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Unit 4 Selected Topics. 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. Spintronics. Spintronics is a branch of electronics based on the spin of the electron.

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spintronic devices
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

spintronics
Spintronics
  • Spintronics is a branch of electronics based on the spin of the electron.
  • Electrons can be “polarized” in two different ways:
    • Spin up +½
    • Spin down -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

giant magneto resistance effect gmr

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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.
  • The thinner the layers are, the stronger the GMR effect is. Each layer is typically <5 nm in thickness.
spin valve
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/

hard disk drives
Hard Disk Drives
  • 90% of hard disk drives made today make use spin valves in their read/write heads.

Read/Write Head

why does the resistance change in a gmr device

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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.
  • In this case both types of electrons are scattered so that R+½ = R-½

ITotal = I+½ + I-½

or

why does the resistance change in a gmr device1
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.

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slide9
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.
pseudo spin valve

Metal Line

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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.
  • Flowing current through the magnetic layers reduces the magnetic field strength required to flip them.
write operation

Metal Line

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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.
read operation

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Metal Line

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”
signal strength
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 5-10%.
magnetic tunnel junction mtj
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.
magnetic tunnel junction device
Magnetic Tunnel Junction Device

Insulating

layer

http://www.research.ibm.com/resources/news/20001207_mramimages.shtml

magnetic tunnel junction device1
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.
biochips
Biochips
  • http://www.nanohub.org/index.php?option=content&task=view&id=176&Itemid=60