Posts Tagged ‘Giant Magnetoresistance’

Nobel lectures: Physics and Chemistry

December 10, 2007

Have fun!

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Nobel 2007: Literature and Physics

October 24, 2007

Uma Mahadevan-DasGupta writes about Dorris Lessing, the 2007 Literature Nobel prize winner:

… even as Harold Bloom derides Lessing’s selection for the Nobel as nothing more than “pure political correctness” and describes her recent work as “fourth-rate science fiction”, Lessing’s latest novel, The Cleft (2007), depicts women as lazy and men as adventurous – to the great irritation of literal-minded feminists. Clearly, her irreverence and creativity (“Laughter is by definition healthy,” she has said famously) continue to explore new frontiers, forcing her readers to think things through for themselves. Typical for a writer whose enduring plea to her readers has always been: “Think wrongly, if you please, but in all cases think for yourself.”

In another piece in the same issue of Frontline,  R Ramachandran explains how giant magnetoresistance (which won the 2007 Physics Nobel) is exploited in hard disk technologies:

Consider the simplest three-layer structure, consisting of a layer of non-magnetic metal sandwiched between two layers of magnetic metal, in which GMR can arise. The current-carrying electrons with different spins experience different resistances within the first magnetic material and at the first interface (between the magnetic and the non-magnetic metals), with larger resistance for electrons that are not aligned in the direction of magnetisation of the metal. As the current enters the non-magnetic material, the resistance is the same for both types, which is generally negligibly lower than that in the magnetic layer. At the second interface and in the second magnetic material too, electrons that are not aligned will experience more resistance than those that are aligned.

In the case where both magnetic metals are magnetised in the same direction (as would be the case in the presence of an applied external magnetic field), the spins of most electrons will be aligned with the direction of magnetisation and the electrons will, therefore, pass through the entire structure without facing much resistance. However, if the magnetisation of the two magnetic layers is opposed (as can be the case in the absence of an external magnetic field), all the electrons will be oppositely aligned in one of the two layers. This means that no electrons will be able to pass through easily and the electric resistance will be at a maximum. An analogy with polaroids may be helpful in understanding this effect. A pair of crossed polaroids shuts off light completely. Similarly, a pair of magnetic layers with crossed magnetic polarities (or magnetisation) offers high resistance to the flow of electric current.

A structure as described above works as follows in a read-out device of an HDD. The magnetisation of the first layer is held fixed, or “pinned”, and the magnetisation of the third layer is free to move. When a weak magnetic field, such as that from a bit on a hard disk, comes under the structure, the magnetisation of the unpinned layer rotates relative to the pinned layer and because of the GMR effect causes a significant change in the electrical resistance and hence in the current signal leaving the read-out head. A high current may represent a binary “1” and a low current a binary “0”.

An important reason why this discovery would not have been possible before techniques to grow nanoscale layers were known is the following. In order to exhibit the GMR effect, the mean free path length of the conduction electrons – the average distance that an electron traverses before it is scattered – has to exceed greatly the interlayer separation so that the electrons can travel through the magnetic layers and pick up the GMR effect. Without the new techniques, it would not have been possible to meet this requirement, and GMR would not have revealed itself. In this context, it may be pointed out that before the work of Fert and Grünberg, there were experimental observations of enhanced MR (of about a few per cent) but none was recognised as a new effect. Nanometre separation between magnetic layers is also important for an effective mutual magnetic coupling between them via the electrons of the non-magnetic layer so that their relative magnetisation is maintained in the absence of an external field.

Giant magnetoresistance wins physics Nobel 2007

October 9, 2007

Albert Fert and Peter Gruenberg are awarded the Physics Nobel 2007

for the discovery of giant magnetoresistance.

The wiki page has a nice write-up on giant magnetoresistance.

Update: Doug at Nanoscale views has a short write-up on GMR.