SPINTRONIC MEMRISTORS

Franklin Geo FrancisE7B

Roll No. 14Guide: Mr. Binesh T

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Contents

• Memristors– Origin– Theory

• Spintronics• Spintronic Memristors– MTJ– Thin Film Element– Spin Valve

• Applications• Advantages

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What Are Memristors?

•A memristor is essentially a resistor with “memory”.

•It expresses the property of memristance.

•A memristor is able to hold to its last resistance value

even if the voltage has been turned off and it can retain it

for a long time.

•It is a fundamental circuit element.

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Fundamental Circuit Quantities

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Origin Of Memristors

• Leon Chua, a professor at UC Berkley, predicted the existence of Memristor, in 1971, based upon symmetry arguments.

• Chua discovered a missing link in the pair wise mathematical equations that relate the four circuit quantities—charge, current, voltage, and magnetic flux—to one another.

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Theory

• The original definition of Memristor is given by

• q is the electric charge,ϕ is the magnetic flux and M is the memristance. From Faraday’s Law we can show that,

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• Memristance is different from ordinary resistance in that it is charge-dependent.

• The definition of Memristor can be generalised to

• The power consumption of a memristor is given by

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Current Voltage Characteristics

• Current Voltage relationship is similar to that of a Lissajous pattern.

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Spintronics

• Spintronics refers to the study of the role played by electron spin, in solid state physics, and its associated magnetic moment, in addition to its fundamental electronic charge.

• Spin is a purely quantum phenomenon roughly akin to the spinning of a child’s top or the directional behavior of a compass needle. The top could spin in the clockwise or counterclockwise direction

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• Spin therefore lends itself elegantly to a new kind of binary logic of ones and zeros. The movement of spin, like the flow of charge, can also carry information among devices.

• One advantage of spin over charge is that spin can be easily manipulated by externally applied magnetic fields, a property already in use in magnetic storage technology.

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Spin Torque Induced Magnetisation Switching• The magnetic tunnel junction

consists of two ferromagnetic metals, separated by an ultrathin layer of insulator.

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

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• One of the ferromagnet is called a reference layer. It is engineered in such a way that its magnetisation remains constant.

• The other ferromagnet is called the free layer. Its magnetisation can be varied through Spin Torque Induced Magnetisation.

• Spin Torque Induced Magnetisation is based on the Conservation of Angular Momentum, which leads to torque.

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Current Induced Domain Wall Motion

• A domain wall is an interface separating magnetic domains. It is a transition between different magnetic moments.

• Spin-polarized electrons in a ferromagnet pass through a domain wall, creates a torque on the electrons tending to align their spin magnetic moments with the magnetization direction.

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• Conservation of angular momentum requires a reaction torque, known as the spin transfer torque, from the electrons to the magnetization that displaces the domain wall in the direction of electron flow.

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Spintronic Memristors

• Memristors implemented using the concept of Spintronics.

• Basically we try to control the magnetisation of a spintronic device through current electron spin changes.

• If the resistance of this spintronic device is determined by its magnetization state, we could have a spintronic memristor with its resistance dependent upon the integral effects of its current profile.

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Magnetic Tunnel Junction

• It consists of two layers, the free layer and the reference layer.

• The polarisation of the reference layer is constant whereas the polarisation of free layer can be varied.

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• MTJ resistance can be written as a function of angle between the free-layer magnetization direction and the pinned layer magnetization direction

• where θ is the angle between the free layer and reference layer, G0 is the MTJ conductance

for θ=∏/2 rads.• TMR= (GH – GL )/ GL

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• High Conductance Both layers parallel.• Low Conductance Both layers anti-parallel.

• The shapes of these I–V curves depend upon the voltage frequencies due to the dependence of resistance upon the historic profile of current or voltage.

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Thin Film Element

• A thin film element with varying width w and thickness t.

• The current induced domain-wall motion in the film-length direction x is considered.

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• The domain-wall velocity in a thin-film element is a function of driving current density

• where J = (I/wt) is the current density. The driving force exerted by electrons on the domain wall corresponds to a reaction force applied to electrons by the domain wall, which manifests itself as a wall resistance,

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• In the previous equation R0 is the hall coefficient, β is a correction factor = 2 and μ is the domain wall mobility, which scales with the thin film aspect ratio. μ =(w/t)K.

• Thus, for a thin-film element with constant thickness t, the domain wall resistance scales with the element width.

• For spatially varying width,

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• Two hysteretic I–V curves at two different voltage frequencies are shown. For thin-film domain-wall motion, memristor effects become significant as the driving voltage frequency is high enough to interact with domain-wall moving speed.

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Spin Valve

• Long spin-valve structure with domain wall motion in the free layer.

• The resistance depends on domain-wall position x: R = [(RLx/D) + (RH(D − x)/D)], where D is the free-layer length.

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• RL low resistance & RH high resistance .• We assume a very sharp domain wall and ignore

the domain-wall resistance. The domain-wall velocity is then proportional to current-density strength, e.g., (dx/dt) =τI.

• Thus memristance is equal to

where q is electronic charge.

• The memristor is realized by electron and spin transport coupling

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Spintronic Memristors Applications

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Multi Bit Data Storage & Logic

• Two Methods– Two horizontal domains are introduced in MTJ free

layer.– Two MTJ are stacked vertically.

• One bit of the two bits cell is hard and requires higher current to switch and the other bit is soft and requires less current to switch

• Switching is achieved by varying current magnitude.

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• We denote the first bit as hard bit and the second bit as soft bit.

• Based upon R-I curve switching behavior; multi-bit MTJ cell has reversible branches and irreversible branches.

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• As a memristor, MTJ resistance switching not only depends upon current magnitude but also depends upon current pulse width

• The longer current pulse width switches the soft sub-cell & shorter current pulse width switches the hard sub-cell.

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Temperature Sensor

• Spin- Valve structure with a free layer and a reference layer.

• The free layer is divided by a domain-wall into two segments that have opposite magnetization directions to each other

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• Domain wall velocity increases as temperature increases.

• A biasing voltage pulse with constant magnitude is applied to the device. Resistance difference before and after voltage pulse is measured. This resistance difference is calibrated to sense temperature.

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• Operated at a region where its electric behavior is sensitive to temperature change.

• Positive feedback between resistance and driving strength.

• It significantly increases the temperature sensing margin.

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Power Management Scheme

• In Series– E =∫VIdt =V∫Idt– ∫Idt can be read out by

memristor resistance.• In Parallel– E = ∫VIdt = I ∫Vdt– ∫Vdt can be read out by

memristor resistance.

Series

Parallel

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• Active control of power can also be achieved.• Negative feedback between memristor

resistance and power i.e. the domain wall moves in a direction to reduce device resistance.

• Memristor resistance is comparable to that of the circuit.

• As circuit power increases memristor resistor increases and this reduces the current and power through circuit.

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Information Security

• Here a scheme is presented for the case that user and administrator has the same authority on writing and reading.

• The device used is a MTJ.• For Writing– High Resistance Fully

Saturated(0)– Low Resistance Partially

Saturated(1)

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• For reading we use two constant voltage pulses– one pulse tries to push domain wall toward high

resistance end– other pulse tries to push domain wall toward low

resistance end• The first is common for both the user and

admin whereas the second is specific for both user and admin.

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• The device reports two values for reading– final state of the device

close to high or low resistance state (High or Low)

– whether device resistance has been significantly changed during reading (Yes or No).

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• For hacking this system the user will have to guess the second pulse strength. Guessing a particular no. in a continuum range is almost impossible.

• The user could try many different cells but the admin needn’t necessarily use same pulse driving strength. The admin can use a random sequence.

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Advantages Of Memristors

• The can function as synapses which can lead to simulation of human brain.

• INSTANT ON computer using memristor memories.

• Less power hungry and faster.• Could lead to the integration of the CPU and

memory into one block.• A single memory for all purposes.

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Conclusion

• Just as the last century belonged to transistors the new century could belong to memristors.

• Wide ranging applications.• Huge opportunities in both the theoretical as

well as practical domain.

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References• L. O. Chua, “Memristor—The missing circuit element,” IEEE Trans.

Circuit Theory, vol. CT-18, no. 5, pp. 507–519, Sep. 1971.• Hiro Akinaga and Hideo Ohno, “Semiconductor Spintronics”, IEEE

Transactions On Nanotechnology, Vol. 1, No. 1, March 2002• X. Wang, Y. Chen, H. Xi, H. Li and D. Dimitrov, “Spintronic memristor

through spin torque induced magnetization motion,” IEEE Device Letters, vol. 30, no. 3, pp. 294-297, 2009.

• X. Wang, Y. Chen, Y. Gu, H. Li, “Spintronic memristor temperature sensor”, IEEE Electron Device Letters, Vol. 31, No. 1, January 2010.

• Spintronic Memristor Devices and Application- Xiaobin Wang, Yiran Chen

• R. Stanley Williams, “How we found the missing memristor”, IEEE Spectrum, December 2008, pp 25-31.

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Thank You