1 Unit 4 Selected Topics

2 Spintronic devices Hard disk drivesHard disk drives –GMR –Spin valve MRAMMRAM –Pseudo-spin valve –Magnetic tunnel junction Spin transistorsSpin transistors

3 Spintronics Spintronics is a branch of electronics based on the spin of the electron.Spintronics is a branch of electronics based on the spin of the electron. Electrons can be “polarized” in two different ways:Electrons can be “polarized” in two different ways: –Spin up +½ –Spin down -½ 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 with opposite spins behave differently in magnetic materials.Spintronics often makes use of the fact that electrons with opposite spins behave differently in magnetic materials.

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

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.

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

7 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 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.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 RIn this case both types of electrons are scattered so that R +½ = R -½ NS NS I Total = I +½ + I -½ or

8 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.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.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 NS

9 MRAM MRAM is a solid state memory device based on spintronics.MRAM is a solid state memory device based on spintronics. There are currently two different types of MRAM: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.PSV is an older technology but MTJ offers the potential for greater density devices. Most current research is in MTJ technology.Most current research is in MTJ technology.

10 Pseudo Spin Valve The pseudo spin valve does not have a pinned layer. Both layers can change magnetic orientation, or flip.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.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.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.Flowing current through the magnetic layers reduces the magnetic field strength required to flip them. NS NS Metal Line

11 Write Operation A device is typically written by flowing mA of current through the metal line and 4-6 mA of current through the device itself.A device is typically written by flowing 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.This produces a strong enough magnetic field to change the orientation of both magnetic layers of the device. NS NS Metal Line

12 Read Operation A device is typically read by flowing mA of current through the metal line and 2 mA of current through the device itself.A device is typically read by flowing 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.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”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” NS NS Metal Line

13 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%.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%.

14 Magnetic Tunnel Junction (MTJ) With MTJ much larger differences in resistance can be achieved compared to PSV.With MTJ much larger differences in resistance can be achieved 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 through the insulating layer.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.Electron tunneling is possible if the distance between the insulating and the conducting layers is small.

15 Magnetic Tunnel Junction Device Insulating layer

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

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