗

arXiv2019

Manufacturable 300mm platform solution for Field-Free Switching SOT-MRAM

We propose a field-free switching SOT-MRAM concept that is integration friendly and allows for separate optimization of the field component and SOT/MTJ stack properties. We demonstrate it on a 300 mm wafer, using CMOS-compatible processes, and we show that device performances are similar to our standard SOT-MTJ cells: reliable sub-ns switching with low writing power across the 300mm wafer. Our concept/design opens a new area for MRAM (SOT, STT and VCMA) technology development.

physics.app-ph↗

arXiv2019

Offset fields in perpendicularly magnetized tunnel junctions

We study the offset fields affecting the free layer of perpendicularly magnetized tunnel junctions. In extended films, the free layer offset field results from interlayer exchange coupling with the reference layer through the MgO tunnel oxide. The free layer offset field is thus accompanied with a shift of the free layer and reference layer ferromagnetic resonance frequencies. The shifts depend on the mutual orientation of the two magnetizations. The offset field decreases with the resistance area product of the tunnel oxide. Patterning the tunnel junction into an STT-MRAM disk-shaped cell changes substantially the offset field, as the reduction of the lateral dimension comes with the generation of stray fields by the reference and the hard layer. The experimental offset field compares best with the spatial average of the sum of these stray fields, thereby providing guidelines for the offset field engineering.

cond-mat.mtrl-sci↗

arXiv2018

SOT-MRAM 300mm integration for low power and ultrafast embedded memories

We demonstrate for the first time full-scale integration of top-pinned perpendicular MTJ on 300 mm wafer using CMOS-compatible processes for spin-orbit torque (SOT)-MRAM architectures. We show that 62 nm devices with a W-based SOT underlayer have very large endurance (> 5x10^10), sub-ns switching time of 210 ps, and operate with power as low as 300 pJ.

cond-mat.mes-hall↗

arXiv2018

Gilbert damping of high anisotropy Co/Pt multilayers

Using broadband ferromagnetic resonance, we measure the damping parameter of [Co(5 Å)/Pt(3 Å)]${\times 6}$ multilayers whose growth was optimized to maximize the perpendicular anisotropy. Structural characterizations indicate abrupt interfaces essentially free of intermixing despite the miscible character of Co and Pt. Gilbert damping parameters as low as 0.021 can be obtained despite a magneto-crystalline anisotropy as large as $10^6~\textrm{J/m}^3$. The inhomogeneous broadening accounts for part of the ferromagnetic resonance linewidth, indicating some structural disorder leading to a equivalent 20 mT of inhomogenity of the effective field. The unexpectedly relatively low damping factor indicates that the presence of the Pt heavy metal within the multilayer may not be detrimental to the damping provided that intermixing is avoided at the Co/Pt interfaces.

cond-mat.mtrl-sci↗