Unit 4 selected topics
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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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Unit 4 Selected Topics

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Unit 4 selected topics

Unit 4 Selected Topics


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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Unit 4 selected topics

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


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