This is based on the study conducted by me for the Btech Seminar the Seminar presentation can be downloaded from here

In our conventional electronic devices charge of electron used to achieve functionalities and also semi conducting materials for logical operation and magnetic materials for storage, but spintronics manipulates the electron spin and resulting magnetic moment,to achieve improved functionalities and also magnetic materials are used for processing and storage. These spintronic devices are more versatile and faster than the present one.

Spintronics (” SPIN TRansport electrONICS “), also known as magneto electronics, is an emerging technology that exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.}

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. During that 50-year period, the world witnessed a revolution based on a digital logic of electrons. From the earliest transistor to the remarkably powerful microprocessor in your desktop computer, most electron IC devices have employed circuits that express data as binary digits, or bits—ones and zeros represented by the existence or absence of electric charge.

Moore’s Law, which holds that microprocessors will double in power every 18 months as electronic devices shrink and more logic is packed into every chip. Moore’s Law has run out of momentum as the size of individual bits approaches the dimension of atoms—this has been called the end of the silicon road map. For this reason and also to enhance the multi-functionality of devices investigators have been eager to exploit another property of the electron—a characteristic known as spin. Spin is a purely quantum phenomenon .

A BRIEF HISTORY

Two experiments in 1920’s suggested spin as an additional property of the electron. One was the closely spaced splitting of Hydrogen spectral lines, called fine structure. The other was Stern –Gerlach experiment, which in 1922 that a beam of silver atoms directed through an inhomogeneous magnetic field would be forced in to two beams. These pointed towards magnetism associated with the electrons.

In 1965, Gordon Moore, Intel’s co-founder, predicted that the number of transistors on an integrated circuit would double every 18 month. That prediction, now known as Moore’s Law, effectively described a trend that has continued ever since, but the end of that trend—the moment when transistors are as small as atoms, and cannot be shrunk any further—is expected as early as 2015.

Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson (more commonly known as Lord Kelvin) in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This effect was later termed Anisotropic Magnetoresistance (AMR) to distinguish it from GMR. Spintronics came into light by the advent of Giant Magneto Resistance (GMR) in 1988. GMR is 200 times stronger than ordinary Magneto Resistance. It results from subtle electron – spin effects in ultra multi-layers of magnetic materials that cause a huge change in electrical resistance. Giant magneto resistance is a quantum mechanical magneto resistance effect observed in thin film structures composed of alternating ferromagnetic and non magnetic layers.

The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grunberg for the discovery of GMR. The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an anti-parallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for anti-parallel alignment. GMR is used by hard disk drive manufactures.

GIANT MAGNETO RESISTANCE

GMR was independently discovered in 1988 in Fe/Cr/Fe trilayers by a research team led by Peter Grunberg, who owns the patent, and in Fe/Cr multilayers by the group of Albert Fert of the University of Paris-Sud, who first saw the large effect in multilayers (up to 50% change in resistance) that led to its naming, and first correctly explained the underlying physics. The discovery of GMR is considered as the birth of Spintronics. Grunberg and Fert have received a number of prestigious prizes and awards for their discovery and contributions to the field of Spintronics, including the Nobel Prize in  Physics in 2007.

Like other magnetoresistive effects, giant magnetoresistance (GMR) is the change in electrical resistance of some materials in response to an applied magnetic field. It was discovered that the application of a magnetic field to magnetic metallic multilayers such as Fe/Cr and Co/Cu, in which ferromagnetic layers are separated by nonmagnetic spacer layers of a few nm thick, results in a significant reduction of the electrical resistance of the multilayer. This effect was found to be much larger than other magnetoresistive effects that had ever been observed in metals and was, therefore, called “giant magnetoresistance”. In Fe/Cr and Co/Cu multilayers the magnitude of GMR can be higher than 100% at low temperatures.

The change in the resistance of the multilayer arises when the applied field aligns the magnetic moments of the successive ferromagnetic layers, as is illustrated schematically in the figure below. In the absence of the magnetic field the magnetizations of the ferromagnetic layers are antiparallel. Applying the magnetic field, which aligns the magnetic moments and saturates the magnetization of the multilayer, leads to a drop in the electrical resistance of the multilayer. Usually resistance of multilayer is measured with the Current in Plane (CIP). For instance, Read back magnetic heads uses this property. But this suffers from several drawbacks such as; shunting and channeling, particularly for uncoupled multilayers and for thick spaced layers diminish the CIP magneto resistance. Diffusive surface scattering reduces the magneto resistance for sandwiches and thin multilayers.

To erase these problems we measure with Current Perpendicular to the Plane (CPP), mainly because electrons cross all magnetic layers, but a practical difficulty is encountered the perpendicular resistance of ultra thin multilayers is too small to be measured by ordinary techniques.

The use of Micro fabrication techniques for CPP measurements, from 4.2 to 300k was first shown for Fe/Cr multilayers, where the multilayers were etched into micropillars to obtain a relatively large resistance (a few milli ohms). These types of measurements have confirmed the larger MR for the CPP configuration, but they suffer from general complexity of realisation and measurement techniques. Experiments using electro deposited nanowires showed CPP MR up to 15% at room temperature

TYPES OF GMR

MULTILAYER:

Two or more ferromagnetic layers are separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe). At certain thicknesses the RKKY¹ coupling between adjacent ferromagnetic layers becomes anti ferromagnetic, making it energetically preferable for the magnetizations of adjacent layers to align in anti-parallel. The electrical resistance of the device is normally higher in the anti-parallel case and the difference can reach more than 10% at room temperature. The interlayer spacing in these devices typically corresponds to the second anti ferromagnetic peak in the AFM-FM oscillation in the RKKY coupling. The GMR effect was first observed in the multilayer configuration, with much early research into GMR focusing on multilayer stacks of 10 or more layers.

GRANULAR:

Granular GMR is an effect that occurs in solid precipitates of a magnetic material in a non-magnetic matrix. In practice, granular GMR is only observed in matrices of copper containing cobalt granules. The reason for this is that copper and cobalt are immiscible, and so it is possible to create the solid precipitate by rapidly cooling a molten mixture of copper and cobalt. Granule sizes vary depending on the cooling rate and amount of subsequent annealing. Granular GMR materials have not been able to produce the high GMR ratios found in the multilayer counterparts.

PSEUDO SPIN VALVE:

Pseudo-spin valve devices are very similar to the spin valve structures. The significant difference is the coercivities of the ferromagnetic layers. In a pseudo-spin valve structure a soft magnet will be used for one layer; where as a hard ferromagnet will be used for the other. This allows an applied field to flip the magnetization of the hard ferromagnet layer. For pseudo-spin valves, the non-magnetic layer thickness must be great enough so that exchange coupling minimized. This reduces the chance that the alignment of the magnetization of adjacent layers will spontaneously change at a later time.

TUNNEL MAGNETORESISTANCE

The Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in magnetic tunnel junctions (MTJs). This is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a few nanometers), electrons can tunnel from one ferromagnet into the other. Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly quantum mechanical phenomenon.

Magnetic tunnel junctions are manufactured in thin film technology. On an industrial scale the film deposition is done by magnetron sputter deposition; on a laboratory scale molecular beam epitaxy, pulsed laser deposition and electron beam physical vapor deposition are also utilized. The junctions are prepared by photolithography.

The direction of the two magnetizations of the ferromagnetic films can be switched individually by an external magnetic field. If the magnetizations are in a parallel orientation it is more likely that electrons will tunnel through the insulating film than if they are in the oppositional (antiparallel) orientation. Consequently, such a junction can be switched between two states of electrical resistance, one with low and one with very high resistance.

The effect was originally discovered in 1975 by M. Jullière (University of Rennes, France) in Fe/Ge-O/Co-junctions at 4.2 K. The relative change of resistance was around 14%, and did not attract much attention. In 1991 T. Miyazaki (University Tohoku, Japan) found an effect of 2.7% at room temperature. Later, in 1994, Miyazaki found 18% in junctions of iron separated by an amorphous aluminum oxide insulator and J. Moodera found 11.8% in junctions with electrodes of CoFe and Co. The highest effects observed to date with aluminum oxide insulators are around 70% at room temperature.

Since the year 2000, tunnel barriers of crystalline magnesium oxide (MgO) are under development. In 2001 Butler and Mathon independently made the theoretical prediction that using iron as the ferromagnet and MgO as the insulator, the tunnel magnetoresistance can reach several thousand percent. The same year, Bowen et al. were the first to report experiments showing a significant TMR in a MgO based magnetic tunnel junction [Fe/MgO/FeCo(001)]. In 2004, Parkin and Yuasa were able to make Fe/MgO/Fe junctions that reach over 200% TMR at room temperature. Today (2009) effects of up to 600% at room temperature and more than 1100% at 4.2 K are observed in junctions of CoFeB/MgO/CoFeB .

The read-heads of modern hard disk drives work on the basis of magnetic tunnel junctions. TMR, or more specifically the magnetic tunnel junction, is also the basis of MRAM, a new type of non-volatile memory. The 1st generation technologies relied on creating cross-point magnetic fields on each bit to write the data on it, although this approach has a scaling limit at around 90-130 nm. There are two 2nd generation techniques currently being developed: Thermal Assisted Switching (TAS) and Spin Torque Transfer (STT) on which several companies are working Further, magnetic tunnel junctions are also used for sensing applications.

SPIN VALVE

Two ferromagnetic layers are separated by a thin (about 3 nm) non-ferromagnetic spacer, but without RKKY coupling. If the coercive fields of the two ferromagnetic electrodes are different it is possible to switch them independently. Therefore, parallel and anti-parallel alignment can be achieved, and normally the resistance is again higher in the anti-parallel case. This device is sometimes also called a spin valve. Spin valve GMR is the configuration that is industrially most useful, and is used in hard drives. Stuart Parkin and two groups of colleagues at IBM’s Almaden Research Center, San Jose, Calif, quickly recognized its potential, both as an important new scientific discovery in magnetic materials and one that might be used in sensors even more sensitive than MR heads. Parkin first wanted to reproduce the Europeans’ results. But he did not want to wait to use the expensive machine that could make multilayers in the same slow-and-perfect way that Grunberg and Fert had. So Parkin and his colleague, Kevin P. Roche, tried a faster and less-precise process common in disk-drive manufacturing: sputtering. To their astonishment and delight, it worked! Parkin’s team saw GMR in the first multilayers they made. This demonstration meant that they could make enough variations of the multilayers to help discover how GMR worked, and it gave Almaden’s Bruce Gurney and co-workers hope that a room-temperature, low-field version could work as a super-sensitive sensor for disk drives.
The key structure in GMR materials is a spacer layer of a non magnetic metal between two magnetic metals. Magnetic materials tend to align themselves in the same direction. So if the spacer layer is thin enough, changing the orientation of one of the magnetic layers can cause the next one to align itself in the same direction. Increase the spacer layer thickness and you’d expect the strength of such “coupling” of the magnetic layers to decrease. But as Parkin’s team made and tested some 30,000 different multilayer combinations of different elements and layer dimensions, they demonstrated the generality of GMR for all transition metal elements and invented the structures that still hold the world records for GMR at low temperature, room temperature and useful fields. In addition, they discovered oscillations in the coupling strength: the magnetic alignment of the magnetic layers periodically swung back and forth from being aligned in the same magnetic direction (parallel alignment) to being aligned in opposite magnetic directions (anti-parallel alignment). The overall resistance is relatively low when the layers were in parallel alignment and relatively high when in anti-parallel alignment. For his pioneering work in GMR, Parkin won the European Physical Society’s prestigious 1997 Hewlett-Packard Europhysics Prize along with Gruenberg and Fert. Searching for a useful disk-drive sensor design that would operate at low magnetic fields, Bruce Gurney and colleagues began focusing on the simplest possible arrangement: two magnetic layers separated by a spacer layer chosen to ensure that the coupling between magnetic layers was weak, unlike previously made structures. They also “pinned” in one direction the magnetic orientation of one layer by adding a fourth layer: a strong anti ferromagnet. When a weak magnetic field, such as that from a bit on a hard disk, passes beneath such a structure, the magnetic orientation of the unpinned magnetic layer rotates relative to that of the pinned layer, generating a significant change in electrical resistance due to the GMR effect. This structure was named the spin valve. Gurney and colleagues worked for several years to perfect the sensor design that is used in the new disk drives. The materials and their tiny dimensions had to be fine-tuned so they 1) could be manufactured reliably and economically, 2) yielded the uniform resistance changes required to detect bits on a disk accurately, and 3) were stable — neither corroding nor degrading — for the lifetime of the drive. “That’s why it’s so important to understand the science,” Parkin says. “IBM’s intensive studies of GMR enabled us to enhance considerably the performance of some low-field sensors.”

The chief source of GMR is “spin dependent” scattering of electrons. Electrical resistance is due to scattering of electrons within a material. By analogy, consider how fast it takes you to drive from one town to another. Without obstacles on a freeway, you can proceed quickly. But if you encounter heavy traffic, accidents, road construction and other obstacles, you’ll travel much slower. Depending on its magnetic direction, a single-domain magnetic material will scatter electrons with “up” or “down” spin differently. When the magnetic layers in GMR structures are aligned anti-parallel, the resistance is high because “up” electrons that are not scattered in one layer can be scattered in the other. When the layers are aligned in parallel, all of the “up” electrons will not scatter much, regardless of which layer they pass through, yielding a lower resistance.

SPIN TRANSFER TORQUE

Spin-transfer torque is an effect in which the orientation of a magnetic layer in a tunnel magnetoresistance or spin valve can be modified using a spin-polarized current. Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic to the carrier. 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, one can produce a spin-polarized current. If a spin-polarized current is directed into a magnetic layer, angular momentum can be transferred to the 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.

Spin-transfer torque can be used to flip the active elements in magnetic random access memory. Spin-transfer torque random access memory, or STT-RAM, has the advantages of lower power consumption and better scalability over conventional MRAM which uses magnetic fields to flip the active elements. The name STT-RAM was first coined by Grandis, Inc. Spin-transfer torque technology has the potential to make possible MRAM devices combining low current requirements and reduced cost; however, the amount of current needed to reorient the magnetization is at present too high for most commercial applications, and the reduction of this current density alone is the basis for current academic research in spin electronics.

Hynix Semiconductor and Grandis formed a partnership in April 2008 to explore commercial development of STT-RAM technology. On August 1, 2011, Grandis announced that it had been purchased by Samsung for an undisclosed sum. Hitachi and Tohoku University demonstrated a 32-Mbit STT-RAM in June 2009.

1It refers to a coupling mechanism of nuclear magnetic moments or localized inner d or f shell electron spins in a metal by means of an interaction through the conduction electrons.

ADVANTAGES

1. Non-Volatile memory

2. Low power Consumption

3. Spintronics does not require unique and specialised semiconductors

4. Spin life time is relatively long on the order of nanoseconds.

5. compared to normal RAM chips,

6. spintronic RAM chips will:

• increase storage densities

• have faster operation

LIMITATIONS

1. Controlling spin for long distances.
2. Difficult to INJECT and MEASURE spin.
3. Interference with nearest field.
4. Control of spin in Silicon is difficult.

CONCLUSION

Interest in spintronics arises, in part, from the looming problem of exhausting the fundamental physical limits of conventional electronics. 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.

This is based on the study conducted by me for the Btech Seminar the Seminar presentation can be downloaded from here

 

REFFERENCES

[1] Stuart A. Wolf,Jiwei Lu, Mircea R.Stan,Eugene Chen and Daryl M. Tregger The Promise of Nanomagnetics and Spintronics for future Logic and Universal memory IEEE,2010.

[2] Michael E. Flatte Spintronics IEEE Transactions on Electronic Devices Vol 54, No.5 ,2007.

[3] Shoji Ikeda, Jun Hayakawa Magnetic tunneling Junctions for spintronics Memories and Beyond IEEE Transactions on Electronic Devices Vol 54, No.5 ,2007.

[4] Feynman, Leighton, Sands Feynman Lectures on Physics ,Volume 3