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Design of MRAM using Spintronics Chetan Deorao Khandalkar B .E. (electronics) Walchand college of Engineering, Sangli [email protected] 9975952851

Abstract: Imagine a Data Storage Device with the size of an atom and working at the speed of light. Imagine a computer memory with thousands of times denser and faster than today’s computer memories and nonvolatile, so it retains its contents even when power is off. This imagination has now converted from fiction to reality through the emerging technology, Spintronics. The hunger for speedy devices is hastily rising. The conventional technologies are not showing up to scratch outcome as the demand for speed is going beyond the margin. However we can’t deny the need and have to find out the way for its achievement. It is well said that “Need is the mother of Invention.” To conquer the need for speed, a new technology has been freshly invented. Spintronics, or spin electronics, refers to the study of the role played by electron spin in solid state physics. This technology uses spin property of electron as a memory in any storage device. The Spintronics is mainly used for data storage, R/W at faster speed and all this in a compact size. This paper throws light on the contribution of Spintronics in the development of different technologies like Magnetoresistive RAM (MRAM) which eliminates computer’s boot-up time and volatility, making it to function with lightening speed. MRAM (Magnetoresistive Random Access Memory) works on the magnetization phenomenon caused due to spintronics. The data is stored in the form of magnetization alignment. The transfer of charge requires more time than switching the spin alignment & hence magnetization direction. MRAM uses the change of magnetization direction to store the data unlike conventional memories. Thus it is Faster than other RAMs. It promises to replace conventional RAMs because of its very high speed, instant access, non volatility, very high integration density, very low power consumption & good endurability.

1. Introduction 1.1 Nanotechnology The branch of science which deals with the study & development of objects at the size of few nanometers for the human welfare is called the nanotechnology. The nanotechnology now a day is being used in each & every field to get optimum performance. There are various sub branches of nanotechnology• Nanoelectronics • Nanorobotics • Nanobiotechnology • NEMS (Nano electromechanical systems ) • Plasmonics • Photonics and many more……. Among the various sub branches of the Nanotechnology, Spintronics is one which is vital. 1.2 What is Spintronics? Spintronics, or spin electronics, refers to the study of the role played by electron spin in solid state physics. It also deals with possible devices that specifically exploit spin properties including or excluding charge degrees of freedom. 1.3 Where it can be used? The Spintronics is mainly used for data storage, speed improvement along with the size reduction. The important applications includes • Magnetoresistive random access memory (MRAM) • Magnetic field sensors. • Read heads for hard drives. • Spin based optoelectronics. • In the Lab-On-Chip in the Nanorobotics. • Spin-FET, Spin-LED. • Spin Logic Circuits & many more…..

1.4 Motivation Today we are using the most advanced and rapidly changing electronic technologies. Among them, semiconductor technology has already reached to its optimum level. Present day transistors dissipate about 0.1 µ W of power per bit flip. Assuming that transistor density is about 109/cm2, now if all the transistors are switched simultaneously, then chip dissipates about 100W/cm2. Further power dissipated in a 10 cm2 chip is 1KW. This would demand heat sink of 1000W which is unmanageable. Thus further achievement in this technology would decrease its power dissipation at the cost of ……… The technology of magnetics dates back to centuries but still it is trying to overcome its own problem like effect of external magnetic field. Moreover the magnetics uses metals so circuit becomes bulky. If we go for the electromagnet then the current requirement increases. On contrary, Spintronics, right from its childhood, has shown the ray of hope in boosting the commercial development. The products developed by this technology are being used by us & are giving the satisfactory result. This technology further can also be coupled with the other technologies like nanotechnology, plasmonics, photonics etc. which would definitely serve the best. The only best promising way to combine Semiconductor technology with the Magnetics is Spintronics.

2. Principle of Spintronics The electron has three basic characteristics 1. Mass. 2. Charge and 3. Spin.

Fig1: Fundamental properties of electron

All fundamental particles including electron have the property called spin which can be oriented in one direction or the other – called ‘spin-up’(+1/2) or

Fig 2: Spins are aligned in particular direction creating ferromagnetism ‘spin-down’(-1/2). It is just like a top spinning anticlockwise or clockwise. When electron spins are aligned (i.e. all spin-up or all spin-down) they create a large-scale net magnetic moment as seen in magnetic materials like iron and cobalt. Magnetism is an intrinsic physical property associated with the spins of electrons in a material. The Spintronics device will function in the following way 1. Information will be stored into spins as a particular spin orientation (up or down) 2. The spins, being attached to mobile electrons, will carry the information along a wire. 3. The information will be read at a terminal. GMR (Giant Magneto Resistance)

Fig3: GMR showing resistance as the function of magnetization alignment of ferromagnetic layers

When the non magnetic conductor/insulator is sandwiched between two ferromagnetic materials the resistance of the non magnetic conductor/insulator becomes the function of magnetization alignment of the two ferromagnetic layers. Ferromagnetic layer has particular magnetic orientation; all electrons in it are having their magnetic moment aligned to a specific direction. This means that if the magnetic moment of two ferromagnetic layers is aligned in the same direction, the electrons in both the layers have their magnetic moment i.e. spin in one particular direction. Thus, the flow of electrons with the opposite spin is facilitated by these layers showing low resistance. Thus parallel alignment offers low resistance (often called magnetoresistance). However, if the two magnetic layers are antialigned, then each layer represents a particular spin. So the electrons with all possible spin (spin up & spin down) are resisted to flow through. Hence, antiparallel alignment offers high resistance. (Fig 3) Experimentally, it was found that the change in resistance of the sandwiched material is measurably large. For non magnetic conductor it is about 40 times the original resistance while for the insulator it is about the 200 times the original resistance. The phenomenon of the GMR was invented in 1988, & we are now using the hard disk for our desktop pc based on this phenomenon. Later on as it was found that the change in the magnetic moment of electrons is responsible for the change in the resistance of the sandwiched material & ferromagnetic layer, researchers turned to find out the ways to manipulate the spin of electron. Ultimately they found that the spin of electron, as has two states, can be effectively used to store the information making the device more compact in size & bulky in information. With further study, it was found that not only the data storage capacity is increased by this technology but the speed also rises beyond imagination. This is the Spintronics. The MRAM uses the principle of change of magnetic alignment of the ferromagnetic layer to store the information using the spintronics. This has enabled to increase the speed of computer.

3. MRAM This will combine all the advantages of today’s existing memories like speed of SRAM, size of DRAM and non volatility of Flash memory. It is predicted that this may replace small Hard Disk of Desktop PCs. The utilization of MRAM would virtually eliminate boot-up time.

3.1 MRAM Principle Magnetoresistive random access memory (MRAM) is a memory technology that uses the magnetic tunnel junction (MTJ) to store information. RAM uses the magnetization orientation of a thin ferromagnetic material to store information, and a bit can be detected by sampling the difference in electrical resistance between the two polarized states of the MTJ. 3.2 MRAM Cell Design

Fig4: the MTJ- Functional unit of MRAM The MTJ stack material consists of two ferromagnetic layers separated by a thin dielectric barrier. The polarization of one of the magnetic layers (lower in figure) is pinned in a fixed direction, while the direction of the other layer (upper in the figure) can be changed using the direction of current in the bitline. The resistance of the MTJ depends on the relative direction of polarization of the fixed and the free layer. When the polarization is anti-parallel, the electrons experience an increased resistance to tunneling through the MTJ stack. Thus, the information stored in a selected memory cell can be read by comparing its resistance with the resistance of a reference memory cell located along the same wordline. The resistance of the reference memory cell always remains at the minimum level. When the magnetization direction of free layer is changed using any of the method specified, the fixed layer remain intact. This is because; the fixed layer consists of high coercive material while free layer consists of low coercive material ready to change the magnetization direction.

3.2.1 Read operation: During read operation, the transistor associated with the cell, the data over which is to be read, is selected i.e. made ON. The word line is made low. The read current is passed over the bit line. Now depending upon the magnetization direction of the free ferromagnetic layer, the resistance is offered by the MTJ of the cell. If the two magnetization directions of the ferromagnetic layers are aligned in parallel fashion, very low resistance will be offered by the MTJ allowing the spin polarized current to flow pulse is sent through the MTJ through it. In contrast to this if magnetization directions are antialigned then the resistance offered by the cell increases reducing the spin polarized current. The current passed through the cell is compared with the reference current which is the measure of resistance offered by the cell. Depending upon the current the logic ‘0’ & logic ‘1’ are detected.

3.2.2 Write operation: There are three methods for writing the data onto the cell a. FIMS (Field Induced Magnetic Switching) At write, the “word lines” and “bit lines”, arranged in cross point architecture on each side of the MTJ, are energized by synchronized current pulses in order to generate a magnetic field on the addressed memory cell. The intensities of the current pulses are chosen such that only the storage layer at the cross-point of

Fig 6: Write operation of MRAM the two lines (the so-called fully selected cell) can be switched, all other cells on any given line or any given column (the so-called half selected cells) being unable to switch. This is possible because of characteristic property of magnetic nanostructures, the Stoner-Wolhfarth asteroid, wherein the switching field is reduced when the magnetic field is applied at an angle with respect to the particle magnetization, as is the case for fully selected cells. To prevent selection errors, the current has to be large enough to ensure switching of all selected cells, but low enough to prevent switching of half-selected cells. This defines the write window, which is primarily determined by the switching field distribution in the array. b. TAS (Thermally Assisted Switching)

Fig5: MRAM cell with logic ‘0’ & logic ‘1’ dependant on the magnetization alignment

current burst is quickly thermalized in the adjacent metallic layers of the MTJ. The resulting decrease of switching field in the storage layer makes it possible to write at reduced current. The selection at write is now mostly temperature-driven so addressing errors are minimized TAS is expected to prove a viable alternative for sub 0.1 µm cell size, with little or no impact on the memory architecture.

cell will have lower resistance and hence more current will flow through the cell thus reducing the current flowing through the sense amplifiers. The bitline associated with the topmost cell experiences a smaller drop in current as the cell has higher resistance compared to the other two cells connected to the selected wordline. This change in current is detected using sense amplifiers, and the stored data is read out. As the wordline is responsible for sinking the current through a number of cells, the wordline driver should be strong to ensure reliable sensing. Alternative sensing schemes have been proposed for MRAM which have increased sensing reliability but also increase the cell area which is undesirable. Fig 7: Write operation of MRAM C. CIS (Current Induced Switching) When a current flows through a magnetic material, it becomes spin polarized, e.g. there is an imbalance between the numbers of up and down spins carrying the current. When this current hits another magnetic layer, this spin imbalance generates some kind of a torque on the local magnetization, which torque can be large enough to induce a complete reversal of the magnetization. This method provides potential for ultra-small cell size as the thermal stability limit materials requirements are now independent of the current induced switching parameters.

Fig9: Memory bank organization

Fig8: CIS (Current Induced Switching)

3.3 MRAM Bank Design Figure, shows an MRAM bank composed of a number of MRAM cells located at the intersection of every bit and word line. During a read operation, current sources are connected to the bit lines and the selected wordline is pulled low by the wordline driver. Current flows through the cells in the selected wordline and the magnitude of the current through each cell depends on its relative magnetic polarity. If the ferromagnetic layers have the same polarity, the

3.4 Speed Improvement of MRAM Compared To Conventional Memories MRAM Flash DRAM SRAM Bit size

4-8 F2

2-4 F2

8 F2

100 F2

Writing time

<2ns

510µs

~1ns

~1ns

Access time

2-40ns

4070ns

40-70ns

6-70ns

Writing voltage

0.3-5V

1018V

2.5-5V

0.8-5V

Writing energy

<100pJ

<300 pJ

10200pJ

<200pJ

Radiationn hardness

Excellent

poor

poor

poor

storage device to turn into dense and compact. Magnetic random access memory operated on lightning speed, also will soon impact another multibillion dollar industry. Imagine…. • Increasing Brain I.Q artificially. • Single sided writable CD containing information in terabytes. • Desktop PCs with zero boot time. • Transistors without heat sink. • Spinplasmonics. All this is possible in coming future through SPINTRONICS

7. Conclusion

The above table shows how the MRAM is better in speed as compared to the other memories. 4. Advantages 1) Spintronics does not require unique and specialized semiconductors; therefore it can be implemented or worked with common metals, such as Copper, Aluminum and Silver. 2) Spintronics devices would consume less power compared to conventional electronics, because the energy needed to change spin is easy compared to energy needed to push charges around. 3) Since Spins don’t change when power is turned off, the memory remains non-volatile 4) This technology increases data processing speed. 5) It increases integration densities. 6) It can be coupled with other technology easily like plasmonics, nanotechnology.

Spintronics is a new field which aims at using both the charge and spin of an electron, to realize devices that have capabilities more advanced than that of semiconductor and magnetic technologies alone. Since a decade or two we have been witnessing how the semiconductor technology reduced the size and increased the performance of everything making objects portable but this has reached to its maximum level and further reduction in size would be paid up with large power dissipation. Magnetic devices are also obsolete due to its bulkiness and effect of external magnetic field. But, now it is predicted that the Spintronics technology alone will keep this rate of development further by following the same tradition, what we have already witnessed since decades, of semiconductor technology. Spintronics gives path for the journey Off the ground to the ab astra

8. References 5. Scope for improvement

1. http://www.sciencemag.org/cgi/reprint/294/5546 1) Spin is not in thermal equilibrium. 2) Electronic spin is not conserved. 3) Use of magnetic material is must. 6. An eye to the Future

/148.pdf

2. www.epsrc.ac.uk/CMSWeb/Downloads/Publicat ions/Other/Spintronics0607.pdf

3. www.brl.ntt.co.jp/event/ms+s2004/abstract.pdf 4. www.research.ibm.com/journal/rd/501/tocpdf.ht ml

The progress toward understanding and implementing the spin degree of freedom in metallic multilayers and more recently, in semiconductors is gaining momentum as more researchers begin to address the relevant challenges from markedly different viewpoints. Spintronic read head sensors are already impacting a multibillion dollar industry due to its smallest current handling capacity causing

5. http://www.wikipedia.com/Spintronics. 6. http://www.google.com/Spintronics