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Supriyo Bandyopadhyay

Publications and source records attributed to Supriyo Bandyopadhyay.

At least 73 records · Page 4Linked to original sources
arXiv2015

Reversible strain-induced magnetization switching in FeGa nanomagnets: Pathway to a rewritable, non-volatile, non-toggle, straintronic memory cell for extremely low energy operation

Reversible straintronic switching of a nanomagnet's magnetization between two stable or metastable states promises ultra-energy-efficient non-volatile memory. Here, we report strain-induced magnetization switching in ~300 nm sized FeGa nanomagnets delineated on a piezoelectric PMN-PT substrate. Voltage of one polarity applied across the substrate generates compressive strain in a nanomagnet and switches its magnetization to one state, while voltage of the opposite polarity generates tensile strain and switches the magnetization back to the original state, resulting in "non-toggle" switching. The two states can encode the two binary bits, and, using the right voltage polarity, one can write either bit deterministically.

cond-mat.mes-hall↗
arXiv2015

Electric field control of magnetic states in isolated and dipole-coupled FeGa nanomagnets delineated on a PMN-PT substrate

We report observation of a "non-volatile" converse magneto-electric effect in elliptical FeGa nanomagnets delineated on a piezoelectric PMN-PT substrate. The nanomagnets are initially magnetized with a magnetic field directed along their nominal major axis and subsequent application of an electric field across the substrate generates strain in the substrate, which is partially transferred to the nanomagnets and rotates the magnetizations of some of them to metastable orientations. There they remain after the field is removed, resulting in "non-volatility". In isolated nanomagnets, the angular separation between the initial and final magnetization directions is < 90 deg, but in a dipole-coupled pair consisting of one "hard" and one "soft" nanomagnet, the soft nanomagnet's magnetization is rotated by > 90 deg from the initial orientation because of the dipole influence of the hard nanomagnet. These effects can be utilized for ultra-energy-efficient logic and memory devices.

cond-mat.mes-hall↗
arXiv2015

Experimental Clocking of Nanomagnets with Strain for Ultra Low Power Boolean Logic

Nanomagnetic implementations of Boolean logic [1,2] have garnered attention because of their non-volatility and the potential for unprecedented energy-efficiency. Unfortunately, the large dissipative losses that take place when nanomagnets are switched with a magnetic field [3], or spin-transfer-torque [4] inhibit the promised energy-efficiency. Recently, there have been experimental reports of utilizing the Spin Hall effect for switching magnets [5-7], and theoretical proposals for strain induced switching of single-domain magnetostrictive nanomagnets [8-12], that might reduce the dissipative losses significantly. Here, we demonstrate, for the first time, that strain-induced switching of single-domain magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate can implement a nanomagnetic Boolean NOT gate and unidirectional bit information propagation in dipole-coupled nanomagnets chains. This portends ultra-low-energy logic processors and mobile electronics that may be able to operate solely by harvesting energy from the environment without ever requiring a battery.

cond-mat.mes-hall↗
arXiv2015

Magneto-elastic universal logic gate: A non-volatile, error-resilient Boolean logic gate with ultra-low energy-delay product

A long-standing goal of computer technology is to process and store digital information with the same device in order to implement new architectures. One way to accomplish this is to use nanomagnetic `non-volatile' logic gates that can perform Boolean operations and then store the output data in the magnetization states of nanomagnets, thereby doubling as both logic and memory. Unfortunately, many proposed nanomagnetic gates do not possess the seven essential characteristics of a Boolean logic gate: concatenability, non-linearity, isolation between input and output, gain, universal logic implementation, scalability and error resilience. More importantly, their energy-delay products and error-rates vastly exceed that of conventional transistor-based logic gates, which is a drawback. Here, we propose a non-volatile voltage-controlled nanomagnetic logic gate that possesses all the necessary characteristics of a logic gate and whose energy-delay product is ~2 orders of magnitude less than that of other nano-magnetic (non-volatile) logic gates and ~1 order of magnitude less than that of (volatile) CMOS-based logic gates. The error-resilience is also superior to that of other known nanomagnetic gates.

cond-mat.mes-hall↗
arXiv2015

Self-similar Magneto-electric Nanocircuit Technology for Probabilistic Inference Engines

Probabilistic graphical models are powerful mathematical formalisms for machine learning and reasoning under uncertainty that are widely used for cognitive computing. However they cannot be employed efficiently for large problems (with variables in the order of 100K or larger) on conventional systems, due to inefficiencies resulting from layers of abstraction and separation of logic and memory in CMOS implementations. In this paper, we present a magneto-electric probabilistic technology framework for implementing probabilistic reasoning functions. The technology leverages Straintronic Magneto-Tunneling Junction (S-MTJ) devices in a novel mixed-signal circuit framework for direct computations on probabilities while enabling in-memory computations with persistence. Initial evaluations of the Bayesian likelihood estimation operation occurring during Bayesian Network inference indicate up to 127x lower area, 214x lower active power, and 70x lower latency compared to an equivalent 45nm CMOS Boolean implementation.

cs.ET↗
arXiv2015

Energy-efficient hybrid spintronic-straintronic reconfigurable bit comparator

We propose a reconfigurable bit comparator implemented with a nanowire spin valve whose two contacts are magnetostrictive with bistable magnetization. Reference and input bits are "written" into the magnetization states of the two contacts with electrically generated strain and the spin-valve's resistance is lowered if they match. Multiple comparators can be interfaced in parallel with a magneto-tunneling junction to determine if an N-bit input stream matches an N-bit reference stream bit by bit. The system is robust against thermal noise at room temperature and a 16-bit comparator can operate at roughly 416 MHz while dissipating at most 420 aJ per cycle.

cond-mat.mes-hall↗
arXiv2015

Straintronic spin-neuron

In artificial neural networks, neurons are usually implemented with highly dissipative CMOS-based operational amplifiers. A more energy-efficient implementation is a 'spin-neuron' realized with a magneto-tunneling junction (MTJ) that is switched with a spin-polarized current (representing weighted sum of input currents) that either delivers a spin transfer torque or induces domain wall motion in the soft layer of the MTJ. Here, we propose and analyze a different type of spin-neuron in which the soft layer of the MTJ is switched with mechanical strain generated by a voltage (representing weighted sum of input voltages) and term it straintronic spin-neuron. It dissipates orders of magnitude less energy in threshold operations than the traditional current-driven spin neuron at 0 K temperature and may even be faster. We have also studied the room-temperature firing behaviors of both types of spin neurons and find that thermal noise degrades the performance of both types, but the current-driven type is degraded much more than the straintronic type if both are optimized for maximum energy-efficiency. On the other hand, if both are designed to have the same level of thermal degradation, then the current-driven version will dissipate orders of magnitude more energy than the straintronic version. Thus, the straintronic spin neuron is superior to current-driven spin neurons.

cond-mat.mes-hall↗
arXiv2014

Highly reliable low-energy writing of bits in non-volatile multiferroic memory based on 180-degree magnetization switching with voltage-generated stress

Rotating the magnetization of a magnetostrictive nanomagnet with electrically generated mechanical strain dissipates miniscule amount of energy compared to any other rotation method and would have been the ideal method to write bits in nonvolatile magnetic memory, except strain cannot ordinarily rotate the magnetization of magnet by more than 90 degrees and "flip" it. Here, we describe a scheme to achieve complete 180 degree rotation of the magnetization of a nanomagnet with strain that will enable writing of binary bits in non-volatile magnetic memory implemented with magneto-tunneling junctions whose soft layers are two-phase magnetostrictive/piezoelectric multiferroics. At room temperature, this writing method results in: (1) energy dissipation < 6200 kT per bit, (2) write error probability < 10^-6, (3) write time of ~ 1 ns, and (4) low read error. This could potentially lead to a new genre of non-volatile memory that is extremely reliable, fast and, at the same time, ultra-energy-efficient.

cond-mat.mtrl-sci↗
arXiv2014

Reducing error rates in straintronic multiferroic dipole-coupled nanomagnetic logic by pulse shaping

Dipole-coupled nanomagnetic logic (NML), where nanomagnets with bistable magnetization states act as binary switches and information is transferred between them via dipole coupling and Bennett clocking, is a potential replacement for conventional transistor logic since magnets dissipate less energy than transistors when they switch in response to the clock. However, dipole-coupled NML is much more error-prone than transistor logic because thermal noise can easily disrupt magnetization dynamics. Here, we study a particularly energy-efficient version of dipole-coupled NML known as straintronic multiferroic logic (SML) where magnets are clocked/switched with electrically generated mechanical strain. By appropriately shaping the voltage pulse that generates strain, the error rate in SML can be reduced to tolerable limits. In this paper, we describe the error probabilities associated with various stress pulse shapes and discuss the trade-off between error rate and switching speed in SML.

cond-mat.mes-hall↗
arXiv2014

Energy-efficient magnetoelastic non-volatile memory

We propose an improved scheme for low-power writing of binary bits in non-volatile (multiferroic) magnetic memory with electrically generated mechanical stress. Compared to an earlier idea [Tiercelin, et al., J. Appl. Phys., 109, 07D726 (2011)], our scheme improves distinguishability between the stored bits when the latter are read with magneto-tunneling junctions. More importantly, the write energy dissipation and write error rate are reduced significantly if the writing speed is kept the same. Such a scheme could be one of the most energy-efficient approaches to writing bits in magnetic non-volatile memory.

cond-mat.mes-hall↗
arXiv2014

Coherent spin transport and suppression of spin relaxation in InSb nanowires with single subband occupancy at room temperature

A longstanding goal of spintronics is to inject, coherently transport, and detect spins in a semiconductor nanowire where a SINGLE quantized subband is occupied at room temperature. Here, we report achieving this goal in 50-nm diameter InSb nanowires by demonstrating both the spin-valve and the Hanle effect. The spin relaxation time in the nanowires was found to have increased by an order of magnitude over what has been reported in bulk and quantum wells due to the suppression of D'yakonov-Perel' spin relaxation as a result of single subband occupancy. These experiments raise hopes for the realization of a room-temperature Datta-Das spin transistor.

cond-mat.mes-hall↗
arXiv2012

Energy dissipation and error probability in fault-tolerant binary switching

The potential energy profile of a binary switch is a symmetric double well. Switching between the wells without energy dissipation requires time-modulating the potential barrier separating them and tilting the profile towards the desired well at the precise juncture when the barrier disappears. This demands perfect timing synchronization and is therefore fault-intolerant, even in the absence of noise. A fault-tolerant strategy that requires no time modulation of the barrier (and hence no timing synchronization) switches by tilting the profile by an amount at least equal to the barrier height and dissipates at least that amount of energy. Here, we present a third strategy that requires a time modulated barrier but no timing synchronization. It is therefore fault-tolerant in the absence of thermal noise and yet it dissipates arbitrarily small energy since an arbitrarily small tilt is required for slow and adiabatic switching. This case is exemplified with stress induced switching of a shape-anisotropic single-domain nanomagnet dipole coupled to a neighbor. We also show by examining various energy profiles and the corresponding probability distributions that when thermal noise is present, the minimum energy dissipated to switch in this scheme is 2kTln(1/p) [p = switching error probability].

cond-mat.mes-hall↗
arXiv2012

Power dissipation in spintronic devices: A general perspective

Champions of spintronics often claim that spin based signal processing devices will vastly increase speed and/or reduce power dissipation compared to traditional charge based electronic devices. Yet, not a single spintronic device exists today that can lend credence to this claim. Here, I show that no spintronic device that clones conventional electronic devices, such as field effect transistors and bipolar junction transistors, is likely to reduce power dissipation significantly. For that to happen, spin-based devices must forsake the transistor paradigm of switching states by physical movement of charges, and instead, switch states by flipping spins of stationary charges. An embodiment of this approach is the single spin logic idea proposed more than 10 years ago. Here, I revisit that idea and present estimates of the switching speed and power dissipation. I show that the Single Spin Switch is far superior to the Spin Field Effect Transistor (or any of its clones) in terms of power dissipation. I also introduce the notion of matrix element engineering which will allow one to switch devices without raising and lowering energy barriers between logic states, thereby circumventing the kTln2 limit on energy dissipation. Finally, I briefly discuss single spin implementations of classical reversible (adiabatic) logic.

cond-mat.mes-hall↗
arXiv2011

Binary switching in a symmetric potential landscape

The general methodology of binary switching requires tilting of potential landscape along the desired direction of switching. The tilt generates a torque along the direction of switching and the degree of tilt should be sufficient enough to beat thermal agitations with a tolerable error probability. However, we show here that such tilt is not necessary. Considering the full three-dimensional motion, we point out that the built-in dynamics can facilitate switching without requiring any asymmetry in potential landscape even in the presence of thermal noise. With experimentally feasible parameters, we theoretically demonstrate such intriguing possibility in electric field-induced magnetization switching of a magnetostrictive nanomagnet.

cond-mat.mes-hall↗
arXiv2011

Energy dissipation and switching delay in spin-transfer torque switching of nanomagnets with low-saturation magnetization in the presence of thermal fluctuations

A common ploy to reduce the switching current and energy dissipation in spin-transfer-torque driven magnetization switching of shape-anisotropic single-domain nanomagnets is to employ magnets with low saturation magnetization $M_s$ and high shape-anisotropy. The high shape-anisotropy compensates for low $M_s$ to keep the static switching error rate constant. However, this ploy increases the switching delay, its variance in the presence of thermal noise, and the dynamic switching error rate. Using the stochastic Landau-Lifshitz-Gilbert equation with a random torque emulating thermal noise, we show that pumping some excess spin-polarized current into the nanomagnet during switching will keep the mean switching delay and its variance constant as we reduce $M_s$, while still reducing the energy dissipation significantly.

cond-mat.mes-hall↗
arXiv2011

Metastable state in a shape-anisotropic single-domain nanomagnet subjected to spin-transfer-torque

We predict the existence of a new metastable magnetization state in a single-domain nanomagnet with uniaxial shape anisotropy. It emerges when a spin-polarized current, delivering a spin-transfer-torque, is injected into the nanomagnet. It can trap the magnetization vector and prevent spin-transfer-torque from switching the magnetization from one stable state along the easy axis to the other. Above a certain threshold current, the metastable state no longer appears. This has important technological consequences for spin-transfer-torque based magnetic memory and logic systems.

cond-mat.mes-hall↗