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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