17
1 Unit 4 Selected Topics

Unit 4 Selected Topics

  • Upload
    taurus

  • View
    55

  • Download
    0

Embed Size (px)

DESCRIPTION

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. - PowerPoint PPT Presentation

Citation preview

Page 1: Unit 4 Selected Topics

1

Unit 4 Selected Topics

Page 2: Unit 4 Selected Topics

2

Spintronic devices

• Hard disk drivesHard disk drives– GMRGMR– Spin valveSpin valve

• MRAMMRAM– Pseudo-spin valvePseudo-spin valve– Magnetic tunnel junctionMagnetic tunnel junction

• Spin transistorsSpin transistors

http://physicsweb.org/articles/news/6/4/12/1

Page 3: Unit 4 Selected Topics

3

Spintronics

• Spintronics is a branch of electronics based on the spin Spintronics is a branch of electronics based on the spin of the electron.of the electron.

• Electrons can be “polarized” in two different ways:Electrons can be “polarized” in two different ways:– Spin up Spin up ++½½– Spin downSpin down - -½½

• The spin of the electron is closely related to magnetism.The spin of the electron is closely related to magnetism.

• Spintronics often makes use of the fact that electrons Spintronics often makes use of the fact that electrons with opposite spins behave differently in magnetic with opposite spins behave differently in magnetic materials.materials.

http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

Page 4: Unit 4 Selected Topics

4

Giant Magneto Resistance Effect (GMR)

• The GMR effect is observed when current is The GMR effect is observed when current is passed through a film stack consisting of two passed through a film stack consisting of two magnetic layers separated by a conductive magnetic layers separated by a conductive layer.layer.

• A small resistance is observed when the A small resistance is observed when the magnetic layers are aligned, with the north and magnetic layers are aligned, with the north and south poles of the magnetic layers pointing in south poles of the magnetic layers pointing in the same direction.the same direction.

• A large resistance is observed when the A large resistance is observed when the magnetic layers are anti-aligned, with the north magnetic layers are anti-aligned, with the north and south poles of each layer pointing in and south poles of each layer pointing in opposite directions.opposite directions.

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

NS

NS

NS

N S

Page 5: Unit 4 Selected Topics

5

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/

Page 6: Unit 4 Selected Topics

6

Hard Disk Drives

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

Read/Write Head

Page 7: Unit 4 Selected Topics

7

Why Does the Resistance Change in a GMR device?

• The yellow line symbolizes the path of a +The yellow line symbolizes the path of a +½ electron traveling in the ½ electron traveling in the GMR stack. Notice that it scatters when it enters the material with a GMR stack. Notice that it scatters when it enters the material with a magnetic field opposite to its own.magnetic field opposite to its own.

• The green line symbolizes the path of a -½ electron. It scatters in the The green line symbolizes the path of a -½ electron. It scatters in the other layer.other layer.

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

NS

N S

ITotal = I+½ + I-½ or1 12 2Total + -

1 1 1 = +

R R R

Page 8: Unit 4 Selected Topics

8

Why Does the Resistance Change in a GMR device?

• When the magnetic fields are aligned the spin +When the magnetic fields are aligned the spin +½ electron does not ½ electron does not scatter in either magnetic layer.scatter in either magnetic layer.

• The - ½ electron scatters in both magnetic layers.The - ½ electron scatters in both magnetic layers.

• This results in RThis results in R+½ < R-½ . This results in a low resistance shunting path for the circuit.

NS

N S

1 12 2Total + -

1 1 1 = +

R R R

Page 9: Unit 4 Selected Topics

9

MRAM

• MRAM is a solid state memory device based on MRAM is a solid state memory device based on spintronics.spintronics.

• There are currently two different types of MRAM:There are currently two different types of MRAM:– Pseudo-Spin Valve (PSV)Pseudo-Spin Valve (PSV)– Magnetic Tunnel Junction (MTJ)Magnetic Tunnel Junction (MTJ)

• PSV is an older technology but MTJ offers the potential PSV is an older technology but MTJ offers the potential for greater density devices.for greater density devices.

• Most current research is in MTJ technology.Most current research is in MTJ technology.

Page 10: Unit 4 Selected Topics

10

Pseudo Spin Valve• The pseudo spin valve does not have a The pseudo spin valve does not have a

pinned layer. Both layers can change pinned layer. Both layers can change magnetic orientation, or flip.magnetic orientation, or flip.

• One layer is typically thinner than the other One layer is typically thinner than the other layer. The thin layer is called the soft layer layer. The thin layer is called the soft layer and the thick layer is called the hard layer.and the thick layer is called the hard layer.

• A metal line is located either above or A metal line is located either above or below the magnetic layers which can be below the magnetic layers which can be used to apply a magnetic field to the used to apply a magnetic field to the magnetic layers by flowing current through magnetic layers by flowing current through the metal line.the metal line.

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

NS

NS

Metal Line

Page 11: Unit 4 Selected Topics

11

Write Operation

• A device is typically written by A device is typically written by flowing 30-40 mA of current flowing 30-40 mA of current through the metal line and 4-6 through the metal line and 4-6 mA of current through the mA of current through the device itself.device itself.

• This produces a strong enough This produces a strong enough magnetic field to change the magnetic field to change the orientation of both magnetic orientation of both magnetic layers of the device.layers of the device.

NS

NS

Metal Line

Page 12: Unit 4 Selected Topics

12

Read Operation• A device is typically read by flowing A device is typically read by flowing

15-20 mA of current through the metal 15-20 mA of current through the metal line and 2 mA of current through the line and 2 mA of current through the device itself.device itself.

• This produces a magnetic field just This produces a magnetic field just strong enough to flip the soft layer strong enough to flip the soft layer without flipping the hard layer.without flipping the hard layer.

• The device current is monitored and a The device current is monitored and a change in resistance of the device change in resistance of the device from high to low indicates the hard from high to low indicates the hard layer is in a certain state for example layer is in a certain state for example “0”. A change in resistance from low “0”. A change in resistance from low to high indicates the other state “1”to high indicates the other state “1”

NS

NS

Metal Line

Page 13: Unit 4 Selected Topics

13

Signal Strength

• The theoretical maximum difference in The theoretical maximum difference in resistance between two states for a resistance between two states for a pseudo-spin valve device is 50% but a pseudo-spin valve device is 50% but a practical maximum is more likely 25%. practical maximum is more likely 25%. Typical differences in resistance are 5-Typical differences in resistance are 5-10%. 10%.

Page 14: Unit 4 Selected Topics

14

Magnetic Tunnel Junction (MTJ)

• With MTJ much larger differences in resistance can be achieved With MTJ much larger differences in resistance can be achieved compared to PSV.compared to PSV.

• MTJ devices can be made much smaller than PSV devices.MTJ devices can be made much smaller than PSV devices.

• An MTJ device is capable of filtering electrons based on their spin.An MTJ device is capable of filtering electrons based on their spin.

• The quantum effect of electron tunneling is used to flow current The quantum effect of electron tunneling is used to flow current through the insulating layer.through the insulating layer.

• Electron tunneling is possible if the distance between the insulating Electron tunneling is possible if the distance between the insulating and the conducting layers is small.and the conducting layers is small.

Page 15: Unit 4 Selected Topics

15

Magnetic Tunnel Junction Device

Insulatinglayer

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

Page 16: Unit 4 Selected Topics

16

Magnetic Tunnel Junction Device

• When the layers are aligned one type of electrons is allowed to pass When the layers are aligned one type of electrons is allowed to pass and the other type is filtered out.and the other type is filtered out.

• When the layers are anti-aligned both types of electron are filtered When the layers are anti-aligned both types of electron are filtered out.out.

Page 17: Unit 4 Selected Topics

17

Biochips

• http://www.nanohub.org/index.php?optionhttp://www.nanohub.org/index.php?option=content&task=view&id=176&Itemid=60=content&task=view&id=176&Itemid=60