Spintronics

ABSTRACT

Control over spins in the solid state forms the basis for nascent spintronics and quantum information technologies. There is a growing interest in the use of electronic and nuclear spins in semiconductor nanostructures as a medium for the manipulation and storage of both classical and quantum information.

Spin-based electronics offer remarkable opportunities for exploiting the robustness of quantum spin states by combining standard electronics with spin-dependent effects that arise from the interactions between Sections, nuclei, and magnetic fields. Here we provide an overview of recent developments in coherent electronic spin dynamics in semiconductors ant quantum structures, including a discussion of temporally- and spatially-resolved magneto- optical measurements that reveal an interesting interplay between electronic and nuclear spins. In particular, we present an electrical scheme for local spin manipulation based on g- tensor modulation resonance (g-TMR), functionally equivalent to electron spin resonance

(ESR) but without the use of time dependent magnetic fields.

The technique of g-TMR enables three-dimensional control of electron spins in nanometer-scale geometries using a single voltage signal. These results provide a compelling proof of concept that quantum spin Information can be locally manipulated using high-speed electrical circuits. Furthermore, recent measurements of hybrid ferromagnet / semiconductor www.thetoppersway.com i Spintronics hetero structures under optical illumination reveal that nuclear spins become highly polarized at low temperatures.

We explore the potential for exploiting this behavior to create complex nuclear domains and arrays in lithographically patterned structures. A time-resolved polarization microscope is used to directly image the nuclear landscape in hybrid nanostructures, demonstrating the ability to design and control polarization patterns in the semiconductor.

These experiments investigate the electronic, photonic, and magnetic manipulation of electron and nuclear spins in a variety of semiconductor structures and focus on investigating the underlying physics for quantum information processing in the solid state.

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CONTENTS

INTRODUCTION 1

ELECTRONICS VS SPINTRONICS 5

SPIN DEVICES 7

GIANT MAGNETO RESISTANCE 8

MEMORYCHIPS 10

SENSORS 12

SPIN-VALVE TRANSISTOR 14

WORKING OF SPINTRONIC DEVICES 18

CONCLUSION 19

REFERENCES 20

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INTRODUCTION

In the weird world of quantum mechanics the fundamental, particle, electron possesses a property Called ‘spin’. It is not the sort of spin used in common everyday speech but, the angular momentum or the rotational momentum of a subatomic particle that creates its own tiny magnetic field. By exploiting this spin property, in a field called spintronics, computer scientists and physicists have the potential to revolutionise the basis of computer processing and storage technologies.

‘Spintronics’ can be a fairly new term for you but the concept isn’t so very exotic this technological discipline aims to exploit the subtle and mind-bending esoteric quantum properties of the electron to develop a new generation of electronic devices. The word itself is a blend of electronics with spin, the quantum property it exploits. Like so many words applied to the subatomic realm, you can refer spin figuratively as a convenient label for a property that has no equivalent in gross matter.

Every electron exists in one of two states, namely, spin-up and spin-down, with its spin either +1/2 or - 1/2 (refer Figs 1 and 2). In other words, an electron can rotate either clockwise or anticlockwise around its own axis with constant frequency. The two possible spin states naturally represent ‘0’ and ‘1’ states in logical operations. And just because of this it is possible to make a sandwich of gold atoms between two thin films of magnetic material www.thetoppersway.com 1 Spintronics that acts as, a filter or a valve permitting only the electrons in one of the two states to pass.

The filter can be changed from one state to the other using a brief and tiny burst of current.

There are total three categories of spintronics based devices: 1) ferromagnetic metallic alloy based devices, 2) semiconductor based devices and 3) the devices that manipulate the quantum spin states of individual electrons for information processing [8].

Ferromagnetic metallic alloy based devices are mainly used in memory and information storage. They are also termed as magnetoelectronics devices [8]. They rely on the giant magnetoresistance (GMR) or tunnelling magnetoresistance effect. Magnetic interaction is well understood in this category of devices [5].

Semiconductor spintronics devices combine advantages of semiconductor with the concept of magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET. To make semiconductor based spintronic devices, researchers need to address several following different problems. A first problem is creation of inhomogeneous spin distribution. It is called spin-polarisation or spin injection. Spin-polarised current is the primary requirement to make semiconductor spintronics based devices. It is also very fragile state. Therefore, the second problem is achieving transport of spin-polarised electrons maintaining their spin-orientation. Final problem, related to application, is relaxation time. This problem is even more important for the last category devices. Spin comes to equilibrium by the phenomenon called spin relaxation. It is important to create long relaxation time for effective spin manipulation, which will allow additional spin degree of freedom to spintronics devices with the electron charge. Utilizing spin degree of freedom alone or add it to mainstream electronics will significantly improve the performance with higher capabilities.

The third category devices are being considered for building quantum computers. Quantum information processing and quantum computation is the most ambitious goal of spintronics research. The spins of electrons and nuclei are the perfect candidates for quantum bits or

www.thetoppersway.com 2 Spintronics qubits. Therefore, electron spin and nuclear based hardwares are some of the main candidates being considered for quantum computers.

Spintronics based devices offers several advantages over conventional charge based devices. Since magnetised materials maintain their spin even without power, spintronics based devices could be the basis of non-volatile memory device. Energy efficiency is another virtue of these devices as spin can be manipulated by low-power external magnetic field. Miniaturisation is also another advantage because spintronics can be coupled with conventional semiconductor and optoelectronic devices.

However, temperature is still a major bottleneck. Practical application of spintronics needs room-temperature ferromagnet in semiconductors. Making such materials represents a substantial challenge for materials scientists.

Spin of Electrons

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From this simple device it’s hoped to make incredibly tiny chips that will act as super- fast memories whose contents will survive loss of power. The adjective is spintronic. The ability to exploit spin in semiconductors promises new logic devices. With enhanced fimctiona1ity higher speed, and reduced power consumption, and might spark a revolution in the semiconductor industry. So far the problem of injecting electrons with a controlled spin direction has held up the realization of such spintronic devices.

Spin up and down allows two electrons for each set of spatial quantum numbers.

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ELECTRONICS VS SPINTRONICS

One of the most inherent advantages of spintronics over electronics is that magnets tend to stay magnetised, which is sparking in the industry an interest for replacing computers’ semiconductor-based components with magnetic ones, starting with the random access memory (RAM), Let me tell you an example: You are in the mid of documenting a project presentation that you need to present tomorrow morning and you face an electric power failure. Your UPS was not recharged and, the worst part of all, you didn’t save your presentation. I am sure that a condition like this is enough, to leave you back, pulling your hair, for now you have to do the same task right from the scratch.

You need to do all this just because all the information that is stored via electron charges is lost as soon as you face the power failure. And that is why before turning a computer off, you are bound to save your new work to a disk.

Imagine a computer that retains all the information put into it: it’s really possible with all-magnetic RAM. Most importantly, there would be no ‘boot-up’ waiting period when the power is first turned on a great advantage, especially for laptop users.

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Another promising feature of spintronics is that it does not require the use of unique and specialised semiconductors, thereby allowing it to work with common metals like copper, alumimum, and silver. So the cost of such devices for you is unlikely to be high even in the beginning.

Magnetic Domains in a Spin Valve

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SPIN DEVICES

A spin-valve device consists of two ferromagnetic layers separated by anon-magnetic metallic spacer layer. The magnetisation direction (or direction of the net magnetic field of the layer) of one of the two ferromagnetic layers is pinned by an adjacent antiferromagnetic layer.

When the sensor is passed over the magnetic medium,-it sees a small applied field that causes the magnetisation of the second ferromagnetic (free) layer to rotate in the plane of the layer to align with this field and causes a reduction in the resistance of the spin-valve device.

The nature of the reversal of the magnetisation is complex due to magnetic interactions between the antiferromagnetic layer and the pinned ferromagnetic layer.

Schematic Diagram of a spin valve

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GIANT MAGNETO RESISTANCE

Giant Magneto resistance (GMR) devices

The read heads in modern hard drives and non-volatile, magnetic random access memory (MRAM) are the two application of GMR effect. In 1988, Albert Fert’s group discovered GMR effect. They observed that when multi layers of alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic field, they exhibit large change in electric resistance, which also known as magnetoresistance .

Figure1: Giant magneto resistance effect; (a) electron transport takes place when magnetization direction of both ferromagnetic regions aligned parallel to each other, (b) electrons are facing high resistance and scattered away near interface when magnetization direction of both ferromagnetic regions are opposite to each other (b).

www.thetoppersway.com 8 Spintronics The change in resistance depends on the relative orientation of the magnetization in magnetic layers. The resistance to passage of current is low when the ferromagnetic layers align in the same direction and transfer of current takes place dynamically (fig 1 (a)). If they align themselves in opposite directions electrons scattering occurs near interface and a high resistance path is produced (fig 1 (b)). The relative orientation of magnetic layers can be altered by the applying external magnetic field . This effect is called spin-valve effect . These multi layers are used to configure the GMR devices.

The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are stored as the minute magnetic areas on the surface of HDD . ‘Zero’ is stored, when the magnetic layers align themselves in one direction and ‘one’ when they align in opposite directions. The read head reads the data by sensing a change in voltage corresponding to a change in resistance. It reads 1 when resistance is higher and 0 when resistance is lower.

Thus, the ability of read head to sense minute changes in voltage corresponding to small changes in magnetic fields will allow data storage at highest packing densities in small magnetic particles. The expected value of storage densities may reach to 100 gigbites per square inch by using synthetic Ferromagnets.

When electron spins are aligned (all spin-up or aft spin-down), these 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.

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Magnetism is already exploited in recording devices such as computer hard disks.

Data is recorded and stored as tiny areas of magnetised iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. This induces corresponding changes in the head’s electrical resistance a phenomenon called magneto resistance.

Spintronics burst on the scene in 1988 when French and German physicists discovered much more powerful giant magneto resistance (GMR). It results from .subtle electron-spin effects in ultra-thin multilayers of magnetic materials which cause huge changes in their electrical resistance when; a magnetic field is applied.

GMR is 200 times stronger than ordinary magneto resistance. IBM soon realized that read heads incorporating GMR materials can sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabytes. In 1997 it launched

GMR read heads into the market worth about $1 billion a year.

The basic GMR device is a three-layer sandwich of a magnetic metal (such as cobalt) with a nonmagnetic metal filling (such as silver). A current passes through the layers consisting of spin-up and spin-down electrons. The electrons oriented in the same direction as the electron spins in a magnetic layer pass through quite easily, while those oriented in the opposite direction are scattered.

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If the orientation of one of the magnetic layers is changed by the presence of a magnetic field, the device will act as a filter or a spin valve, letting through more electrons when the spin orientations in the two layers are the same and fewer electrons when the spin orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.

How magneto resistance works

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MEMORY CHIPS

Physicists have been quick to see further possibilities of spin valves. -The spin valves are not only the highly sensitive magnetic sensors but these can also be made to act as switches by flipping the magnetization in one of the layers. This allows information to be stored as 0s and 1s (magnetisations of the layers parallel or anti parallel) as in a conventional transistor memory device. An obvious application is the magnetic version of the RAM used in your computer.

The advantage of magnetic random access memory (MRAM) is that it is nonvolatile, i.e. information isn’t lost when the system is switched off The main advantages of MRAM devices include lower cost, smaller size, faster speed, and less power consumption. These devices would be much more robust in extreme conditions such as high temperature and high level radiation or interference. The US electronics company Honeywell has already shown that arrays of linked MRAMs could be made to work. The potential market for MRAMs is worth $100 billion annually.

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For the past three years or so, researchers around the world have been working hard on a range of MRAM devices. A particularly promising device is the magnetic tunnel junction that has two magnetic layers separated by an insulating metal-oxide layer Electrons can tunnel from one layer to the other only when magnetizations of the two layers in the same direction. Other wise the resistance is high in fact, a thousand times higher than in the standard spin valve.

Even-more interesting are the devices that combine the magnetic layers with semiconductors like silicon. The advantage of using silicon is that it is still a favorite with the electronics industry and is likely to remain so- Such hybrid devices could be made to behave more A Ttke conventional transistors. These could be used as non-volatile logic elements that could be reprogrammed using software during actual processing to create an entirely new type of very fast computing.

Inductive write/GMR read head

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SENSORS

GMR sensors are already being developed in the UK. These have a wide range of applications and their market is worth $8 billion a year.

Applications include:

• Fast and accurate position and motion sensing of mechanical components in precision

engineering and robotics.

• All kinds of automotive sensors for fuel handling systems, electronicengine control,

anti-skid systems, speed control, and navigation.

• Missile guidance.

• Position and motion sensing in computer video games.

• Key-hole surgery and post-operative care.

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SPIN-VALVE TRANSISTOR

A new type of magnetic field sensor is the spin-valve transistor (Fig. 5). This transistor is based on the magneto resistance found in. multilayers (for example, in

Co/Cu/Co). Usually, the resistance of a multiplayer is measured with the current-in-plane

(CIP). The CIP configuration suffers from several drawbacks; for example, the CIP magneto resistance is diminished by shunting and diffusive surface scattering. Hence the fundamental parameters of the spin-valve effect, such as the relative contributions of interface and bulk spin-dependent scattering, are difficult to obtain using the CIP geometry items, mainly because the electrons cross all magnetic layers. But a practical difficulty is encountered: the perpendicular resistance of the ultra-thin multilayers is too small to be measured by ordinary techniques.

Band structure of the spin

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Measuring with the current perpendicular to plane (CPP) solves most problems, mainly because the electrons crass all magnetic layers. But a practical difficulty is encountered; the perpendicular resistance of the ultra-thin multilayer is too small to be measured by ordinary techniques.

Schematic cross-section of the spin-valve transistor

Fabrication

The spin-valve transistor consists of silicon emitter, a magnetic multi-layer as the base and silicon collector (Fig. 6). Electrons are injected from the emitter, passing the first

Schottky barrier (semiconductor-metal interface) into the base. Because of the thin base multilayer (10 nm), most electrons are not directed to the base contact and travel perpendicular through the multilayer across the second Schottky barrier. These electrons form the collector-current.

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Figure Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in transport channel (a) it enables current flow from source to drain. With applied gate voltage, electrons change their spin state from parallel to anti parallel to the direction of magnetization of ferromagnetic layer (b) this offers high resistance to flow of current. Therefore, electron scattering occurs at drain and no current flow from source to drain .

A Co/Cu multilayer is sputtered on one of the two silicon substrates and these are pressed together at the last second of the sputter deposition. Because of the smoothness and freshness of the metal surfaces, spontaneous adhesion occurs at room temperature. A metal layer between two crystalline semiconductors is accomplished and the bond proves stronger than silicon. Through lithography processes and wet chemical etching of the top substrate and the metal base, spin-valve transistors are fabricated.

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Magnetic sensitivity

The number of electrons that reach the collector increases exponentially with the mean free path of the electrons in the base. The mean free path varies with the applied magnetic field; hence the collector current becomes strongly magnetic field-dependent.

Example of a LED structure device with magnetic semi conductor

The collector current variation at low temperatures is more than 400 per cent. Even larger variations are expected with higher-quality bases; (The hysteresis is caused by the hysteresis of the magnetic layers.) lie extreme magneto sensitivity makes the transistor an interesting device for high technology read-heads for high-density hard disks and magnetic

RAMs.

Spin injection and spin-polarized transport

The spintronic device requires efficient transport of generated non-equilibrium spin (spin- polarized current) across the electrode/sample interface. The transport of non-equilibrium spin across interface is called spin injection. The non-equilibrium spin can be injected by www.thetoppersway.com 19 Spintronics driving ordinary current through ferromagnetic electrode to sample. The current can be driven in plane plan of interface called 'current in plane (CIP) geometry' (fig 3(a)) or perpendicular to the interface called 'current perpendicular to plane (CIP) geometry' (fig 3(b)). The spin can be also injected by optical method. The efficiency of spin injection is determined by rate of accumulation of non-equilibrium spin in sample. There are several proposed ways to transport spin-polarized current across interface. These are: (1) formation of Ohmic contact between electrode-sample interface, (2) Ballistic electron injection, (3) electron tunneling from space charge region and, (4) Hot spin injection.

Figure 3 current flow across interfaces; current Figure 4 Spin injection in non ferromagnetic flow in the plane geometry (CIP) (a), current region via ferromagnetic region; equivalent flow perpendicular to the plane geometry (CPP) circuit diagram for ferromagnet/non- ferromagnet interface (a) accumulation of (b) [4] nonequilibrium spin at non-ferromagnetic region (b) non-equilibrium spin state in non- ferromagnetic region (c) [4].

Ohmic injection

The most basic approach to spin injection is through the perfect Ohmic contact between ferromagnetic/non-magnetic (F/N) interfaces (fig 4 (a)). The interface can be produce by taking metals or semiconductors or superconductors as non-magnetic region with ferromeget. The degree of spin injection in non-magnetic region depends on the ratio of the conductivities of ferromagnetic region (F) and non-magnetic region (N). For typical conductivity mismatch, when conductivity of F region ≤ N region, higher the spin injection efficiency (fig 4(b) and www.thetoppersway.com 20 Spintronics (c)). When conductivity of F region ≥ non-magnetic region, smaller the spin injection efficiency. This phenomenon is called “conductivity mismatch”. In the case of ferromagnet/semiconductor interface, Ohmic contacts resulted from the doping of semiconductor surface. However, doping leads to loss of spin polarization by spin-flip scattering. The electrochemical potential of N region increases with spin injection. The difference of spin dependent electrochemical potentials generates effective resistance δR on either side of F/N interface. In superconductor/F interface, increase in total resistance with spin injection results in switching superconducting state to normal state of much higher resistance.

WORKING OF SPINTRONIC DEVICES

The information is stored (written) into spins as a particular spin orientation (up or down). The spins, being attached to mobile electrons, catty the information along a wire, and the information is read at a terminal.

Spin orientation of conduction electrons survives for a relatively long time

(nanoseconds, compared to term of nanoseconds during which electron momentum decays).

This makes spintronic devices particularly attractive for memory storage and magnetic sensor applications, and for quantum computing where electron spin would represent a bit (called qubit) of the information.

www.thetoppersway.com 21 Spintronics Spin Relaxation

Non-equilibrium spin accumulates in non-magnetic region due to process of spin injection. It comes to equilibrium by the phenomenon called spin relaxation . The rate of accumulation of non-equilibrium spin depends on the spin relaxation. Electrons can remember their spin state for finite period of time before relaxing. That finite time period is called ‘Spin lifetime’. Longer lifetime is more desirable for data communication application while shorter for fast switching. The distance traveled by the electron without loosing spin state is called ‘Spin diffusion length. It is most important variable in spintronic devices, which determines maximum allowable thickness of the non-magnetic region in device. It is also depend on spin lifetime. There are four proposed ways by which conduction electrons of metals and semiconductors relax: (A) The Elliott-Yafet mechanism, (B) The D’yakonov-Perel’ mechanism, (C) The Bir-Aronov-Pikus mechanism, and (D) hyperfine-interaction.

Elliot-Yafet Mechanism

Elliot (1954) first suggested that electron spin relaxation occurs via momentum scattering. Momentum scattering occurs when lattice ions or photons bring on spin-orbital coupling in the electron wave function. This spin-orbital coupling introduces wave functions of opposite spin. Now, electron wave functions with related spin have an admixture of the opposite spin state. These combinations of spin-up and spin-down momentum lead to relaxation of electron spin. The mechanism is dominant in small-gap semiconductors with large spin-orbit splitting.

D’yakonove-Perel’ Mechanism

This mechanism comes into play, when the systems lack inversion symmetry. The electrons feel an effective magnetic field, resulting from the lack of inversion symmetry, and from spin-orbit interaction. These fluctuating magnetic fields randomly change the magnitude and direction of electron spin precession. They also randomize the spin. This spin randomization is more effective than momentum scattering. Therefore, spin dephasing occurs because of the www.thetoppersway.com 22 Spintronics momentum dependent spin precession along with momentum scattering. This mechanism plays important role with increase in temperature and increase in band gap.

Bir-Aronov-Pikus Mechanism

The holes also possess spin. The spin of hole can be exchange with conduction electrons. These exchanges proceed through scattering and lead to spin relaxation of conduction electron in p-doped semiconductors (Bir, 1975). Holes have shorter spin coherence time and spin exchange between electrons and holes is very effective. Ultimately, it will leads to spin decoherence. This mechanism is of importance at low temperatures.

Hyperfine-interaction Mechanism

Hyperfine-interaction comes from the magnetic interaction between the magnetic momentum of nuclei and electrons. In semiconductor hetrostructures, this mechanism is responsible for spin dephasing of localized or confined electron spins.

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CONCLUSION

Spintronics is still in its infancy and it’s difficult to predict how it will evolve. New physics is being discovered and new materials are being developed, such as magnetic semiconductors and exotic oxides that manifest an even more extreme effect called colossal magneto resistance.

So what is the future of Spintronics? Two years ago, several experiments demonstrated huge progress in transporting spins over long distances and in high electric fields. This year, electrical spin injection, one of the main remaining obstacles of Spintronics, is on the way to be solved.

Injecting spins with an efficiency of a few per cent using planar contacts or with the tip of a scanning tunneling microscope is useless for commercial devices. But techniques for highly efficient spin injection with planar contacts are being developed by various groups and will probably prove successful in the near future.

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REFERENCES

1. Robert Matthews “Take a spin” New Scientist, Feb. 28, 1998, P. 24-28. 2. Peter Rodgers “Giants in their field” New Scientist, Feb. 10, 1996, P. 34-37. 3. Stuart A. Wolf and Daryl Treger “Spintronics: A new paradigm for electronics for the new millennium” IEEE transactions on magnetics, Vol, 36, No.5 September 2000. 4. S. Das Sharma, Jaroslav Fabian, Igor zutic “Spintronics: Fundamental and applications” Reviews of modern Physics, Vol. 76, No. 2, April 2004. 5. M Oestreich, M Bender, JH¨ubner, D H¨agele,WWR¨uhle, Th Hartmann, P J Klar, W Heimbrodt, M Lampalzer, K Volz and WStolz “Spin injection, Spin transport and spin coherence” Semiconductor science and Technology 17 (2002) P. 285-297 6. S.A. Wolf, D. D. Awschalom, D. M. Treger “Spintronics: A spin-based electronics vision for the future” Science Vol. 294, Nov. 2001. 7. Robert F. Service “Spintronics innovation bids to bolster bids” Science Vol. 297, July 5 2002.

1. www.seminarsonly.com

2. www.googIe.com

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