113 research outputs found
A True Random Number Generator for Probabilistic Computing using Stochastic Magnetic Actuated Random Transducer Devices
Magnetic tunnel junctions (MTJs), which are the fundamental building blocks
of spintronic devices, have been used to build true random number generators
(TRNGs) with different trade-offs between throughput, power, and area
requirements. MTJs with high-barrier magnets (HBMs) have been used to generate
random bitstreams with 200~Mb/s throughput and pJ/bit energy
consumption. A high temperature sensitivity, however, adversely affects their
performance as a TRNG. Superparamagnetic MTJs employing low-barrier magnets
(LBMs) have also been used for TRNG operation. Although LBM-based MTJs can
operate at low energy, they suffer from slow dynamics, sensitivity to process
variations, and low fabrication yield. In this paper, we model a TRNG based on
medium-barrier magnets (MBMs) with perpendicular magnetic anisotropy. The
proposed MBM-based TRNG is driven with short voltage pulses to induce
ballistic, yet stochastic, magnetization switching. We show that the proposed
TRNG can operate at frequencies of about 500~MHz while consuming less than
100~fJ/bit of energy. In the short-pulse ballistic limit, the switching
probability of our device shows robustness to variations in temperature and
material parameters relative to LBMs and HBMs. Our results suggest that
MBM-based MTJs are suitable candidates for building fast and energy-efficient
TRNG hardware units for probabilistic computing.Comment: 10 pages, 10 figures, Accepted at ISQED 2023 for poster presentatio
Magnetic Tunnel Junction Random Number Generators Applied to Dynamically Tuned Probability Trees Driven by Spin Orbit Torque
Perpendicular magnetic tunnel junction (pMTJ)-based true-random number
generators (RNG) can consume orders of magnitude less energy per bit than CMOS
pseudo-RNG. Here, we numerically investigate with a macrospin
Landau-Lifshitz-Gilbert equation solver the use of pMTJs driven by spin-orbit
torque to directly sample numbers from arbitrary probability distributions with
the help of a tunable probability tree. The tree operates by dynamically
biasing sequences of pMTJ relaxation events, called 'coinflips', via an
additional applied spin-transfer-torque current. Specifically, using a single,
ideal pMTJ device we successfully draw integer samples on the interval 0,255
from an exponential distribution based on p-value distribution analysis. In
order to investigate device-to-device variations, the thermal stability of the
pMTJs are varied based on manufactured device data. It is found that while
repeatedly using a varied device inhibits ability to recover the probability
distribution, the device variations average out when considering the entire set
of devices as a 'bucket' to agnostically draw random numbers from. Further, it
is noted that the device variations most significantly impact the highest level
of the probability tree, iwth diminishing errors at lower levels. The devices
are then used to draw both uniformly and exponentially distributed numbers for
the Monte Carlo computation of a problem from particle transport, showing
excellent data fit with the analytical solution. Finally, the devices are
benchmarked against CMOS and memristor RNG, showing faster bit generation and
significantly lower energy use.Comment: 10 pages, 8 figures, 2 table
A Study on Performance and Power Efficiency of Dense Non-Volatile Caches in Multi-Core Systems
In this paper, we present a novel cache design based on Multi-Level Cell
Spin-Transfer Torque RAM (MLC STTRAM) that can dynamically adapt the set
capacity and associativity to use efficiently the full potential of MLC STTRAM.
We exploit the asymmetric nature of the MLC storage scheme to build cache lines
featuring heterogeneous performances, that is, half of the cache lines are
read-friendly, while the other is write-friendly. Furthermore, we propose to
opportunistically deactivate ways in underutilized sets to convert MLC to
Single-Level Cell (SLC) mode, which features overall better performance and
lifetime. Our ultimate goal is to build a cache architecture that combines the
capacity advantages of MLC and performance/energy advantages of SLC. Our
experiments show an improvement of 43% in total numbers of conflict misses, 27%
in memory access latency, 12% in system performance, and 26% in LLC access
energy, with a slight degradation in cache lifetime (about 7%) compared to an
SLC cache
Probabilistic computing with voltage-controlled dynamics in magnetic tunnel junctions
Probabilistic (p-) computing is a physics-based approach to addressing computational problems
which are difficult to solve by conventional von Neumann computers. A key requirement for
p-computing is the realization of fast, compact, and energy-efficient probabilistic bits. Stochastic
magnetic tunnel junctions (MTJs) with low energy barriers, where the relative dwell time in each
state is controlled by current, have been proposed as a candidate to implement p-bits. This
approach presents challenges due to the need for precise control of a small energy barrier across
large numbers of MTJs, and due to the need for an analog control signal. Here we demonstrate
an alternative p-bit design based on perpendicular MTJs that uses the voltage-controlled
magnetic anisotropy (VCMA) effect to create the random state of a p-bit on demand. The MTJs
are stable (i.e. have large energy barriers) in the absence of voltage, and VCMA-induced
dynamics are used to generate random numbers in less than 10 ns/bit. We then show a compact
method of implementing p-bits by using VC-MTJs without a bias current. As a demonstration of
the feasibility of the proposed p-bits and high quality of the generated random numbers, we solve
up to 40 bit integer factorization problems using experimental bit-streams generated by VCMTJs. Our proposal can impact the development of p-computers, both by supporting a fully
spintronic implementation of a p-bit, and alternatively, by enabling true random number
generation at low cost for ultralow-power and compact p-computers implemented in
complementary metal-oxide semiconductor chips
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High Frequency Dynamics of Magnetic Tunnel Junctions
Magnetism has been utilized since ancient civilizations to build tools for day to day life for example, magnetic compass. Understanding of nature behind magnets has ever since evolved passing significant milestones such as, discovery of interplay between magnetism and electricity leading to invention of electric motor and dynamo and, microscopic insight developed following discovery of quantum mechanics. About twenty years ago we passed arguably the most recent milestone with the prediction of the ability to manipulate a nano sized magnet with the aid of the spin of an electronic current. With the advent of magnetic tunnel junctions (MTJ), one of the main applications of this has been spin transfer torque nano oscillators (STNO) that have attracted great attention. Recently, a new emerging field regarding magnetic skyrmions in MTJs has shown even more promising aspects such as higher energy efficiency for real world applications. In the first half of this thesis, we present a tri- layer MTJ based STNO offering 6 GHz microwave emission for both current polarities at zero external magnetic field which is the highest frequency achieved in absence of any bias field to-date, to the best of our knowledge. We have investigated into spin dynamics of this STNO discussing about out of plane (OOP) precessions along with micromagnetic simulations. In the second half of this thesis, we present observation of skyrmionic signature in an MTJ at cryogenic temperature producing random telegraph signal (RTS). We have studied the dependencies of this RTS upon various physical parameters. RTS seen in MTJs involving ferromagnetic states has attracted attention in the research community as it is an ideal candidate to realize neuromorphic computers which are inspired on human brain offering tremendous improved performance in specific tasks over conventional boolean computing. We believe this investigation into RTS involving skyrmionic states in an MTJ will further electrify this on going expedition
Electrical coupling of superparamagnetic tunnel junctions mediated by spin-transfer-torques
In this work, the effect of electrical coupling on stochastic switching of
two in-plane superparamagnetic tunnel junctions (SMTJs) is studied, using
experimental measurements as well as simulations. The coupling mechanism relies
on the spin-transfer-torque (STT) effect, which enables the manipulation of the
state probability of an SMTJ. Through the investigation of time-lagged
cross-correlation, the strength and direction of the coupling are determined.
In particular, the characteristic state probability transfer curve of each SMTJ
leads to the emergence of a similarity or dissimilarity effect. The
cross-correlation as a function of applied source voltage reveals that the
strongest coupling occurs for high positive voltages for our SMTJs. In
addition, we show state tuneability as well as coupling control by the applied
voltage. The experimental findings of the cross-correlation are in agreement
with our simulation results
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