Nobel Prize in Physics

Giant magnetoresistance makes life much easier

At the end of the 80's, discovery of phenomenon known as giant magnetoresistance (GMR) changed not only the musical and computing habits of the entire planet but also altered the very landscape of how people think about information, and the ways in which music, movies and ideas can be shared. This year's Nobel Prize in Physics goes to Albert Fert of the Université Paris-Sud in Orsay, France, and Peter Grünberg of the Forschungszentrum in Jülich, Germany, for their discovery of this phenomenon


By MARIJA MITROVIĆ
from Belgrade, SERBIA


Imagine your computer with hard disks as big as your living room. Can you live everyday life without mobile phone, laptop or iPod? Just two decades ago, this was reality. At the end of the 80's , discovery of phenomenon know as giant magnetoresistance (GMR) changed not only the musical and computing habits of the entire planet but also altered the very landscape of how people think about information, and the ways in which music, movies and ideas can be shared. This year's Nobel Prize in Physics goes to Albert Fert of the Université Paris-Sud in Orsay, France, and Peter Grünberg of the Forschungszentrum in Jülich, Germany, for their discovery of this phenomenon. The two scientists independently discovered the phenomenon and published their results in 1988 and 1998, respectively. Applications of this phenomenon have revolutionized techniques for retrieving data from hard disks. The discovery also plays a major role in various magnetic sensors as well as for the development of a new generation of electronics. The use of giant magnetoresistance can be regarded as one of the first major applications of nanotechnology.

Portable computers, music players, and powerful search engines, all require hard disks where the information are very densely packed. Information on a hard disk are stored in the form of differently magnetized areas. A certain direction of magnetization corresponds to the binary zero, and opposite 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 bite becomes weaker and harder to read. A more tightly packed hard disk thus requires a more sensitive read-out technique.

Magnetoresistance is nothing new in science - it is the change in electrical resistance of a material when it is in the presence of an external magnetic field. It was measured 150 years ago by W. Thomson (Lord Kelvin), who found that the resistance of iron and nickel (ferromagnetic materials) would change depending upon the orientation of the magnetic field relative to the material. This is a weak phenomenon known as anisotropic magnetoresistance, changes the conduction in a material by only a few percent at most. Even though this phenomenon is weak, it led to the development of many important technologies, including the parts that allow us to read and write to disks. Today this technology is still in use for writing information onto the disk. For the read-out function, however, GMR proved better suited.

In a metal conductor, electricity is transported in the form of electrons which can move freely through the material. The current is conducted because of the movement of electrons in a specific direction, the straighter the path of the electrons, the greater the conductance of the material. Electric resistance is due to electrons diverging from their straight path. The more the electrons scatter, the higher the resistance. In a magnetic material the scattering of electrons is influenced by the direction of magnetization. Every electron has intrinsic angular momentum called spin. Spin of free electron can be oriented in two directions, parallel or anti-parallel to magnetic field. Most of the spins in magnetic material point in the same direction (in parallel). A smaller number of spins, however, always point in the opposite direction, anti-parallel to the general magnetization. This imbalance gives rise not only to the magnetization as such, but also to the fact that electrons with different spin are scattered to a smaller or greater degree against irregularities and impurities, and especially in the interfaces between materials. Material properties will determine which type of electron is scattered the most.

GMR-effect can be measured in nanometer thickness layer of nonmagnetic metal sandwiched between two layers of a ferromagnetic material. Under the proper circumstance, that the two layers will align their spin in different directions, at zero external magnetic field. In the presence of the external field both layers will align in the same direction. Within the magnetic material, and especially at the interface between the magnetic and the nonmagnetic material, electrons with different spins are scattered differently. For example in some material 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. In the non-magnetic material all electrons scatter to the same degree, independent of their spin direction. 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. This means that no electrons can move easily through the system and the total resistance will therefore be high.

So how can we use this structure in read-out head scanning a hard disk? The magnetization of one of the layer is pinned, while the magnetization of other layer is free to move and so can be influenced by the varying magnetic fields on the hard disk. The magnetization of the two magnetic layers in the read-out head will then 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.

Thank you Peter Grünberg and Albert Fert you made my life much easier, I mean literally.


(Published: 09.11.2007.)

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