Download fn1_unit_4_topics_mram

Unit 4 Selected Topics
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Spintronic devices
• Hard disk drives
– GMR
– Spin valve
• MRAM
– Pseudo-spin valve
– Magnetic tunnel junction
• Spin transistors
http://physicsweb.org/articles/news/6/4/12/1
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Spintronics
• Spintronics is a branch of electronics based on the spin
of the electron.
• Electrons can be “polarized” in two different ways:
– Spin up
– Spin down
+½
-½
• The spin of the electron is closely related to magnetism.
• Spintronics often makes use of the fact that electrons
with opposite spins behave differently in magnetic
materials.
http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html
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Giant Magneto Resistance Effect (GMR)
• The GMR effect is observed when current is
passed through a film stack consisting of two
magnetic layers separated by a conductive
layer.
• A small resistance is observed when the
magnetic layers are aligned, with the north and
south poles of the magnetic layers pointing in
the same direction.
• A large resistance is observed when the
magnetic layers are anti-aligned, with the north
and south poles of each layer pointing in
opposite directions.
S
N
S
N
S
N
N
S
• The thinner the layers are, the stronger the
GMR effect is. Each layer is typically <5 nm in
thickness.
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Spin Valve
• A spin valve consists of a pinned magnetic layer whose
magnetic field only points in one direction and a free magnetic
layer which can flip directions based on an external magnetic
field.
• Spin valves are typically used to read hard disk drives.
• If the magnetic field direction of the disk is different from the
magnetic field direction of the free layer it will flip and change
the resistance of the spin valve.
• Current is flowed through the spin valve and a change in
current flow indicates the “1” or “0” state of the bit.
http://www.hitachigst.com/hdd/research/recording_head/headmaterials/
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Hard Disk Drives
• 90% of hard disk drives made today make
use spin valves in their read/write heads.
Read/Write Head
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Why Does the Resistance Change
in a GMR device?
•
The yellow line symbolizes the path of a +½ electron traveling in the
GMR stack. Notice that it scatters when it enters the material with a
magnetic field opposite to its own.
•
The green line symbolizes the path of a -½ electron. It scatters in the
other layer.
S
N
N
S
ITotal = I+½ + I-½
•
1
or
RTotal
1
1
=
+
R+ 12 R- 12
In this case both types of electrons are scattered so that R+½ = R-½
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Why Does the Resistance Change
in a GMR device?
• When the magnetic fields are aligned the spin +½ electron does not
scatter in either magnetic layer.
• The - ½ electron scatters in both magnetic layers.
• This results in R+½ < R-½ . This results in a low resistance shunting
path for the circuit.
S
N
N
S
1
RTotal
=
1
1
+
R+ 12 R- 12
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MRAM
• MRAM is a solid state memory device based on
spintronics.
• There are currently two different types of MRAM:
– Pseudo-Spin Valve (PSV)
– Magnetic Tunnel Junction (MTJ)
• PSV is an older technology but MTJ offers the potential
for greater density devices.
• Most current research is in MTJ technology.
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Pseudo Spin Valve
• The pseudo spin valve does not have a
pinned layer. Both layers can change
magnetic orientation, or flip.
• One layer is typically thinner than the other
layer. The thin layer is called the soft layer
and the thick layer is called the hard layer.
• A metal line is located either above or
below the magnetic layers which can be
used to apply a magnetic field to the
magnetic layers by flowing current through
the metal line.
S
N
S
N
Metal Line
• Flowing current through the magnetic
layers reduces the magnetic field strength
required to flip them.
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Write Operation
• A device is typically written by
flowing 30-40 mA of current
through the metal line and 4-6
mA of current through the
device itself.
• This produces a strong enough
magnetic field to change the
orientation of both magnetic
layers of the device.
S
N
S
N
Metal Line
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Read Operation
• A device is typically read by flowing
15-20 mA of current through the metal
line and 2 mA of current through the
device itself.
• This produces a magnetic field just
strong enough to flip the soft layer
without flipping the hard layer.
• The device current is monitored and a
change in resistance of the device
from high to low indicates the hard
layer is in a certain state for example
“0”. A change in resistance from low
to high indicates the other state “1”
S
N
S
N
Metal Line
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Signal Strength
• The theoretical maximum difference in
resistance between two states for a
pseudo-spin valve device is 50% but a
practical maximum is more likely 25%.
Typical differences in resistance are 510%.
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Magnetic Tunnel Junction (MTJ)
• With MTJ much larger differences in resistance can be achieved
compared to PSV.
• MTJ devices can be made much smaller than PSV devices.
• An MTJ device is capable of filtering electrons based on their spin.
• The quantum effect of electron tunneling is used to flow current
through the insulating layer.
• Electron tunneling is possible if the distance between the insulating
and the conducting layers is small.
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Magnetic Tunnel Junction Device
Insulating
layer
http://www.research.ibm.com/resources/news/20001207_mramimages.shtml
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Magnetic Tunnel Junction Device
• When the layers are aligned one type of electrons is allowed to pass
and the other type is filtered out.
• When the layers are anti-aligned both types of electron are filtered
out.
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Biochips
• http://www.nanohub.org/index.php?option=
content&task=view&id=176&Itemid=60
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