<p>Novel
computational paradigms based on non-von Neumann architectures are being
extensively explored for modern data-intensive applications and big-data
problems. One direction in this context is to harness the intrinsic physics of
spintronics devices for the implementation of nanoscale and low-power building
blocks of such emerging computational systems. For example, a Probabilistic
Spin Logic (PSL) that consists of networks of p-bits has been proposed for
neuromorphic computing, Bayesian networks, and for solving optimization
problems. In my work, I will discuss two types of device-components required
for PSL: (i) p-bits mimicking binary stochastic neurons (BSN) and (ii) compound
synapses for implementing weighted interconnects between p-bits. Furthermore, I
will also show how the integration of recently discovered van der Waals
ferromagnets in spintronics devices can reduce the current densities required
by orders of magnitude, paving the way for future low-power spintronics
devices.</p>
<p>First, a
spin-device with input-output isolation and stable magnets capable of
generating tunable random numbers, similar to a BSN, was demonstrated. In this
device, spin-orbit torque pulses are used to initialize a nano-magnet with
perpendicular magnetic anisotropy (PMA) along its hard axis. After removal of
each pulse, the nano-magnet can relax back to either of its two stable states,
generating a stream of binary random numbers. By applying a small Oersted field
using the input terminal of the device, the probability of obtaining 0 or 1 in
binary random numbers (P) can be tuned electrically. Furthermore, our work
shows that in the case when two stochastic devices are connected in series, “P”
of the second device is a function of “P” of the first p-bit and the weight of
the interconnection between them. Such control over correlated probabilities of
stochastic devices using interconnecting weights is the working principle of
PSL.</p>
<p>Next my
work focused on compact and energy efficient implementations of p-bits and
interconnecting weights using modified spin-devices. It was shown that unstable
in-plane magnetic tunneling junctions (MTJs), i.e. MTJs with a low energy
barrier, naturally fluctuate between two states (parallel and anti-parallel)
without any external excitation, in this way generating binary random numbers.
Furthermore, spin-orbit torque of tantalum is used to control the time spent by
the in-plane MTJ in either of its two states i.e. “P” of the device. In this
device, the READ and WRITE paths are separated since the MTJ state is read by
passing a current through the MTJ (READ path) while “P” is controlled by
passing a current through the tantalum bar (WRITE path). Hence, a BSN/p-bit is
implemented without energy-consuming hard axis initialization of the magnet and
Oersted fields. Next, probabilistic switching of stable magnets was utilized to
implement a novel compound synapse, which can be used for weighted
interconnects between p-bits. In this experiment, an ensemble of nano-magnets
was subjected to spin-orbit torque pulses such that each nano-magnet has a
finite probability of switching. Hence, when a series of pulses are applied,
the total magnetization of the ensemble gradually increases with the number of
pulses</p>
<p>applied similar to the
potentiation and depression curves of synapses. Furthermore, it was shown that
a modified pulse scheme can improve the linearity of the synaptic behavior,
which is desired for neuromorphic computing. By implementing both neuronal and
synaptic devices using simple nano-magnets, we have shown that PSL can be
realized using a modified Magnetic Random Access Memory (MRAM) technology. Note
that MRAM technology exists in many current foundries.</p>
<p>To further
reduce the current densities required for spin-torque devices, we have
fabricated heterostructures consisting of a 2-dimensional semiconducting
ferromagnet (Cr<sub>2</sub>Ge<sub>2</sub>Te<sub>6</sub>) and a metal with
spin-orbit coupling metal (tantalum). Because of properties such as clean
interfaces, perfect crystalline nanomagnet structure and sustained magnetic
moments down to the mono-layer limit and low current shunting, 2D ferromagnets
require orders of magnitude lower current densities for spin-orbit torque
switching than conventional metallic ferromagnets such as CoFeB.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13356956 |
| Date | 14 December 2020 |
| Creators | Vaibhav R Ostwal (9751070) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/SPINTRONIC_DEVICES_FROM_CONVENTIONAL_AND_EMERGING_2D_MATERIALS_FOR_PROBABILISTIC_COMPUTING/13356956 |
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