Magnetic Memory:Data Storage and Nanomagnets

Magnetic Memory:Data Storage and Nanomagnets

Mark Tuominen UMass

Kathy AidalaMount Holyoke College

Data

Data is information

iTunes

How do we store data digitally?Everything is reduced to binary, a “1” or a “0”.

We look for ways to represent 1 or 0, which means we need to find physical systems with two distinct states.

We have to be able to switch the state of the system if we want to “write” data.

The bit has to stay that way for long enough.

We have to be able to “read” if the bit is a zero or one to use the data.

What physical systems have these properties??

10 GB2001

20 GB2002

40 GB2004

80 GB2006

160 GB2007

Data Storage. Example: Advancement of the iPod

Hard driveMagnetic data storage

Uses nanotechnology!

Review

MAGNETISM

Electrical current produces a magnetic field: "electromagnetism"

www.ndt-ed.org/EducationResources

B

I

MAGNETISM

www.eia.doe.gov

www.how-things-work-science-projects.com

myfridge

Refrigerator magnets provide an external magnetic field, permanently; no wires, no power supply and no current needed.

Permanent Magnets = FERROMAGNETS

Ferromagnetuniform magnetization

anisotropy axis("easy" axis)

Electron magnetic moments ("spins")

Aligned by "exchange interaction"

Bistable ! Equivalent energy for "up" or "down” statesIron, nickel, cobalt

and many alloys are ferromagnets

The Bistable Magnetization of a Nanomagnet

• A single-domain nanomagnet with a single “easy axis” (uniaxial anisotropy) has two stable magnetization states

“topview”shorthand

z

or H

Mz Mz

Mz

H

Bistable! Ideal for storing data - in principle, even one nanomagnet per bit.

hysteresis curve

E = K1sin2•H switching field

“Writing” data to a ferromagnet

?

Ferromagnet with unknown magnetic state

Current

N

S

‘0’

S

Current

N

‘1’

Magnetic Data StorageA computer hard drive stores your data magnetically

Disk

N S

direction of disk motion

“ Write”Head

0 0 1 0 1 0 0 1 1 0 _ _

“ Bits” ofinformation

NS

“ Read”Head

Signalcurrent

Scaling Down to the Nanoscale

Increases the amount of data stored on a fixed amount of “real estate” !

Now ~ 100 billion bits/in2, future target more than 1 trillion bits/in2

25 DVDs on a disk the size of a quarter.

Nanofabrication with self-assembled“cylindrical phase” diblock copolymer films

DepositionTemplate

Remove polymerblock within cylinders(expose and develop)

UMass/IBM: Science 290, 2126 (2000)

Filling the Template: Making Cobalt Nanorods by Electrochemical Deposition

WE REF

electrolyte

CE

Co2+

Co

metal

Binary Representation of Data

one bit “ 1” or “0” only 2 choices

two bits 00, 01, 10, 11 4 choices

three bits 000, 001, 010, 011,100, 101, 110, 111

8 choices

n bits has 2n choices

For example, 5 bits has 25 = 32 choices… more than enough to represent all the letters of the alphabet

or

Binary representationof lower case letters

5-bit "Super Scientist" code:

For example, k = 01011

0 1 0 1 1

S

N

S

N

S

N

N

S

N

S

OR

(Coding Activity: Use attractive and repulsive forces to "read" the magnetic data!)

Ferromagnetic Nanorings as Memory

"0" "1"

Vortex Magnetization

Nanotechnology(2008); PRB (2009)

Pt solid tip

AFM: Electromagnetic Forces

Lift height

• Anything that creates a force on the tip can be “imaged”

• Electromagnetic force is long range, but generally weaker than the repulsive forces at the surface

• Image electromagnetic forces 10 – 100nm above the surface

Magnetic Force Microscopy

magnetic tip

Computer Hard Drive

Magnetic Force Microscopy

magnetic tip

Computer Hard Drive

Topography

Magnetic Force Microscopy

Lift height

magnetic tip

Computer Hard Drive

Topography

Magnetic Force Microscopy

Lift height

magnetic tip

Computer Hard Drive

Topography

Magnetism

Magnetic Force Microscopy

dB/dz smalldB/dz large, negative

dB/dz large, positive

Image contrast is proportional to the derivative of the magnetic field

200 nm

Magnetic state

MFM simulation

MFM of Ring StatesSymmetric Rings

MFM of Ring StatesSymmetric Rings

vortex

onion

onion

• No contrast in the vortex state in a perfect ring. Cannot determine circulation (CW or CCW)

• Light and Dark spots indicate Tail to Tail and Head to Head domain walls.

Switching: Onion to Vortex1 um

1 2

3 4

T. Yang, APL, 98, 242505 (2011).

1 um

1 2

3 4

Stronger field(40 mA = 178 Oe)

Weaker field(30 mA = 133 Oe)

T. Yang, APL, 98, 242505 (2011).

Switching: Onion to Vortex

1 um

1 2

3 4

Stronger field(40 mA = 178 Oe)

Weaker field(30 mA = 133 Oe)

T. Yang, APL, 98, 242505 (2011).

Switching: Onion to Vortex

Improved MRAM Proposal

Zhu, Proceedings of the IEEE 96(11), 1786 (2008)

Trapped DWs lead to lower switching current

Proof of Principle

Cobalt, 12nm thick

Nanotechnology, 22 (2011) 485705

Ferromagnetic Nanorings as Memory

"0" "1"

Vortex Magnetization

Nanotechnology(2008); PRB (2009)

Aidala and Tuominen, APL (2011); Nanotech. 2011; J.A.P. 2012

Manipulation of magnetization with local circular field

Pt solid tip