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The Giant within Small Devices
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Lying at the heart of the computer which you are using to read this article is a memory retrieval system based on the discoveries for which the 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg. They discovered, quite independently, a new way of using magnetism to control the flow of electrical current through sandwiches of metals built at the nanotechnology scale.
150 years ago, William Thomson observed very small changes in the electrical properties of metals when they were placed in a magnetic field, a phenomenon he named 'Magnetoresistance'. In due course, his finding found application, magnetically-induced current fluctuations becoming the underlying principle for reading computer memories. Then, in 1988, Fert and Grünberg, working with specially-constructed stacks made from alternating layers of very thinly-spread iron and chromium, unexpectedly discovered that they could use magnetic fields to evoke much greater increases in electrical resistance than Thomson, or anyone since, had observed. Recognizing the novelty of the effect, Fert named it 'Giant magnetoresistance', and it was only a few years before the improvements, and the miniaturization, it offered led to its adoption in favour of classical magnetoresistance.
Giant magnetoresistance is essentially a quantum mechanical effect depending on the property of electron spin. Using an applied magnetic field to cause the electrons belonging to atoms in alternate metal layers to adopt opposite spins results in a reduction in the passage of electric current, in a similar fashion to the way that crossed polarizing filters block the passage of sunlight. When, however, magnetic fields are used to align the electron spins in different layers, current passes more easily, just as light passes through polarizers aligned in the same direction.
The application of this discovery has been rapid and wide-ranging, dramatically improving information storage capacity in many devices, from computers to car brakes. And while quietly pervading the technology behind our daily lives, the principles of giant magnetoresistance are now being used to tackle problems in wider fields, for instance in the selective separation of genetic material.
(by Adam Smith, Editor-in-Chief, Nobelprize.org)
Advanced Information
The phenomenon called magnetoresistance (MR) is the change of resistance of a conductor when it is placed in an external magnetic field. For ferromagnets like iron, cobalt and nickel this property will also depend on the direction of the external field relative to the direction of the current through the magnet. Exactly 150 years ago W. Thomson (Lord Kelvin) measured the behaviour of the resistance of iron and nickel in the presence of a magnetic field. He wrote “I found that iron, when subjected to a magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electricity across, the lines of magnetization”. This difference in resistance between the parallel and perpendicular case is called anisotropic magnetoresistance (AMR). It is now known that this property originates from the electron spin-orbit coupling. In general magnetoresistance effects are very small, at most of the order of a few per cent.
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The MR effect has been of substantial importance technologically, especially in connection with readout heads for magnetic disks and as sensors of magnetic fields. The most useful material has been an alloy between iron and nickel, Fe20Ni80 (permalloy). In general, however, there was hardly any improvement of the performance of magnetoresistive materials since the work of Kelvin. The general consensus in the 1980s was that it was not possible to significantly improve on the performance of magnetic sensors based on magnetoresistance.
Therefore it was a great surprise when in 1988 two research groups independently discovered materials showing a very large magneto resistance, now known as giant magnetoresistance (GMR). These materials are so called magnetic multilayers, where layers of ferromagnetic and non-magnetic metals are stacked on each other. The widths of the individual layers are of nanometre size — i.e. only a few atomic layers thick. In the original experiments leading to the discovery of GMR one group, led by Peter Grünberg, used a trilayer system Fe/Cr/Fe, while the other group, led by Albert Fert, used multilayers of the form (Fe/Cr)n where n could be as high as 60. Grünberg also reported low temperature magnetoresistance measurements for a system with three iron layers separated by two chromium layers and found a resistance decrease of 10%.
Not only did Fert and Grünberg measure strongly enhanced magnetoresistivities, but they also identified these observations as a new phenomenon, where the origin of the magnetoresistance was of a totally new type. The title of the original paper from Fert’s group already referred to the observed effect as “Giant Magnetoresistance”. Grünberg also realized at once the new possibilities for technical applications and patented the discovery. From this very moment the area of thin film magnetism research completely changed direction into magnetoelectronics /…/
The resistance of a GMR device can be understood from the following somewhat simplified way. A plot of the magnetic configuration for the FM/NM/FM (ferromagnetic/non-magnetic/ferromagnetic) multilayer is made together with the corresponding electron density of state for the two ferromagnetic sides (FM). In the absence of a magnetic field the two FM layers are separated from each other in such a way that they have opposite magnetization directions. In the presence of a magnetic field the magnetizations of the two FM layers will be parallel. An electrical current is sent through the system for both configurations. As already mentioned above the current through the FM layer is composed of two types — one spin up current and one spin down current — and the resistance for these two currents will differ. When an electron leaves the first iron layer and enters the non-magnetic metal there will be additional scattering processes giving rise to extra resistance. Since the spin up and spin down particles have different density of states at the Fermi level (or rather, they originate from energy levels having different character), the resistance not only within the FM layers, but also that originating from the FM/NM interface will be different for the two spins.
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Inside the NM layer the up and down spins will experience the same resistance, but generally this is low compared to those in the FM layers and FM/NM interfaces and can here be neglected. When the electrons enter the second iron layer they will again experience spin dependent scattering at the NM/FM interface. Finally the spin up and spin down electrons go through the second iron layer with the same resistance as in the first iron layer, which still of course differs for the two spins. For simplicity the resistance for the spin up (down) electrons through the FM layer and the scattering at the interface to the NM layer will be called R↑ (R↓). Thus when the two layers have parallel spin polarizations (magnetizations), i.e. in the presence of an external magnetic field (H), the resistance for the spin up channel is 2 R↑ and for the spin down channel it is 2 R↓. Standard addition of resistances for a parallel current configuration gives the following total resistance, RH, in the presence of an external magnetic field; RH = 2R↑R↓/(R↑ + R↓)./…/
Since magnetoresistance deals with electrical conductivity it is obvious that it is the behaviour of the electrons at the Femi surface (defined by the Fermi energy) which is of primary interest. The more spin-polarized the density of states (DOS) at the Fermi energy, i.e., the more N↑ (EF) deviates from N↓(EF), the more pronounced one expects the efficiency of the magnetoelectronic effects to be. In this respect a very interesting class of materials consists of what are called half-metals, a concept introduced by de Groot and co-workers. Such a property was then predicted theoretically for CrO2 by Schwarz in 1986. The name half-metal originates from the particular feature that the spin downband is metallic while the spin up band is an insulator. It is clear that there is a 100% spin polarization at the Fermi level. The theoretical prediction for CrO2 was later confirmed by experiment./…/
Another variation of multilayers in the present context is to grow layered materials with an alternation between metallic and insulating layers. Here the insulating material should be only a few atomic layers thick so that there is a significant probability that electrons can quantum mechanically tunnel through the insulating barrier. In this manner a current can be sent through the multilayer. The first publication on such a system was made by Julliere. This work was done for a trilayer junction with the following structure Fe/amorphous Ge/Co. The experiments were done at low temperature and an effect of about 14% was reported /…/
The discovery by Albert Fert and Peter Grünberg of giant magnetoresistance (GMR) was very rapidly recognized by the scientific community. Research in magnetism became fashionable with a rich variety of new scientific and technological possibilities. GMR is a good example of how an unexpected fundamental scientific discovery can quickly give rise to new technologies and commercial products. The discovery of GMR opened the door to a new field of science, magnetoelectronics (or spintronics), where two fundamental properties of the electron, namely its charge and its spin, are manipulated simultaneously. Emerging nanotechnology was an original prerequisite for the discovery of GMR, now magnetoelectronics is in its turn a driving force for new applications of nanotechnology. In this field, demanding and exciting scientific and technological challenges become intertwined, strongly reinforcing progress.
(http://www.nobelprize.org/nobel_prizes/physics/laureates/2007/advanced.html)
Exercises
1) Translate the terms into Russian: AMR, GMR, electron spin-orbit coupling, electron density, Fermi level, parallel spin polarization, scattering process, energy level, magnetic multilayer
2) Answer the questions:
a) How did W. Thomson (Lord Kelvin) contribute to studying the phenomenon of magnetoresistance?
b) Can you define anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR)?
c) What metals did the groups led by Peter Grünberg and Albert Fert use in their groundbreaking experiments in the 1980s? What did they achieve?
d) Can you describe the resistance of a GMR device?
e) What are half-metals?
f) What is tunneling magnetoresistance?
g) What new field of science did the discovery of GMR lead to?
h) Which new technological possibilities did GMR reveal?
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