arXiv subjects

Kuntal Roy

Publications and source records attributed to Kuntal Roy.

At least 19 recordsLinked to original sources

arXiv2023

Spin-circuit representation of spin pumping into topological insulators and determination of giant spin Hall angle and inverse spin Hall voltages

Topological insulators and giant spin-orbit toque switching of nanomagnets are one of the frontier topics for the development of energy-efficient spintronic devices. Spin-circuit representations involving different materials and phenomena are quite well-established now for its prowess of interpreting experimental results and then designing complex and efficient functional devices. Here, we construct the spin-circuit representation of spin pumping into topological insulators considering both the bulk and surface states with parallel channels, which allows the interpretation of practical experimental results. We show that the high increase in effective spin mixing conductance and inverse spin Hall voltages cannot be explained by the low-conductive bulk states of topological insulators. We determine high spin Hall angle close to the maximum magnitude of one from experimental results and with an eye to design efficient spin devices, we further employ a spin-sink layer in the spin-circuit formalism to increase the effective spin mixing conductance at low thicknesses and double the inverse spin Hall voltage.

cond-mat.mes-hall↗

arXiv2020

Colossal stability of antiferromagnetically exchange coupled nanomagnets

Bistable nanomagnets store a binary bit of information. Exchange coupled nanomagnets can increase the thermal stability at low dimensions. Here we show that the antiferromagnetically (AFM) coupled nanomagnets can be highly stable at low dimensions than that of the ferromagnetically (FM) coupled nanomagnets. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics at room temperature, we analyze the stability of the exchange coupled nanomagnets in the presence of correlated, uncorrelated, and anti-correlated noise. The results show that the correlated noise can make the stability of the AFM coupled nanomagnets very high. Such finding will lead to very high-density non-volatile storage and logic devices in our future information processing systems.

cond-mat.mes-hall↗

arXiv2020

Spin-circuit representation of spin-torque ferromagnetic resonance

Spin-torque ferromagnetic resonance (ST-FMR) particularly using magnetic insulators and heavy metals possessing a giant spin Hall effect (SHE) has gotten a lot of attention for the development of spintronic devices. To devise complex functional devices, it is necessary to construct the equivalent spin-circuit representations of different phenomena. Such representation is useful to translate physical equations into circuit elements, benchmarking experiments, and then proposing creative and efficient designs. We utilize the superposition principle in circuit theory to separate the spin Hall magnetoresistance and spin pumping contributions in the ST-FMR experiments. We show that the proposed spin-circuit representation reproduces the standard results in literature. We further consider multilayers like a spin-valve structure with an SHE layer sandwiched by two magnetic layers and show how the corresponding spin-circuit representation can be constructed by simply writing a vector netlist and solved using circuit theory.

cond-mat.mes-hall↗

arXiv2020

Determining complex spin mixing conductance and spin diffusion length from spin pumping experiments in magnetic insulator/heavy metal bilayers

Magnetic insulators are promising materials for the development of energy-efficient spintronics. Unlike metallic counterparts, the magnetic insulators are characterized by imaginary part of the interfacial spin mixing conductance as well in a bilayer with heavy metals and it is responsible for the field-like toque in spin-orbit torque devices. Here, we study the underlying theoretical constructs and develop a general strategy to determine the complex spin mixing conductance from the experimental results of ferromagnetic resonance and spin pumping. The results show that the imaginary part of the spin mixing conductance can be one order more than the real part and it matches the critical trend of spin mixing conductance with thickness of the heavy metal. The interpretation of experimental results also indicates that at small thicknesses the interface contribution becomes significant and bulk diffusion model cannot explain the results. A thickness-dependent spin diffusion length is necessary too that is tantamount to Elliott-Yafet spin relaxation mechanism in the heavy metals. Also, we effectively explain the experimental results while inserting a copper layer with varying thicknesses in between the magnetic insulator and the heavy metal using spin-circuit formalism.

cond-mat.mes-hall↗

arXiv2017

Estimating spin diffusion length and spin Hall angle from spin pumping-induced inverse spin Hall voltages

There exists considerable confusion in estimating the spin diffusion length of materials with high spin-orbit coupling from spin pumping experiments. For designing functional devices, it is important to determine the spin diffusion length with sufficient accuracy from experimental results. An inaccurate estimation of spin diffusion length also affects the estimation of other parameters (e.g., spin mixing conductance, spin Hall angle) concomitantly. The spin diffusion length for platinum (Pt) has been reported in literature in a wide range of 0.5 - 14 nm, and particularly it is a constant value independent of Pt's thickness. Here, the key reasonings behind such wide range of reported values of spin diffusion length have been identified comprehensively. Particularly, it is shown here that a thickness-dependent conductivity and spin diffusion length is necessary to simultaneously match the experimental results of effective spin mixing conductance and inverse spin Hall voltage due to spin pumping. Such thickness-dependent spin diffusion length is tantamount to Elliott-Yafet spin relaxation mechanism, which bodes well for transitional metals. This conclusion is not altered even when there is significant interfacial spin memory loss. Furthermore, the variations in the estimated parameters are also studied, which is important for technological applications.

cond-mat.mes-hall↗

arXiv2017

Spin circuit representation of spin pumping

Circuit theory has been tremendously successful in translating physical equations into circuit elements in organized form for further analysis and proposing creative designs for applications. With the advent of new materials and phenomena in the field of spintronics and nanomagnetics, it is imperative to construct the spin circuit representations for different materials and phenomena. Spin pumping is a phenomenon by which a pure spin current can be injected into the adjacent layers. If the adjacent layer is a material with a high spin orbit coupling, a considerable amount of charge voltage can be generated via inverse spin Hall effect, allowing spin detection. Here we develop the spin circuit representation of spin pumping. We then combine it with the spin circuit representation for the materials having spin Hall effect to show that it reproduces the standard results in literature. We further show how complex multilayers can be analyzed by simply writing a netlist.

cond-mat.mes-hall↗

arXiv2017

Ultralow Energy Analog Straintronics Using Multiferroic Composites

Electric field-induced magnetization switching in multiferroics holds profound promise for ultra-low-energy computing in beyond Moore's law era. Bistable nanomagnets in the multiferroics are usually deemed to be suitable for storing a binary bit of information and switching between the two stable states allows us to process digital information. However, it requires to process continuous analog signals too for seamless integration of nanomagnetic devices in our future information processing systems. Here, we show that it is possible to harness the analog nature in the magnetostrictive nanomagnets, contrary to writing a digital bit of information. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics at room-temperature, we demonstrate such possibility and show that there exists a transistor-like high-gain region in the input-output characteristics of the magnetostrictive nanomagnets in strain-mediated multiferroic composites. This can be the basis of ultra-low-energy analog and mixed signal precessing in our future information processing systems and it eliminates the requirement of using charge-based transistors.

cond-mat.mes-hall↗

arXiv2015

Comment on some articles regarding magnetization switching and computing using strain-mediated multiferroic composites

This posting addresses the incorrect information (about my papers) and analysis provided in the following articles: arXiv:1405.4000/Nanotechnology 26, 245202 (2015), arXiv:1412.7765/Scientific Reports 4, 7553 (2014), Appl. Phys. Lett. 105, 176101 (2014), arXiv:1407.2140/Appl. Phys. Lett. 105, 072408 (2014), arXiv:1402.5356/Appl. Phys. Lett. 104, 232403 (2014), Appl. Phys. Lett. 103, 232401 (2013), arXiv:1005.5358/Appl. Phys. Lett. 97, 173105 (2010)

cond-mat.mes-hall↗

arXiv2015

Ultra-low-energy straintronics using multiferroic composites

The primary impediment to continued improvement of traditional charge-based electronic devices in accordance with Moore's law is the excessive energy dissipation that takes place in the devices during switching of bits. One very promising solution is to utilize strain-mediated multiferroic composites, i.e., a magnetostrictive nanomagnet strain-coupled to a piezoelectric layer, where the magnetization can be switched between its two stable states in sub-nanosecond delay while expending a minuscule amount of energy of ~1 attojoule at room-temperature. Apart from devising digital memory and logic, these multiferroic devices can be also utilized for analog signal processing, e.g., voltage amplifier. First, we briefly review the recent advances on multiferroic straintronic devices and then we show here that in a magnetostrictive nanomagnet, it is possible to achieve the so-called Landauer limit (or the ultimate limit) of energy dissipation of amount "kT ln(2)" compensating the entropy loss, thereby linking information and thermodynamics.

cond-mat.mes-hall↗

arXiv2015

Separating read and write units in multiferroic devices

Strain-mediated multiferroic composites, i.e., piezoelectric-magnetostrictive heterostructures, hold profound promise for energy-efficient computing in beyond Moore's law era. While reading a bit of information stored in the magnetostrictive nanomagnets using a magnetic tunnel junction (MTJ), a material selection issue crops up since magnetostrictive materials in general cannot be utilized as the free layer of the MTJ. This is an important issue since we need to achieve a high magnetoresistance for technological applications. We show here that magnetically coupling the magnetostrictive nanomagnet and the free layer e.g., utilizing the magnetic dipole coupling between them can circumvent this issue. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics in the presence of room-temperature thermal fluctuations, we show that such design can eventually lead to a superior energy-delay product.

cond-mat.mes-hall↗

arXiv2015

Ultra-low-energy computing paradigm using giant spin Hall devices

Spin Hall effect converts charge current to spin current, which can exert spin-torque to switch the magnetization of a nanomagnet. Recently, it is shown that the ratio of spin current to charge current using spin Hall effect can be made more than unity by using the areal geometry judiciously, unlike the case of conventional spin-transfer-torque switching of nanomagnets. This can enable energy-efficient means to write a bit of information in nanomagnets. Here, we study the energy dissipation in such spin Hall devices. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics in the presence of room temperature thermal fluctuations, we show a methodology to simultaneously reduce switching delay, its variance and energy dissipation, while lateral dimensions of the spin Hall devices are scaled down.

cond-mat.mes-hall↗

arXiv2015

Landauer limit of energy dissipation in a magnetostrictive particle

According to Landauer's principle, a minimum amount of energy proportional to temperature must be dissipated during the erasure of a classical bit of information compensating the entropy loss, thereby linking the information and thermodynamics. Here we show that the Landauer limit of energy dissipation is achievable in a shape-anisotropic single-domain magnetostrictive nanomagnet having two mutually anti-parallel degenerate magnetization states that store a bit of information. We model the magnetization dynamics using stochastic Landau-Lifshitz-Gilbert equation in the presence of thermal fluctuations and show that on average the Landauer bound is satisfied, i.e., it accords to the generalized Landauer's principle for small systems with stochastic fluctuations.

cond-mat.mes-hall↗

arXiv2015

Area-Delay-Energy Tradeoffs of Strain-Mediated Multiferroic Devices

Multiferroic devices hold profound promise for ultra-low energy computing in beyond Moore's law era. The magnetization of a magnetostrictive shape-anisotropic single-domain nanomagnet strain-coupled with a piezoelectric layer in a multiferroic composite structure can be switched between its two stable states (separated by an energy barrier) with a tiny amount of voltage via converse magnetoelectric effect. With appropriate choice of materials, the magnetization can be switched with a few tens of millivolts of voltages in sub-nanosecond switching delay while spending a miniscule amount of energy of ~1 attojoule at room-temperature. Here, we analyze the area-delay-energy trade-offs of these multiferroic devices by solving stochastic Landau-Lifshitz-Gilbert equation in the presence of room-temperature thermal fluctuations. We particularly put attention on scaling down the lateral area of the magnetostrictive nanomagnet that can increase the device density on a chip. We show that the vertical thickness of the nanomagnet can be increased while scaling down the lateral area and keeping the assumption of single-domain limit valid. This has important consequence since it helps to some extent preventing the deterioration of the induced stress-anisotropy energy in the magnetostrictive nanomagnet, which is proportional to the nanomagnet's volume. The results show that if we scale down the lateral area, the switching delay increases while energy dissipation decreases. Avenues available to decrease the switching delay while still reducing the energy dissipation are discussed.

cond-mat.mes-hall↗

arXiv2015

Dynamical systems study in single-phase multiferroic materials

Electric field induced magnetization switching in single-phase multiferroic materials is intriguing for both fundamental studies and potential technological applications. Here we develop a framework to study the switching dynamics of coupled polarization and magnetization in such multiferroic materials. With the coupling term between the polarization and magnetization as an invariant dictated by the Dzyaloshinsky-Moriya vector, the dynamical systems study reveals switching failures and oscillatory mode of magnetization if the polarization and magnetization relax slowly during switching.

cond-mat.mes-hall↗

arXiv2015

Ultra-low-energy straintronics using multiferroic composites

This paper reviews the recent developments on building nanoelectronics for our future information processing paradigm using multiferroic composites. With appropriate choice of materials, when a tiny voltage of few tens of millivolts is applied across a multiferroic composite, i.e. a piezoelectric layer stain-coupled with a magnetostrictive layer, the piezoelectric layer gets strained and the generated stress in the magnetostrictive layer switches the magnetization direction between its two stable states. We particularly review the switching dynamics of magnetization and calculation of associated metrics like switching delay and energy dissipation. Such voltage-induced magnetization switching mechanism dissipates a minuscule amount of energy of only ~1 attojoule in sub-nanosecond switching delay at room-temperature. The performance metrics for such non-volatile straintronic devices make them very attractive for building not only memory devices but also building logic, so that they can be deemed suitable for computational purposes. Hence, multiferroic straintronics has profound promise of contributing to Beyond Moore's law technology, i.e. of being possible replacement of conventional charge-based electronics, which is reaching its performance limit specifically due to excessive energy dissipation.

cond-mat.mes-hall↗

arXiv2015

Electric field-induced magnetization switching in interface-coupled multiferroic heterostructures: A highly-dense, non-volatile, and ultra-low-energy computing paradigm

Electric-field induced magnetization switching in multiferroic magnetoelectric devices is promising for beyond Moore's law computing. We show here that interface-coupled multiferroic heterostructures, i.e., a ferroelectric layer coupled with a ferromagnetic layer, are particularly suitable for highly-dense, non-volatile, and ultra-low-energy computing. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics in the presence of room-temperature thermal fluctuations, we demonstrate that error-resilient switching of magnetization is possible in sub-nanosecond delay while expending a minuscule amount of energy of $\sim$1 attojoule. Such devices can be operated by drawing energy from the environment without the need for an external battery.

cond-mat.mes-hall↗

arXiv2015

Critical analysis and remedy of switching failures in straintronic logic using Bennett clocking in the presence of thermal fluctuations

Straintronic logic is a promising platform for beyond Moore's law computing. Using Bennett clocking mechanism, information can propagate through an array of strain-mediated multiferroic nanomagnets exploiting the dipolar coupling between the magnets without having to physically interconnect them. Here we perform a critical analysis of switching failures, i.e., error in information propagation due to thermal fluctuations through a chain of such straintronic devices. We solved stochastic Landau-Lifshitz-Gilbert equation considering room-temperature thermal perturbations and show that magnetization switching may fail due to inherent magnetization dynamics accompanied by thermally broadened switching delay distribution. Avenues available to circumvent such issue are proposed.

cond-mat.mes-hall↗

arXiv2015

Ultra-low-energy non-volatile straintronic computing using single multiferroic composites

The primary impediment to continued downscaling of traditional charge-based electronic devices in accordance with Moore's law is the excessive energy dissipation that takes place in the device during switching of bits. One very promising solution is to utilize multiferroic heterostructures, comprised of a single-domain magnetostrictive nanomagnet strain-coupled to a piezoelectric layer, in which the magnetization can be switched between its two stable states while dissipating minuscule amount of energy. However, no efficient and viable means of computing is proposed so far. Here we show that such single multiferroic composites can act as universal logic gates for computing purposes, which we demonstrate by solving the stochastic Landau-Lifshitz-Gilbert (LLG) equation of magnetization dynamics in the presence of room-temperature thermal fluctuations. The proposed concept can overwhelmingly simplify the design of large-scale circuits and portend a highly dense yet an ultra-low-energy computing paradigm for our future information processing systems.

cond-mat.mes-hall↗