Unit 4 Selected Topics

1. Unit 4 Selected Topics

2. 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

3. 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

4. S N N S S N S N 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.

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/

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

7. S N N S 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

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. • 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

9. 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.

10. Metal Line S N S N 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.

11. Metal Line S N S N 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.

12. S N S N 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”

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%.

14. 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.

15. Magnetic Tunnel Junction Device Insulating layer http://www.research.ibm.com/resources/news/20001207_mramimages.shtml

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 anti-aligned both types of electron are filtered out.

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