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 Breakthrough in nanoresearch - Quantum chains in graphene nanoribbons
09.08.2018 | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt

nachricht UNH Researchers find seed coats could lead to strong, tough, yet flexible materials
08.08.2018 | University of New Hampshire

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: New interactive machine learning tool makes car designs more aerodynamic

Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.

When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...

Im Focus: Robots as 'pump attendants': TU Graz develops robot-controlled rapid charging system for e-vehicles

Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.

Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....

Im Focus: The “TRiC” to folding actin

Proteins must be folded correctly to fulfill their molecular functions in cells. Molecular assistants called chaperones help proteins exploit their inbuilt folding potential and reach the correct three-dimensional structure. Researchers at the Max Planck Institute of Biochemistry (MPIB) have demonstrated that actin, the most abundant protein in higher developed cells, does not have the inbuilt potential to fold and instead requires special assistance to fold into its active state. The chaperone TRiC uses a previously undescribed mechanism to perform actin folding. The study was recently published in the journal Cell.

Actin is the most abundant protein in highly developed cells and has diverse functions in processes like cell stabilization, cell division and muscle...

Im Focus: Lining up surprising behaviors of superconductor with one of the world's strongest magnets

Scientists have discovered that the electrical resistance of a copper-oxide compound depends on the magnetic field in a very unusual way -- a finding that could help direct the search for materials that can perfectly conduct electricity at room temperatur

What happens when really powerful magnets--capable of producing magnetic fields nearly two million times stronger than Earth's--are applied to materials that...

Im Focus: World record: Fastest 3-D tomographic images at BESSY II

The quality of materials often depends on the manufacturing process. In casting and welding, for example, the rate at which melts solidify and the resulting microstructure of the alloy is important. With metallic foams as well, it depends on exactly how the foaming process takes place. To understand these processes fully requires fast sensing capability. The fastest 3D tomographic images to date have now been achieved at the BESSY II X-ray source operated by the Helmholtz-Zentrum Berlin.

Dr. Francisco Garcia-Moreno and his team have designed a turntable that rotates ultra-stably about its axis at a constant rotational speed. This really depends...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Within reach of the Universe

08.08.2018 | Event News

A journey through the history of microscopy – new exhibition opens at the MDC

27.07.2018 | Event News

2018 Work Research Conference

25.07.2018 | Event News

 
Latest News

'Building up' stretchable electronics to be as multipurpose as your smartphone

14.08.2018 | Information Technology

During HIV infection, antibody can block B cells from fighting pathogens

14.08.2018 | Life Sciences

First study on physical properties of giant cancer cells may inform new treatments

14.08.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>