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S. Rao

Publications and source records attributed to S. Rao.

At least 19 recordsLinked to original sources
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↗
arXiv2025

An improved argument principle root-search method for modes of slab waveguides, optical fibers, and spheres

We update our root-search method for transcendental equations. Our method is globally convergent and is guaranteed to locate all complex roots within a specified search domain, since it is based on Cauchy's residue theorem. We extend the implementation to treat the dispersion relations of slab waveguides and the resonances of a sphere, in addition to step-index fibers. We also implement other improvements, such as to the contour selection procedure and using non-dimensional search variables, to ensure the method remains reliable even in challenging parameter regimes. We also extend the algorithm to identify leaky modes in terms of propagation constant eigenvalue modes, revealing, to the first time to our knowledge, a discontinuity across the light line in the dispersion plot.

physics.comp-ph↗
arXiv2021

Storage Rings and Gravitational Waves: Summary and Outlook

We report some highlights from the ARIES APEC workshop on ``Storage Rings and Gravitational Waves'' (SRGW2021), held in virtual space from 2 February to 18 March 2021, and sketch a tentative landscape for using accelerators and associated technologies for the detection or generation of gravitational waves.

physics.acc-ph↗
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

Kohn-Sham Density Functional Theory of Abelian Anyons

We develop a density functional treatment of non-interacting abelian anyons, which is capable, in principle, of dealing with a system of a large number of anyons in an external potential. Comparison with exact results for few particles shows that the model captures the behavior qualitatively and semi-quantitatively, especially in the vicinity of the fermionic statistics. We then study anyons with statistics parameter $1+1/n$, which are thought to condense into a superconducting state. An indication of the superconducting behavior is the mean-field result that, for uniform density systems, the ground state energy increases under the application of an external magnetic field independent of its direction. Our density-functional-theory based analysis does not find that to be the case for finite systems of anyons, which can accommodate a weak external magnetic field through density transfer between the bulk and the boundary rather than through transitions across effective Landau levels, but the "Meissner repulsion" of the external magnetic field is recovered in the thermodynamic limit as the effect of the boundary becomes negligible. We also consider the quantum Hall effect of anyons, and show that its topological properties, such as the charge and statistics of the excitations and the quantized Hall conductance, arise in a self-consistent fashion.

cond-mat.str-el↗
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

Baseline predictions of elliptic flow and fluctuations at the RHIC Beam Energy Scan using response coefficients

Currently the RHIC Beam Energy Scan is exploring a new region of the Quantum Chromodynamic phase diagram at large baryon densities that approaches nuclear astrophysics regimes. This provides an opportunity to study relativistic hydrodynamics in a regime where the net conserved charges of baryon number, strangeness, and electric charge play a role, which will significantly change the theoretical approach to simulating the baryon-dense Quark-Gluon Plasma. Here we detail many of the important changes needed to adapt both initial conditions and the medium to baryon-rich matter. Then, we make baseline predictions for the elliptical flow and fluctuations based on extrapolating the physics at LHC and top RHIC energies to support future analyses of where and how the new baryon-dense physics causes these extrapolations to break down. First we compare eccentricities across beam energies, exploring their underlying assumptions; we find the the extrapolated initial state is predicted to be nearly identical to that at AuAu $\sqrt{s_{NN}}=200$ GeV. Then the final flow harmonic predictions are based on linear+cubic response. We discuss preliminary STAR results in order to determine the implications that they have for linear+cubic response coefficients at the lowest beam energy of AuAu $\sqrt{s_{NN}}=7$ GeV.

nucl-th↗
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

Ultracentral Collisions of Small and Deformed Systems at RHIC: ${U} {U}$, ${d} {Au}$, ${}^{9}{Be} {Au}$, ${}^{9}{Be} {}^{9}{Be}$, ${}^{3}{He} {}^{3}{He}$, and ${}^{3}{He} {Au}$ Collisions

In this paper, we study a range of collision systems involving deformed ions and compare the elliptic and triangular flow harmonics produced in a hydrodynamics scenario versus a color glass condensate (CGC) scenario. For the hydrodynamics scenario, we generate initial conditions using TRENTO and work within a linear response approximation to obtain the final flow harmonics. For the CGC scenario, we use the explicit calculation of two-gluon correlations taken in the high-$p_T$ ``(semi)dilute-(semi)dilute'' regime to express the flow harmonics in terms of the density profile of the collision. We consider ultracentral collisions of deformed ions as a testbed for these comparisons because the difference between tip-on-tip and side-on-side collisions modifies the multiplicity dependence in both scenarios, even at zero impact parameter. We find significant qualitative differences in the multiplicity dependence obtained in the initial conditions+hydrodynamics scenario and the CGC scenario, allowing these collisions of deformed ions to be used as a powerful discriminator between models. We also find that sub-nucleonic fluctuations have a systematic effect on the elliptic and triangular flow harmonics which are most discriminating in $0-1\%$ ultracentral symmetric collisions of small deformed ions and in $0-10\%$ $\mathrm{d} {}^{197}\mathrm{Au}$ collisions. The collision systems we consider are ${}^{238}\mathrm{U} {}^{238}\mathrm{U}$, $\mathrm{d} {}^{197}\mathrm{Au}$, ${}^{9}\mathrm{Be} {}^{197}\mathrm{Au}$, ${}^{9}\mathrm{Be} {}^{9}\mathrm{Be}$, ${}^{3}\mathrm{He} {}^{3}\mathrm{He}$, and ${}^{3}\mathrm{He} {}^{197}\mathrm{Au}$.

hep-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↗
arXiv2017

Material developments and domain wall based nanosecond-scale switching process in perpendicularly magnetized STT-MRAM cells

We investigate the Gilbert damping and the magnetization switching of perpendicularly magnetized FeCoB-based free layers embedded in tunnel junctions adequate for spin-torque operated memories. We study the influence of the boron content in MgO / FeCoB /Ta systems alloys on their Gilbert damping after crystallization annealing. Increasing the boron content from 20 to 30\% increases the crystallization temperature, thereby postponing the onset of elemental diffusion within the free layer. This reduction of the interdiffusion of the Ta atoms helps maintaining the Gilbert damping at a low level of 0.009 without any penalty on the anisotropy and the magneto-transport properties up to the 400$^\circ$C annealing required in CMOS back-end of line processing. In addition, we show that dual MgO free layers of composition MgO/FeCoB/Ta/FeCoB/MgO have a substantially lower damping than their MgO/FeCoB/Ta counterparts, reaching damping parameters as low as 0.0039 for a 3 Å thick Tantalum spacer. This confirms that the dominant channel of damping is the presence of Ta impurities within the FeCoB alloy. On optimized tunnel junctions, we then study the duration of the switching events induced by spin-transfer-torque. We focus on the sub-threshold thermally activated switching in optimal applied field conditions. From the electrical signatures of the switching, we infer that once the nucleation has occurred, the reversal proceeds by a domain wall sweeping though the device at a few 10 m/s. The smaller the device, the faster its switching. We present an analytical model to account for our findings. The domain wall velocity is predicted to scale linearly with the current for devices much larger than the wall width. The wall velocity depends on the Bloch domain wall width, such that the devices with the lowest exchange stiffness will be the ones that host the domain walls with the slowest mobilities.

cond-mat.mtrl-sci↗
arXiv2005

Evidence for the presence of dust in intervening QSO absorbers from the Sloan Digital Sky Survey

We find evidence for dust in the intervening QSO absorbers from the spectra of QSOs in the Sloan Digital Sky Survey Data Release 1. No evidence is found for the 2175 A feature which is present in the Milky Way dust extinction curve. The extinction curve resembles the SMC extinction curve. The observed Delta(g-i) excess for QSOs with strong absorption systems appears to be a result of the reddening due to dust in the intervening absorbers.

astro-ph↗
arXiv2001

The BOOMERANG North America Instrument: a balloon-borne bolometric radiometer optimized for measurements of cosmic background radiation anisotropies from 0.3 to 4 degrees

We describe the BOOMERANG North America (BNA) instrument, a balloon-borne bolometric radiometer designed to map the Cosmic Microwave Background (CMB) radiation with 0.3 deg resolution over a significant portion of the sky. This receiver employs new technologies in bolometers, readout electronics, millimeter-wave optics and filters, cryogenics, scan and attitude reconstruction. All these subsystems are described in detail in this paper. The system has been fully calibrated in flight using a variety of techniques which are described and compared. It has been able to obtain a measurement of the first peak in the CMB angular power spectrum in a single balloon flight, few hours long, and was a prototype of the BOOMERANG Long Duration Balloon (BLDB) experiment.

astro-ph↗
arXiv2001

Cosmology from Maxima-1, Boomerang and COBE/DMR CMB Observations

Recent results from BOOMERANG-98 and MAXIMA-1, taken together with COBE-DMR, provide consistent and high signal-to-noise measurements of the CMB power spectrum at spherical harmonic multipole bands over $2<\ell\lta800$. Analysis of the combined data yields 68% (95%) confidence limits on the total density, $Ω_{\rm {tot}}\simeq 1.11 \pm 0.07 (^{+0.13}_{-0.12})$, the baryon density, $Ω_b h^2\simeq 0.032^{+0.005}_{-0.004} (^{+0.009}_{-0.008})$, and the scalar spectral tilt, $n_s\simeq1.01^{+0.09}_{-0.07} (^{+0.17}_{-0.14})$. These data are consistent with inflationary initial conditions for structure formation. Taken together with other cosmological observations, they imply the existence of both non-baryonic dark matter and dark energy in the universe.

astro-ph↗
arXiv2000

Detection of anisotropy in the Cosmic Microwave Background at horizon and sub-horizon scales with the BOOMERanG experiment

BOOMERanG has recently resolved structures on the last scattering surface at redshift $\sim$ 1100 with high signal to noise ratio. We review the technical advances which made this possible, and we focus on the current results for maps and power spectra, with special attention to the determination of the total mass-energy density in the Universe and of other cosmological parameters.

astro-ph↗