seminar report on wireless spintronics modulation

Seminar Report 2015 Wireless Spintronics Modulation

Dept. of Electronics and Communication 1 METS school of Engg, Mala

1. INTRODUCTION

Wireless spintronics modulation is a new communication concept that can be adapted

to STNO characteristics. By exploiting nanotechnology, spintronics devices can be easily

realized in a STNO array structure, and each STNO can directly be assigned to many RF

frequencies. First, data are carried at each frequency of each STNO. Many channels can be

assigned due to wideband operation. Second, we select OOK modulation,which is not only a

non-coherent communication method for wireless transmission but also allows wireless

transmission of data via air without changing frequency, even in the case of STNO non-

linearity phenomena.

We then propose a spin RF-direct FDM modulation concept that is adaptable to

STNO.It facilitates STNO communication at higher data rates according to the number of

STNOs within an array, because OOK is executed with each frequency assigned at each

STNO with the targeted frequency.

It has been reported that spintronics oscillators can feasibly be used for wireless

communication by using frequency shift keying (FSK) modulation. Because a spintronics

oscillator has characteristics of wideband operation, as well as fast rising and falling time

when the signal is settling, it can transmit at high data rates with multiband operation in the

air. Moreover, devices applying a spin torque nano-oscillator (STNO) can be realized with a

small form factor because the STNO can be fabricated with hundreds of nanometer size.

However, spintronics oscillators still have a low signal level and poor linewidth thus

impeding their use as a substitute for LC oscillators for wireless communication. Another

difficulty for communication applications is nonlinearity of the output signal when frequency

modulation is applied in the spintronics oscillator . The current flowing through a STNO not

only changes the amplitude ofthe oscillation signal but also shifts its frequency. This presents

challenges for wireless communication with an STNO at an assigned frequency.

Communication is targeted near field communication with low power consumption

by not amplifying the signal at the low power level of the STNO. The proposed scheme is

expected to be competitively executed in near field communication (NFC) with high speed

communication, low power consumption, and small size at relatively low cost.Wireless

communication is the transfer of information between two or more points that are not

connected by an electrical conductor.The most common wireless technologiesuse radio.



With a radio waves distances can be short, such as a few meters for the television or as far as

thousands or even millions of kilometers for deep space radio communications. It

encompasses various types of fixed, mobile, and portable applications, including two-way

radios, cellular telephones, personal digital assistants (PDAs), and wireless networking.

The prosposed communication uses on off keying modulation,which the simplest

form of amplitude-shift keying (ASK) modulation that represents digital data as the presence

or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific

duration represents a binary one, while its absence for the same duration represents a binary

zero. Some more sophisticated schemes vary these durations to convey additional

information. It is analogous to unipolar encoding line code.

On-off keying is most commonly used to transmit Morse code over radio frequencies

(referred to as CW (continuous wave) operation), although in principle any digital encoding

scheme may be used. OOK has been used in the ISM bands to transfer data between

computers. OOK is more spectrally efficient than frequency-shift keying, but more sensitive

to noise when using a regenerative receiver or a poorly implemented superheterodyne

receiver.



2. WIRELESS COMMUNICATION

Wireless communication is the transfer of information between two or more points that

are not connected by an electrical conductor.The most common wireless technologies use

radio. With radio waves distances can be short, such as a few meters for television or as far

as thousands or even millions of kilometers for deep-space radio communications. It

encompasses various types of fixed, mobile, and portable applications, including two-way

radios, cellular telephones, personal digital assistants (PDAs), and wireless networking.

Other examples of applications of radio wireless technology include GPS units, garage

door openers, wireless computer mice, keyboards and headsets, headphones, radio receivers,

satellite television, broadcast television and cordless telephones.Some what less common

methods of achieving wireless communications include the use of other electromagnetic

wireless technologies, such as light, magnetic, or electric fields or the use of sound. Wireless

operations permit services, such as long-range communications, that are impossible or

impractical to implement with the use of wires.

Fig 2.1 Wireless communication concept



Fig 2.2 Devices in wireless network

The term is commonly used in the telecommunications industry to refer to

telecommunications systems (e.g. radio transmitters and receivers, remote controls etc.)

which use some form of energy (e.g. radio waves, acoustic energy, etc.) to transfer

information without the use of wires.Information is transferred in this manner over both short

and long distances

2.1 ADVANTAGES

Communication has enhanced to convey the information quickly to the

consumersWorking professionals can work and access Internet anywhere and anytime

without carrying cables or wires wherever they go. This also helps to complete the work

anywhere on time and improves the productivity.

Doctors, workers and other professionals working in remote areas can be in touch with

medical centres through wireless communication.Urgent situation can be alerted through

wireless communication. The affected regions can be provided help and support with the help

of these alerts through wireless communication.Wireless networks are cheaper to install and

maintain.



3. SPINTRONICS

The basic principle involved is the usage of spin of the electron in addition to mass

and charge of electron. Electrons like all fundamental particles have a property called spin

which can be orientated in one direction or the other called spin-up or spin-down like a top

spinning anticlockwise or clockwise. Spin is the root cause of magnetism and is a kind of

intrinsic angular momentum that a particle cannot gain or lose. The two possible spin states

naturally represent 0and 1in logical operations. Spin is the characteristics that makes the

electron a tiny magnet complete with north and south poles .The orientation of the tiny

magnet north-south poles depends on the particles axis of spin.

Fig 3.1 Electron spinning

Conventional electronic devices rely on the transport of electrical charge carriers

electrons in a semiconductor such as silicon. Now, however, physicists are trying to exploit

the spin of the electron rather than its charge to create a remarkable new generation of

spintronic devices which will be smaller, more versatile and more robust than those

currently making up silicon chips and circuit elements .

Spintronics is a technology which deals with spin dependent properties of an electron

instead of its charge dependent properties. Conventional electronics devices rely on the

transport of electric charge carries electrons. But there is other dimensions of an electron

other than its charge and mass i.e. spin. This dimension can be exploited to create remarkable

generation of spintronic devices. It is believed that in the near future spintronics could be

more revolutionary than any other technology.



To enhance the multifunctionality of an devices (for example, carrying out processing

and data storage on the same chip), investigators have been eager to exploit another property

of the electron known as spin. Spin is a purely quantum phenomenon roughly akin to the

spinning of a childs top or the directional behavior of a compass needle. The top could spin

in the clockwise or counter clockwise direction; electrons have spin of a sort in which their

compass needles can point either up or down in relation to a magnetic field.

Spin therefore lends itself elegantly to an 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.

Fig 3.2 Concept for spin electronics



3.1. LOGIC OF SPIN

Spin relaxation (how spins are created and disappear) and spin transport (how spins

move in metals and semiconductors) are fundamentally important not only as basic physics

questions but also because of their demonstrated value as phenomena in electronic

technology.Researchers and developers of spintronic devices currently take two different

approaches.

In the first, they seek to perfect the existing GMR-based technology either by

developing new materials with larger populations of oriented spins (called spin polarization)

or by making improvements in existing devices to provide better spin filtering. The second

effort, which is more radical, focuses on finding novel ways both to generate and to utilize

spin-polarized currents that is, to actively control spin dynamics.

The intent is to thoroughly investigate spin transport in semiconductors and search for

ways in which semiconductors can function as spin polarizers and spin valves. This is crucial

because, unlike semiconductor transistors, existing metal-based devices do not amplify

signals (although they are successful switches or valves).

If spintronic devices could be made from semiconductors, however, then in principle

they would provide amplification and serve, in general, as multi-functional devices. Perhaps

even more importantly, semiconductor-based devices could much more easily be integrated

with traditional semiconductor technology. In addition to the near-term studies of various

spin transistors and spin transport properties of semiconductors, a long-term and ambitious

subfield of spintronics is the application of electron and nuclear spins to quantum

information processing and quantum computation.

The quantum mechanics may provide great advantages over classical physics in

computation. However, the real boom started after 20 Peter Shor of Bell Labs devised a

quantum algorithm that would factor very large numbers into primes, an immensely difficult

task for conventional computers and the basis for modern encryption. It turns out that spin

devices may be well suited to such tasks, since spin is an intrinsically quantum property.



Fig 3.3 Spins arrangements

The simplest method of generating a spin-polarised current in a metal is to pass the

current through a ferromagneticmaterial. The most common applications of this effect

involve giant magnetoresistance (GMR) devices. A typical GMR device consists of at least

two layers of ferromagnetic materials separated by a spacer layer. When the two

magnetization vectors of the ferromagnetic layers are aligned, the electrical resistance will be

lower (so a higher current flows at constant voltage) than if the ferromagnetic layers are anti-

aligned.



4. SPINTRONICS OSCILLATOR

Spintronics oscillators can feasibly be used for wireless communication by using

frequency shift keying (FSK) modulation . Because a spintronics oscillator has characteristics

of wideband operation, as well as fast rising and falling time when the signal is settling, it

can transmit at high data rates with multiband operation in the air.

Moreover, devices applying a spin torque nano-oscillator (STNO) can be realized

with a small form factor because the STNO can be fabricated with hundreds of nanometer

size .However, spintronics oscillators still have a low signal level(e.g., dBm measured in our

fabricated STNO) and poor linewidth (e.g., 130 MHz measured in our fabricated

STNO,meaning the phase noise is dBc at an offset of 130 MHz),thus impeding their use as a

substitute for LC oscillators for wireless communication.

Another difficulty for communication applications is nonlinearity of the output signal

when frequency modulation is applied in the spintronics oscillator . The current flowing

through a STNO not only changes the amplitude of the oscillation signal but also shifts its

frequency. This presents challenges for wireless communication with an STNO at an as-

signed frequency.



5. SPIN TORQUE NANO OSCILLATOR

Spin torque oscillators (STO) are a new class of tunable Radio frequency oscillators

in the 165GHZ range with potential use in cellphones,wireless devices,satellite

communication, and vehicle radar.

Fig 5.1 spin torque transfer

A fundamental obstacle to the rapid commercialization of this technology is the very

limited Output power of the signal. spintronics oscillators still have a low signal level and

poor linewidth thus impeding their use as a substitute for LC oscillators for wireless

communication.Difficulty for communication applications is nonlinearity of the output signal

when frequency modulation is applied in the spintronics oscillator .The current flowing

through a STNO not only changes the amplitude ofthe oscillation signal but also shifts its

frequency. This presents challenges for wireless communication with an STNO at an

assigned frequency.

Fig 5.2 Frequency vs magnetic field



It has been reported that spintronics oscillators can feasibly be used for wireless

communication by using frequency shift keying (FSK) modulation. Because a spintronics

oscillator has characteristics of wideband operation, as well as fast rising and falling time

when the signal is settling, it can transmit at high data rates with multiband operation in the

air. Moreover, devices applying a spin torque nano-oscillator (STNO) can be realized with a

small form factor because the STNO can be fabricated with hundreds of nanometer size.By

exploiting nanotechnology, spintronics devices can be easily realized in a STNO array

structure, and each STNO can directly be assigned to many RF frequencies.

5.1. SPIN TORQUE TRANSFER

Spin-transfer torque is an effect in which the orientation of a magnetic layer in a

magnetic tunnel junction or spin valve can be modified using a spin-polarized current. A

simple model of spin-transfer torque for two anti-aligned layers. Current flowing out of the

fixed layer is spin-polarized. When it reaches the free layer the majority spins relax into

lower-energy states of opposite spin, applying a torque to the free layer in the process.Charge

carriers (such as electrons) have a property known as spin which is a small quantity of

angular momentum intrinsic to the carrier.

Fig 5.3 Simple model of spin-transfer torque for two anti-aligned layers

An electrical current is generally unpolarized (consisting of 50% spin-up and 50%

spin-down electrons); a spin polarized current is one with more electrons of either spin. By

passing a current through a thick magnetic layer (usually called the fixed layer), one can

produce a spin-polarized current. If this spin-polarized current is directed into a second,

thinner magnetic layer (the free layer), angular momentum can be transferred to this layer,

changing its orientation. This can be used to excite oscillations or even flip the orientation of

the magnet. The effects are usually only seen in nanometer scale devices.



6. ON OFF KEYING (OOK) MODULATION

On-off keying (OOK) denotes the simplest form of amplitude-shift keying (ASK)

modulation that represents digital data as the presence or absence of a carrier wave. In its

simplest form, the presence of a carrier for a specific duration represents a binary one, while

its absence for the same duration represents a binary zero. Some more sophisticated schemes

vary these durations to convey additional information. It is analogous to unipolar encoding

line code.

On-off keying is most commonly used to transmit Morse code over radio frequencies

(referred to as CW (continuous wave) operation), although in principle any digital encoding

scheme may be used. OOK has been used in the ISM bands to transfer data between

computers. OOK is more spectrally efficient than frequency-shift keying, but more sensitive

to noise when using a regenerative receiver or a poorly implemented superheterodyne

receiver.

Fig 6.1 OOK modulation concept

OOK modulation is a very popular modulation used in control applications.This is

due to its simplicity and low implementation costs. OOK modulation has the advantage of

allowing the transmitter to idle during the transmission of a zero, therefore conserving

power. The disadvantage of OOK modulation arises in the presence of an undesired signal



7. FREQUENCY DIVISION MULIPLEXING

In telecommunications, frequency-division multiplexing (FDM) is a technique by

which the total bandwidth available in a communication medium is divided into a series of

non-overlapping frequency sub-bands, each of which is used to carry a separate signal. These

sub-bands can be used independently with completely different information streams, or used

dependently in the case of information sent in a parallel stream. This allows a single

transmission medium such as the radio spectrum, a cable or optical fiber to be shared by

multiple separate signals.

The most natural example of frequency-division multiplexing is radio and television

broadcasting, in which multiple radio signals at different frequencies pass through the air at

the same time. Another example is cable television, in which many television channels are

carried simultaneously on a single cable. FDM is also used by telephone systems to transmit

multiple telephone calls through high capacity trunklines, communications satellites to

transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL

modems to transmit large amounts of computer data through twisted pair telephone lines,

among many other uses.

Fig 7.1 FDM channels

An analogous technique called wavelength division multiplexing is used in fiber optic

communication, in which multiple channels of data are transmitted over a single optical fiber

using different wavelengths (frequencies) of light.



8. CONCEPT:SPIN RF- DIRECT FDM WITH STNO ARRAY

Figure shows the concept of spin RF-direct modulation using a frequency division

multiplex (FDM) with a spin torque nano-oscillator (STNO) array.

Fig 8. 1 The concept of a spin RF-direct FDM modulation with a STNO array.

Here, Dn, In, and fn are nth data, current, and frequency; STNOn is the nth STNO

and MNn is a matching network for the nth frequency at the nth STNO. Wideband MN

denotes a wideband matching network and Ant is antenna. .

For the proposed modulation, two STNO are bonded at each branch of a T-junction

on a PCB, where a high pass filter (HPF) and a low pass filter (LPF) are connected on each

branch of the T-junction to transmit the modulated signal into one antenna. Each STNO

modulates by on-off keying (OOK) with digital data directly after setting on the separation of

channels for two STNOs into 700 MHz, one STNO at 3.5 GHz frequency and the other at 4.2

GHz, with consideration of the minimum sensitivity and interference related with isolation



between channels. A data rate of up to 400 Kbps is obtained at a distance of 10 mm, and the

dc power consumption is 3 mW per STNO, including logic circuit operation.

8.1.DESIGN OF SPIN M ODULATOR

In order to realize a spin RF-direct FDM modulation, we executed experiments with

two STNOs to check whether wireless communication can be realized with spintronics

oscillation, and also assessed whether the spin RF-direct FDM modulation concept was

practicable.

Fig 8.2 Frequency vs magnetic field

Figure shows the STNO oscillation characteristics of frequency depending on the

magnetic field. The signal level, at the offset point corresponding with the channel separation

between two channels from each carrier frequency, should be selected to be below the

minimum sensitivity, dBm, considering the linewidth and signal level. We thus selected

frequencies of 3.5 and 4.2 GHz with 700 MHz channel separation for two STNOs for the

channel separation to be at least three times the linewidth.A data rate of up to 400 Kbps is

obtained at a distance of 10 mm, and the dc power consumption is 3 mW per STNO,



including logic circuit operation. The PCB size is as small as 28 27 mm, including the 4.2 2.1

mm STNO array.

8.2. MATCHING THE CIRCUIT ON EACH STNO

A T-junction structure is applied to the PCB for assigning thefrequencies generated

on two STNOs to transmit a signal in the air through a single antenna.We designed matching

networks with a LPF circuit for a 3.5 GHz STNO, and a HPF circuit for a 4.2 GHz STNO to

provide operation without interference with each other, and then each output port of the

matching circuit is connected with other ports of the T-junction,taking into consideration an

antenna that will be situated at the center port of the T-junction. The STNO has the anti-

parallel magnetic resistance (MR) of 80 , where the STNO provides good signal oscillation.

A HPF circuit is applied to obtain higher isolation at 3.5 GHz as well as low insertion

loss at 4.2 GHz, and vice versa in the LPF circuit. Higher isolation and low insertion loss

cause all of the signal to flow into the antenna port of the T-junction without signal loss. We

targeted insertion loss approaching 3 dB, and signal isolation below dB in the T-junction.

Fig. 3 shows the measurement results of the T-junction. The antenna is located immediately

after the T-junction and it has a band from 3.1 to 5.2 GHz, and 2 dBi gain.

The power level transmitted through the antenna is dBm at 4.5 GHz with consider-

ation of the T-junction loss and antenna gain, and the thermal noise level is dBm in the 130

MHz STNO linewidth. The SNR in the transmitter is thus 20.4 dB. The transmission margin

is 11.4 dB, because the required SNR is 9.0 dB for the bit error rate (BER) in the OOK

system. This margin corresponds to supporting transmission up to 27 mm distance in the air

in the condition of using a receiver having no noise or gain

8.3. MODULATION AND BIASING ON STNO

An oscillation signal is generated by the current flowing through the STNO. Current

can be supplied via a current mirror circuit as a bias for a stable current supply.The current

mirror circuit is designed with two PMOS. The ratio of the transistor width for the current

mirror circuit must be set at 1:8 to reduce the leakage current to 1/8th of the current flowing

through the STNO.



Fig 8.3 Measurement results of the T-junction matching circuit.

The supplied current drives the STNO to generate oscillation. We then modulate the

STNOwith the OOK method,where the current supplied from the current mirror circuit is

switched on/off. This function is executed through a NMOS switch that enables operation of

the current mirror circuit.



9. MEASUREMENT RESULTS

Fig. 4 shows the PCB board with soldered components together with STNOs

structured in a 2 4 array for transmitting multi channel signals to one antenna.We selected

two out of the 8 STNOs and set the target frequency by controlling the magnitude of the

magnetic field, the angle of the applied magnetic field to the STNO, and the current quantity

based on STNO operation.

Fig 9.1 Fabricated PCB board with soldered components with a STNO array

We situated a RF switch between the T-junction and the antenna to monitor the

STNO output signal. The RF switch is controlled by a dip switch.To carry out the

measurements, the fabricated PCB is placed on a C-type magnetic closed circuit to apply a

magnetic field around the DUT. Constant current is applied to the electro magnet bya

precision current source,and afringingfield is generated depending on the current quantity.

The fabricated STNO could change the frequency by only around 220 MHz according to the

current flow to the STNO from 1.0 to 2.5 mA in our fabricated STNO. Thus the fringing field

is adjusted by the current quantity, and the direction of the magnetic field is also changed by

manually making the C-type magnetic closed circuit rotate for each STNO to be set to an

assigned frequency of 3.5 and 4.2 GHz, respectively.Under these measurement conditions,

we tested the transmission operation on dual frequencies modulating OOK into one antenna.



Then design a dual channel OOK receiver for verification of the transmitted signal.

The air loss is 5 dB in a distance of 10 mm, and the commercial demodulator sensitivity we

tested is dBm. We obtained a communication margin of 14 and 16 dB, because the received

signal level is -62 dBm and -60 dBm at the receiver with a system NF of 4.3 and 4.5 dB and

system gain of 15 and 17 dB at 3.5 and 4.5 GHz, respectively.This demonstrates that the

wireless technology of spintronics modulation is feasible for use in wireless communication.

Although reported that current clearly modulates pulse repetition frequency (PRF) up

to 5 MHz, in the experiments reported, in this letter the usable frequency of PRF is limited by

the bias delay driving current at the transmitter, and by a falling time of 180 nsec and a rising

time of 100 nsec for detection at the receiver. This work is verified by using the LandauLif-

shitz Gilbert Slonczewski (LLGS) equation for the STNO with a MATLAB tool,

considering parallel MR of 50 and anti-parallel MR of 80 , when 500 pF is added in the bias-

T for cancelling modulation noise.

A data rate of up to 200 kbps is obtained at each frequency in the experiment results

at a distance of 10 mm, and the power consumption is under 3 mW per STNO at these

measurement conditions. It corresponds to an energy efficiency of 15 nJ/bit.This result shows

that a data rate up of up to 400 Kbps is acquired at the STNO array.

The 15 nJ/bit obtained from the acquired consumption result is not a remarkable

achievement due to the use of commercial circuits supplied with 2 VDC; however, recent

numerical simulations suggest that current densities may be reduced to five orders of

magnitude lower than the tested STNO .If nano-scaled CMOS and new spintronics

technologies are applied, the size and power consumption of the wireless technology in this

work could approach those of the core block, i.e.,STNO. It is observed that 0.18 mW power

consumption occurs at one STNO.The core in this work has three times larger tuning range

,5.5 times smaller power consumption and much smaller size



10. ADVANTAGES

Some of the advantagesof wireless spintronics modulation are

Provide near field communication with low power consumption by not amplifying the

signal at the low power level of the STNO

Increases data rates remarkably by using a STNO array, which features the wide band

range even exceeding 100%

Low dc consumption at the micro-watt level

Nano-sized realization

Small size at relatively low cost.



11. APPLICATIONS

Can be used in near field communication (NFC)

Can implemented in mobile telephony

Adopt as a alternative for wireless communication methods



12.CONCLUSION

We demonstrated a new communication technique with spintronics technology. This

is the first report of the wireless spintronics modulation with a STNO array. The obtained

results demonstrate the possibility of increasing data rates remarkably by using a STNO

array, which features a wide band range even exceeding 100%, low dc consumption at the

micro-watt level, and nano-sized realization. However, many barriers must still be

surmounted. In view of modulation, each STNO has to be designed with operation of a

differenct frequency on the same magnetic field in order to easily realize a spin RF-direct

FDM modulation at the STNO array.This has been accomplished by the STNO employing

wideband frequency variation according to the dc bias current, or a different value or

direction of the anisotrophic field.Nevertheless, the proposed scheme is expected to be

competitively executed in near field communication (NFC) with high speed communication,

low power consumption, and small size at relatively low cost.



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seminar report on wireless spintronics modulation