The 2007 Nobel Prize for physics has been awarded to Albert Fert and Peter Grünberg for the discovery of giant magnetoresistance.
iPods, laptops and even Google are among the modern icons made possible by the discovery that is recognized by this year’s Nobel Prize in Physics. Giant magnetoresistance is the effect by which digital information stored in the tiny magnetic domains of a computer’s hard disk is converted to an electrical signal that can be processed by its silicon chips. Without it, the performance, bulk and cost of computer storage would probably have severely hampered the development of many technological innovations that we now take for granted. And it has inspired the birth of an entire field, known as spintronics.
Giant magnetoresistance, discovered independently by Albert Fert1 and Peter Grünberg2 in 1988, arises from the fact that electrons travelling through a ferromagnetic conductor will scatter differently depending on the relative orientation of their spin to the magnetization direction of the conductor — with those oriented parallel scattering less often than those oriented antiparallel. This causes electrons injected from a magnetic conductor into a non-magnetic conductor to be preferentially oriented in one direction. If these electrons then encounter a second ferromagnetic layer, they will pass into it freely from the non-magnetic metal, without strong scattering, only if their preferred orientation is parallel to the magnetization of the second layer. Consequently, the resistance of a trilayer stack of two ferromagnetic metal layers either side of a non-magnetic metal layer depends sensitively on the relative magnetization direction of the two ferromagnetic layers.
This phenomenon is the basis of operation of the read heads that are used in modern computer hard disks. These read heads consist of multiple ferromagnetic and non-magnetic metal layers, but with alternating soft and hard ferromagnetic layers. The magnetization of the hard magnetic layers are fixed during manufacture, but that of the soft layers will be affected by the presence of an external magnetic field. So by placing a read head close to the fields generated by the magnetically encoded bits of a hard disk, the orientation of those bits can be determined through the resistance of the multilayer stack.
Although giant magnetoresistance is ultimately about the flow (or otherwise) of electrons, the fact that their spin is used successfully to mediate this flow is the inspiration for the new field of spintronics. Just as electronics encodes, transmits and processes information in the form of electronic charge, spintronics promises a new generation of devices, circuits and systems that does the same with spin. Not only might this enable computers to become faster and more energy efficient, but it might also enable new functionalities to be developed, such as quantum information processing, that are unavailable through the use of charge alone.
From Nature Physics:
ARTICLE Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field O. Boulle, V. Cros, J. Grollier, L. G. Pereira, C. Deranlot, F. Petroff, G. Faini, J. Barna and A. Fert
The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin-transfer phenomenon, which is the reorientation of the magnetization of a ferromagnetic domain by spin-polarized current. Here we report transport and microwave power measurements on specially designed nanopillars, for which a non-standard angular dependence of the spin-transfer torque is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field. This is also predicted by simulations including a 'wavy' angular dependence of the torque. This type of nanopillar, which is able to generate microwave oscillations in zero applied magnetic field, could represent an interesting method for the implementation of spin-transfer oscillators. We also emphasize the theoretical implications of our results on the angular dependence of the torque. Nature Physics 3, 492–497 (2007)
NEWS AND VIEWS Spintronics: Field-free ringing of nanomagnets Thomas J. Silva
That the magnetic orientation of ferromagnets can be changed using magnetic fields has been known for centuries. But the exploration of magnetization control without any additional magnetic field has only just begun. Nature Physics 3, 447–448 (2007)
REVIEW Challenges for semiconductor spintronics David D. Awschalom and Michael E. Flatté
High-volume information-processing and communications devices are at present based on semiconductor devices, whereas information-storage devices rely on multilayers of magnetic metals and insulators. Switching within information-processing devices is performed by the controlled motion of small pools of charge, whereas in the magnetic storage devices information storage and retrieval is performed by reorienting magnetic domains (although charge motion is often used for the final stage of readout). Semiconductor spintronics offers a possible direction towards the development of hybrid devices that could perform all three of these operations, logic, communications and storage, within the same materials technology. By taking advantage of spin coherence it also may sidestep some limitations on information manipulation previously thought to be fundamental. This article focuses on advances towards these goals in the past decade, during which experimental progress has been extraordinary. Nature Physics 3, 153-159 (2007)
From Nature Materials:
Tunnel junctions with multiferroic barriers Martin Gajek, Manuel Bibes, Stéphane Fusil, Karim Bouzehouane, Josep Fontcuberta, Agnès Barthélémy and Albert Fert Nature Materials 6, 296-302 (2007)
Transformation of spin information into large electrical signals using carbon nanotubes Luis E. Hueso, José M. Pruneda, Valeria Ferrari, Gavin Burnell, José P. Valdés-Herrera, Benjamin D. Simons, Peter B. Littlewood, Emilio Artacho, Albert Fert and Neil D. Mathur Nature 445, 410-413 (25 January 2007)
Baibich, M. N. et al. Phys. Rev. Lett. 61, 2472–2475 (1988)
Binasch, G. Grünberg, P. Saurenbach, F. & Zinn, W. Phys. Rev. B 39, 4828–4830 (1989)
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