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NOBEL PRIZE - PHYSICS

Today’s read-out technology is based on an effect that they observed about 20 years ago

GMR technology may also be regarded as one of the first major applications of nanotechnology

— Photo: AFP

Peter Gruenberg, Professor at Institute of Solid State Research, Germany.

Portable computers, music players, and powerful search engines, all require hard disks where the information is very densely packed. Information on a hard disk is stored in the form of differently magnetized areas. A certain direction of magnetization corresponds to the binary zero, and another direction corresponds to the binary value of one. In order to access the information, a read-out head scans the hard disk and registers the different fields of magnetization.

When hard disks become smaller, each magnetic area must also shrink. This means

that the magnetic field of each byte becomes weaker and harder to read. A more tightly packed hard disk thus requires a more sensitive read-out technique.

Towards the end of the 1990s a totally new technology became standard in the read-out heads of hard disks. This is of crucial importance to the accelerating trend of hard disk miniaturization. Today’s read-out technology is based on a physical effect that these two Nobel Laureates first observed almost 20 years ago. The Frenchman Albert Fert and the German Peter Gruenberg, simultaneously and independently, discovered what is called Giant Magnetoresistance, GMR. It is for this discovery that the two now share the Nobel Prize in Physics.

Originally, induction coils where used in read-out heads, exploiting the fact that a changing magnetic field induces a current through an electric coil. Induction coils are still used for writing information onto the disk. For the read-out function, however, magnetoresistance soon proved better suited.

Electric resistance of materials such as iron may be influenced by a magnetic field. The resistance diminishes along the lines of magnetization when a magnetic field is applied to a magnetic conductor. If the magnetic field is applied across the conductor the resistance increases instead. This (anisotropic) magnetoresistance (MR) was the direct predecessor to giant magnetoresistance as a standard technology in read-out heads.

A prerequisite for the discovery of the GMR-effect was provided by the new possibilities of producing fine layers of metals on the nanometre scale which started to develop in the 1970s.

The GMR technology may also be regarded as one of the first major applications of nanotechnology.

In a metal conductor, electricity is transported in the form of electrons which can move freely through the material and straighter the path of the electrons, the greater the conductance. Electric resistance is due to scattering of electrons. In a magnetic material the scattering of electrons is influenced by the direction of magnetization.

In GMR the very strong connection between magnetization and resistance arises because of the intrinsic rotation of the electron that induces a magnetic moment called spin — which is directed in either one of two opposite directions. In a magnetic material, most of the spins point in the same direction.

A smaller number of spins, however, always point in the opposite direction, anti-parallel to the general magnetization. This imbalance gives rise to magnetization and scattering of electrons to a smaller or greater degree especially in the interfaces between materials.

GMR works on the following principle. It consists of a layer of non-magnetic metal sandwiched between two layers of a magnetic metal. Within the magnetic material, and especially at the interface between the magnetic and the non-magnetic material, electrons with different spins are scattered differently.

Here we will regard the case where electrons scatter more if their spin is anti-parallel to the general direction of magnetization.

This implies that the resistance will be larger for these electrons than for those with a spin which is parallel to the direction of magnetization. When, next, the electrons enter the magnetic material they all scatter to the same degree, independent of their spin direction.

At the second interface and within the last layer of magnetic material, electrons with anti-parallel spin will once again scatter more than electrons with parallel spin.

In the case when both the magnetic layers are magnetized in the same direction, most electrons will have a parallel spin and move easily through the structure. The total resistance will hence be low. However, if the magnetizations of the two layers are opposed, all electrons will be in the state of anti-parallel spin in one of the two layers leading to nil electron movement and total resistance.

In a read-out head scanning a hard disk the magnetization of the two magnetic layers will be alternately in parallel and in anti-parallel to one another. This will lead to a variation in the resistance, and the current, through the read-out head. If the current is the signal leaving the read-out head, a high current may signify a binary ‘one’ and a low current may signify a zero.

GMR quickly became the standard.

Scientists realised in mid-1980s what new possibilities nanometre-sized layers might offer. Albert Fert and his colleagues created some thirty alternating layers of iron and chromium — composed of just a few strata of atoms each using complex technique. Peter Gruenberg’s group created a somewhat simpler system composed of just two or three layers of iron with a layer of chromium sandwiched in between.

Partly because he had used many more layers, Fert registered a greater magnetoresistance

than Gruenberg. The French group saw a magnetization-dependent change of resistance of up to 50 per cent, whereas the German group saw a 10 per cent difference at the most. The basic effect and the physics behind it were however identical in the two cases.

For the new technology to be commercialized it was however necessary to find an industrial process to create the layers. Sputtering technology was the answer.

And GMR technology became a standard in hard disks.

GMR technology had other spin-offs. The first one was spintronics which uses not only the electron’s spin but also the electrical charge.

Spintronics uses electrically insulating material, instead of a non-magnetic metal, sandwiched between two layers of a magnetic metal.

No electric current should be able to pass through the insulating layer, but if it is thin enough, electrons may sneak through, using a quantum mechanical effect called tunnelling.

Therefore this new system is called TMR, Tunnelling Magnetoresistance.

With TMR an even larger difference in resistance can be created by very weak magnetic fields, and the newest generation of read-out heads uses this technology.

Another application of spintronics, which has already begun to emerge — a magnetic working memory called MRAM.

To supplement the hard disk, where information is stored permanently, computers need a faster working memory.

This is usually called RAM, random access memory, where information can be lost unless saved into a hard disk.

MRAM uses TMR to read and write information and hence be used as a working memory.

It could thus develop into a universal memory which would replace both the traditional RAM and the hard disk. The compactness of such a system may prove to be particularly useful in small embedded computer systems — in everything from kitchen stoves to automobiles.

The discovery of the GMR effect followed by spintronics is a perfect example of how fundamental science and new technology intertwine and reinforce each other. — Excerpted from public information available on http://nobelprize.org

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