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April 04, 2013

All-optical switching promises terahertz-speed hard drive and RAM memory

Researchers at the U.S. Department of Energy’s Ames Laboratory, Iowa State University, and the University of Crete in Greece have found a new way to switch magnetism that is at least 1000 times faster than currently used in magnetic memory technologies. Magnetic switching is used to encode information in hard drives, magnetic random access memory and other computing devices. The discovery, reported in the April 4 issue of Nature, potentially opens the door to terahertz (10^12 hertz) and faster memory speeds.

Ames Laboratory physicist Jigang Wang and his team used short laser pulses to create ultra-fast changes in the magnetic structure, within quadrillionths of a second (femtosecond), from anti-ferromagnetic to ferromagnetic ordering in colossal magnetoresistive materials, which are promising for use in next-generation memory and logic device.


Magnetic structure in a colossal magneto-resistive manganite is switched from antiferromagnetic to ferromagnetic ordering during about 100 femtosecond (10^-15 s) laser pulse photo-excitation. With time so short and the laser pulses still interacting with magnetic moments, the magnetic switching is driven quantum mechanically– not thermally. This potentially opens the door to terahertz (10^12 hertz) and faster memory writing/reading speeds.



The fast switching speed and huge magnetization that Wang observed meet both requirements for applying colossal magnetoresistive (CMR) materials in ultra-fast, terahertz magnetic memory and logic devices.

“Our strategy is to use all-optical quantum methods to achieve magnetic switching and control magnetism. This lays the groundwork for seeking the ultimate switching speed and capabilities of CMR materials, a question that underlies the entire field of spin-electronics,” said Wang. “And our hope is that this means someday we will be able to create devices that can read and write information faster than ever before, yet with less power consumed.”

Science - Femtosecond switching of magnetism via strongly correlated spin–charge quantum excitations

he technological demand to push the gigahertz (10^9 hertz) switching speed limit of today’s magnetic memory and logic devices into the terahertz (10^12 hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics—in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states—femtosecond-laser-excited coherence between electronic states can switch magnetic order by ‘suddenly’ breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10^−15 seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10^−12 seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators and non-equilibrium phase transitions of strongly correlated electrons in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.



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