Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Solving a Spintronic Mystery

28.02.2012
Berkeley Lab Researchers Resolve Controversy Over Gallium Manganese Arsenide that Could Boost Spintronic Performance
A long-standing controversy regarding the semiconductor gallium manganese arsenide, one of the most promising materials for spintronic technology, looks to have been resolved. Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab)in collaboration with scientist from University of Notre Dame have determined the origin of the charge-carriers responsible for the ferromagnetic properties that make gallium manganese arsenide such a hot commodity for spintronic devices. Such devices utilize electron spin rather than charge to read and write data, resulting in smaller, faster and much cheaper data storage and processing.

Wladek Walukiewicz, a physicist with Berkeley Lab’s Materials Sciences Division and Margaret Dobrowolska, a physicist at Notre Dame, led a study in which it was shown that the holes (positively-charged energy spaces) in gallium manganese arsenide that control the Curie temperature, the temperature at which magnetism is lost, are located in an impurity energy band rather than a valence energy band, as many scientists have argued. This finding opens the possibility of fabricating gallium manganese arsenide so as to expand the width and occupation of the impurity band and thereby boost the Curie temperature to improve spintronic potential.

“Our results challenge the valence band picture for gallium manganese arsenide and point to the existence of an impurity band, created by even moderate to high doping levels of manganese,” Walukiewicz says. “It is the location and partially localized nature of holes within this impurity band that drives the value of the Curie temperature.”

The results of this study have been published in the journal Nature Materials in a paper titled “Controlling the Curie temperature in (Ga,Mn)As through location of the Fermi level within the impurity band.” Co-authoring the paper with Walukiewicz and Dobrowolska were Kritsanu Tivakornsasithorn, Xinyu Liu, Jacek Furdyna, Mona Berciu and Kin Man Yu.

As a commercial semiconductor, gallium arsenide is second only to silicon. Substitute some of the gallium atoms with atoms of manganese and you get a ferromagnetic semiconductor that is well-suited for spintronic devices. While it has been established that the ferromagnetism of gallium manganese arsenide is hole-mediated, the nature of the hole-states, which has a direct and crucial bearing on its Curie temperature, has been vigorously debated.

In semiconductors and other solid-state materials, the valence band is the range of energies in which the movement of charge is determined by availability of holes. Doping gallium arsenide with manganese can create an impurity band that depletes the valence band and shifts the Fermi level, the energy level at which the electronic states below are filled and those states above are empty.

Wladek Walukiewicz and Kin Man Yu at Berkeley Lab’s Rutherford Backscattering Spectrometry laboratory.

“The question has been whether the holes mediating the interactions of manganese spins reside in a delocalized valence band, or in a manganese-derived partially localized impurity band,” Walukiewicz says. “The valence band model assumes that a separate impurity band does not exist for manganese concentrations higher than about two-percent.”

Walukiewicz and his co-authors addressed the issue through channeling experiments that measured the concentrations of manganese atoms and holes relevant to the ferromagnetic order in gallium manganese arsenide. These experiments were carried out at Berkeley Lab’s Rutherford Backscattering facility, which is operated under the direction of co-author Kin Man Yu. The results of these experiments were then combined with magnetization, transport and magneto-optical data performed at the University of Notre Dame.

“We were able to determine where the manganese atoms were located, what fraction of this total replaced gallium and acted as electron acceptors (meaning they created ferromagnetic-mediating holes), and what fraction was in the interstitial sites, acting as positively-charged double donors compensating for a fraction of manganese acceptors,” Walukiewicz says. “Taking all our data together, we find that the Curie temperature of gallium manganese arsenide can be understood only by assuming that its ferromagnetism is mediated by holes residing in the impurity band, and that it is the location of the Fermi level within the impurity band that determines the Curie temperature.”

Electron spin is a quantum mechanical property arising from the magnetic moment of a spinning electron. Spin carries a directional value of either “up” or “down” and can be used to encode data in the 0s and 1s of the binary system. Walukiewicz says that understanding the factors that control the Curie temperature can serve as a guide for strategies to optimize ferromagnetic materials for spintronic applications.

“For example, with appropriate control of the manganese ions, either co-doping with donor ions, or modulation doping, we can engineer the location of the Fermi level within the impurity band to best the advantage,” he says.

Walukiewicz says the findings of this study further suggest that it should be possible to optimize magnetic coupling and Curie temperature for the whole family of ferromagnetic semiconductors by tuning the binding energy of the acceptor ions.

This research was supported in part by the DOE Office of Science, and by grants from the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

Lynn Yarris | EurekAlert!
Further information:
http://www.lbl.gov

More articles from Materials Sciences:

nachricht Move over, Superman! NIST method sees through concrete to detect early-stage corrosion
27.04.2017 | National Institute of Standards and Technology (NIST)

nachricht Control of molecular motion by metal-plated 3-D printed plastic pieces
27.04.2017 | Ecole Polytechnique Fédérale de Lausanne

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Fighting drug resistant tuberculosis – InfectoGnostics meets MYCO-NET² partners in Peru

28.04.2017 | Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

 
Latest News

Wireless power can drive tiny electronic devices in the GI tract

28.04.2017 | Medical Engineering

Ice cave in Transylvania yields window into region's past

28.04.2017 | Earth Sciences

Nose2Brain – Better Therapy for Multiple Sclerosis

28.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>