Alternative storage technologies

Simon Greaves1

1Research Institute of Electrical CommunicationTohoku University, Japan

4/2019

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Alternative storage technologies

Racetrack memory, ratchets, skyrmions

Ferroelectric and flash memory

Other storage technologies

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Racetrack memory,

ratchets, skyrmions

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Racetrack memory I

Magnetic nanowires are used in this memory device. Domains in

the wire are used to represent bits of information. The domains can

be moved by passing a current through the wire.

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Racetrack memory II

The domain wall velocity depends on the current. The length of the

current pulse determines the amount the domains move. The

image shows a 12 µm long wire. Here v ≈ 150 m/s.

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Racetrack memory III

Faster domain wall motion can be achieved in ferrimagnets, such

as GdCo. Here, TM = compensation temperature. TA = angular

momentum compensation temperature.

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Racetrack memory IV

Experimental measurements of domain wall velocity in GdCo show

a peak at the angular momentum compensation temperature. The

maximum velocity is ten times faster than in a ferromagnetic wire.

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Magnetic ratchets I

These are somewhat similar to racetrack memory. A stack of

anti-ferromagnetically coupled magnetic layers is formed. Solitons

are used to store information in the stack.

Solitons are created when parts of

the stack with different order

parameters meet.

The solitons can be propagated

along the stack by applying a

rotating magnetic field.

Placing a sensor somewhere along

the stack allows the solitons to be

detected as they pass by.

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Magnetic ratchets II

The sense of rotation of the external magnetic field determines

whether the solitons move up or down the stack. Arrays of stacks

may be formed to create a high-density memory.

Propagation of a soliton An array of thin film stacks

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Magnetic ratchets III

Schematic of ratchet

SEM image of ratchet array

Ratchet devices were fabricated. The Pt layer thickness was varied

to control the antiferromagnetic coupling strength between CoFeB

layers.

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Magnetic ratchets IV

Propagation of solitonApplied field sequence and

ratchet magnetisation

The soliton, indicated by a ‘∗’ propagates up the stack when fields

are applied in the sequence shown.

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Skyrmions I

A “hedgehog” skyrmion

Skyrmions are chiral objects that could be used to store data. They

can be moved along magnetic nanowires using an electric current.

Skyrmions form when ferromagnets are in contact with heavy

metals, giving rise to a Dzyaloshinskii-Moriya interaction.

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Skyrmions II

The strength of the DMI

interaction is given by

HDM = ~Dij · (~Si ×~Sj)

If the exchange interaction between spins ~Si and ~Sj is transferred

via a third ion then ~Dij ∝ ~rij × ~x .

The DMI interaction promotes the formation of skyrmions and can

occur at the surfaces and interfaces of thin films.

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Skyrmions III

Writing and erasing skyrmions in a PdFe bilayer on Ir(111) with a

spin-polarised scanning tunneling microscope (from Romming et

al.).

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Ferroelectric and flash

memory

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Ferroelectric memory I

This uses a single crystal of ferroelectric material, such as LiTaO3

(lithium tantalate). Recording of data is achieved using a probe and

applying voltage pulses

Recording information on a ferroelectric medium

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Ferroelectric memory II

Scanning non-linear dielectric microscopy (SNDM) can be used to

observe the polarisation distribution in the medium.

Data recorded on LiTaO3

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Ferroelectric memory III

SNDM readback is very slow (≈ 2 Mb/s). Recently, readback using

a near field transducer has been proposed with estimated

readback speeds of 400 Mb/s.

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Flash memory I

Flash memory cells are similar to MOSFETs, but with two gates: a

control gate and a floating gate. Oxide layers separate the gates,

preventing current from flowing through the device. The source and

drain are n-type and the substrate is p-type.

A single flash memory cell

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Flash memory II

Applying a positive voltage to the word line and bit lines allows a

current to flow from the source to the drain. Some electrons tunnel

through the lower oxide layer and are trapped in the floating gate.

The stored charge represents a “1”. Applying a negative voltage to

the word line empties the floating gate: this state represents a “0”.

Writing a “1” state to a flash memory cell

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Flash memory III

Flash memory cells are coupled together so that they can be

erased in blocks. Subsequently, individual cells can be written. This

makes the memory very fast.

However, after repeated writes the oxide layers can degrade and

become “leaky”, leading to eventual cell failure. Longevity is

typically from 10000 to 1000000 writes, depending on the type of

cell.

An array of flash memory cells

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Other storage

technologies

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Millipede I

The millipede is (was) a nanomechanical AFM-based data storage

system. An array of cantilevers use heat to record data on a thin

polymer film. The cantilevers can be positioned with

nanometer-scale accuracy over the surface of the medium.

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Millipede II

To write data a tip is heated, melting the medium and forming a pit.

To read data the resistance of the tip is measured. This depends on

the tip temperature, which depends on the area of the tip in contact

with the medium: more when the tip is in a pit, less when it isn’t.

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Phase change memory I

Phase change memory uses similar materials to re-writeable

optical discs, i.e. chalcogenides such as GeBsTe.

In phase change random access memory (PRAM) the electrical

resistance is used instead of the optical reflectance.

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Phase change memory II

The electrical resistance ratio varies by a ratio of 1:100 to 1:1000.

Transitions between phases controlled by heating and cooling.

Write time ≈ 10 -150µs, read time 100 - 300 ns.

Durability of the order 106 writes.

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Phase change memory III

Intel has been selling 3D XPoint memory for the past few years.

Intel claims this is not a phase change memory, but information

storage is based on the resistance of the memory cells.

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DNA data storage I

Information can also be stored in DNA base molecules.

One DNA sequence contains 150 - 154 base molecules.

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DNA data storage II

In experiments a 40 bit message “HELLO” was encoded in DNA

and then decoded

Encoding and decoding took 21 hours: about 20 hours to

synthesise the DNA molecules and 1 hour to read the sequence.

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Conclusions

At the moment hard disc drives store the vast majority of data.

However, there are many alternative data storage technologies

which may, or may not, be used in the future.

Novel ideas are constantly being proposed: technologies which can

take advantage of all three dimensions to maximise the amount of

data stored in a unit volume should ultimately be successful.

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Sources

S. S. P. Parkin et al, “Magnetic domain-wall racetrack memory”,

Science 320, p190, (2008).

L. Caretta et al., “Fast current-driven domain walls and small

skyrmions in a compensated ferrimagnet”, Nature Nanotech 13,

p1154, (2018).

J. H. Lee et al., “Soliton propagation in micron-sized magnetic

ratchet elements”, Appl. Phys. Lett. 104, p232404, (2014).

R. Lavrijsen et al., “Magnetic ratchet for three-dimensional

spintronic memory and logic”, Nature 493, p647, (2013).

N. Romming et al., “Writing and deleting single magnetic

skyrmions”, Science 341, p636, (2013).

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Sources

T. Matsumoto et al., “System and method for reading data recorded

on ferroelectric storage medium by means of a near-field

transducer”, US Patent 10,283,146.

C. N. Takahashi et al., “Demonstration of end-to-end automation of

DNA data storage”, Sci. Rep. 9, 4998, (2019).

L. Organick et al., “Random access in large scale DNA data

storage”, Nature Biotech. 36, p242, (2018).

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