MRAM

1

MTJ

Consists of two layers of magnetic metal, such as cobalt-iron, separated by an ultra thin layer of insulator, typically aluminum oxide with a thickness of about 1 nm.

The insulating layer is so thin that electrons can tunnel through the barrier if a a voltage is applied between the two metal electrodes.

The current depends on the relative orientation of magnetizations of the two ferromagnetic layers, which can be changed by an applied magnetic field. This phenomenon is called tunneling magneto resistance (TMR).

Antiparallelmagnetizations

Parallelmagnetizations

Ferromagnet

Ferromagnet

I nsulator

Resistance: R

R

RRTMR=

Tunneling current

Resistance: R

Tunnel Magnetoresistance

• Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effect

• TMR junctions have resistance ratio of about 70%• MgO barrier junctions have produced 230% MR

Tunnel Magnetoresistance Magnetic tunnel junction has two

magnetic layers separated by an insulating

metal-oxide layer. Is similar to a GMR spin valve except that

a very thin insulator layer is sandwitched

between magnetic layers instead of metal

layer . The difference in resistance between the

spin-aligned and nonaligned cases is much

greater than for GMR device – infact 1000

times higher than the standard spin valve.

4

MRAM

Magnetic Tunnel Junctions

MRAM stands for magnetic random access memory.

It places the magnetic domains on the surface of a silicon chip and places a magneto-resistive sensor beneath everyone of them.

5

CONTD…Just like a hard disk,the

information stored in the magnetic domains is non volatile.It is not lost when the power goes off.

The two main concept that govern MRAM technology are:Giant

Magnetoresistance effect(GMR)

The Tunneling Magneto Resistive effect(TMR)

GMR Valves

Magnetoresistive Random Access Memory (MRAM)

MRAM uses magnetic storage elements.The elements are mostly tunnel junctions formed from two

ferromagnetic plates, each of which can hold a magnetic field,

separated by a thin insulating layer.

SRAM VS DRAM VS MRAM

SRAM

DRAM

MRAM

Advantage• Fast read & write speed.• Low power

• High density• Fast read &write speed.

• Fast read &write speed.• Low power• High density• Non Volatile

Disadvantage• Volatile• Low density

• Volatile• High power

• None ??

Comparison with DRAM & SRAM In DRAM & SRAM, a bit is represented as charge stored in capacitor. In MRAM, data is stored as magnetic alignment of electrons in a ferromagnetic material. Spin up represents ‘0’ and spin down represents ‘1’. MRAM promises:• Density of DRAM• Speed of SRAM• Non-volatility like flash memory.

That’s why its called universal memory.

256 K MRAM

Journey of MRAM Problems encountered:

1. The density of bits was low.

2. Cost of chips was high. Improved designs to overcome these problems would work

only at liquid nitrogen temperature. An important breakthrough was made in the year 2009. Scientists at the North Carolina State University discovered

a semiconductor material ‘ Galium manganese nitride’ that

can store & retain spin orientation at room temperature. And research is still going on…

Detection of spin polarization in silicon

Ferro magnet

Al2O3

Tunnel barrier

e-

n type Silicon

Spin accumulation

u

Tunnel resistance in proportional to u

I = G * ( V - u/2)

I = G * ( V + u/2)

MRAMMagneto resistive RAM

Reading process• Measurement of the

bit cell resistance by applying a current in the ‘bit line’

• Comparison with a reference value mid-way between the bit high and low resistance values

MRAMMagneto resistive RAM

Writing process• Currents applied in

both lines : 2 magnetic fields

• Both fields are necessary to reverse the free layer magnetization

• When currents are removed : Same configuration

MRAMMagneto resistive RAM

Array structure of MRAM• Reading: transistor of

the selected bit cell turned ‘on’ + current applied in the bit line

• Writing: transistor of the selected bit cell turned ‘off’ + currents applied in the bit and word lines

• Need of 2 magnetic fields for writing

MRAMMagneto resistive RAM

• MTJ test structures developed at SPINTEC: the die area with 1x5 μm

• 0.2 μm width isolated MTJ element after etch

MRAM• MRAM uses magnetic storage elements instead of

electric used in conventional RAM• Tunnel junctions are used to read the information

stored in Magnetoresistive Random Access Memory, typically a”0” for zero point magnetization state and “1” for antiparallel state

MRAM

• Attempts were made to control bit writing by using relatively large currents to produce fields

• This proves unpractical at nanoscale level

Spin Transfer• Current passed through a magnetic field becomes spin

polarized• This flipping of magnetic spins applies a relatively large

torque to the magnetization within the external magnet

• This torque will pump energy to the magnet causing its magnetic moment to precess

• If damping force is too small, the current spin momentum will transfer to the nanomagnet, causing the magnetization will flip

• Unwanted effect in spin valves• Possible applications in memory writing

MRAM

• The spin transfer mechanism can be used to write to the magnetic memory cells

• Currents are about the same as read currents, requiring much less energy

MRAM• MRAM promises:– Density of DRAM– Speed of SRAM– Non-volatility like flash

Spin Transistor• Ideal use of MRAM would utilize control of the

spin channels of the current• Spin transistors would allow control of the spin

current in the same manner that conventional transistors can switch charge currents

• Using arrays of these spin transistors, MRAM will combine storage, detection, logic and communication capabilities on a single chip

• This will remove the distinction between working memory and storage, combining functionality of many devices into one

Datta Das Spin Transistor

• The Datta Das Spin Transistor was first spin device proposed for metal-oxide geometry

• Emitter and collector are ferromagnetic with parallel magnetizations

• The gate provides magnetic field

• Current is modulated by the degree of precession in electron spin

Magnetic Semiconductors• Materials like magnetite are magnetic

semiconductors• Development of materials similar to conventional • Research aimed at dilute magnetic semiconductors• Manganese is commonly doped onto substrate– However previous manganese-doped GaAs has

transition temp at -88oC• Curie temperatures above room must be

produced.

23

TRANSPINNORS• A Transpinnor is a bridge of four

electrically connected GMR films whose resistance is controlled by the magnetic field from the current in one of more input strip lines electrically isolated from GMR films.

• Transpinnors can be used as selection matrix elements for magnetic memories,for logic elements of all kinds(e.g..AND,OR,NAND , NOT),for amplifiers,differential amplifiers.

24

ADVANTAGES

• The various advantages of spintronics is as follows:

• Does not require unique and specialized semiconductors;can be implemented with common metals such as Cu,Al.

• Spintronic devices consume less power.• The memory remains non-volatile.

25

LOOPHOLES

• The various disadvantages are as follows:

• The fringe fields generated interfere with the neighboring elements.

• Hazards of holes• Much remains to be understood about

the behavior of electron spins in materials for technological applications

26

CONCLUSION

• Spintronics is a rapidly emerging field of science and technology that will most likely have a significant impact on the future of all aspects of electronics as we continue to move into the 21st century.

27

CONTD…Conventional electronics are based on the charge of the electron. Attempts to use the other fundamental property of an electron, its spin, have given rise to a new, rapidly evolving field, known as spintronics, an acronym for spin transport electronics that was first introduced in 1988.

Applications

GMR sensors find a wide range of applications:

Fast and accurate position and motion sensing of mechanical components in precision engineering and robotics.

Missile Guidance Position and motion sensing in

computer video games. Key Hole Surgery and post operative

care. Automotive sensors for fuel handling

system, speed control and navigation etc.

ApplicationsSpin Valve Transistors:It is based on magneto resistance, found in multi layers (Co-Cu-Co) forming the base region. The collector current becomes strongly field dependent, the extreme magneto sensitivity makes the transistor, an interesting device for high technology hard disks and magnetic RAMs.

ConclusionWith lack of dissipation, spintronics may be the best mechanism for creating ever-smaller devices. The potential market is enormous, In maybe a 10-year timeframe, spintronics will be on par with electronics. That's why there's a huge race going on around the world In exploring Spintronics.

Conclusion• Spin property of electrons are yet to mastered.• Researcher and scientist are taking keen interest. • Universities and electronic industries collaborating .• Span of last two decade major milestones. • It holds vast opportunities for physics , material & device

engineering & technology• Last year PTB, Germany, have achieved a (2GBit/s) write cycle• Potential of the field is colossal and continuous development is required.

Current Research

• Material science– Many methods of

magnetic doping

• Spin transport in semiconductors

ConclusionInterest in spintronics arises, in part, from the looming

problem of exhausting the fundamental physical limits of conventional electronics.

However, complete reconstruction of industry is unlikely and spintronics is a “variation” of current technology

The spin of the electron has attracted renewed interest because it promises a wide variety of new devices that combine logic, storage and sensor applications.

Moreover, these "spintronic" devices might lead to quantum computers and quantum communication based on electronic solid-state devices, thus changing the perspective of information technology in the 21st century.