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  • richardmitnick 3:20 pm on June 25, 2014 Permalink | Reply
    Tags: , , Spintronics   

    From Berkeley Lab- “Advanced Light Source Provides New Look at Skyrmions: Results Hold Promise for Spintronics” 

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    Berkeley Lab

    June 25, 2014
    Lynn Yarris

    Skyrmions, subatomic quasiparticles that could play a key role in future spintronic technologies, have been observed for the first time using x-rays. An international collaboration of researchers working at Berkeley Lab’s Advanced Light Source (ALS) observed skyrmions in copper selenite (Cu2SeO3) an insulator with multiferroic properties. The results not only hold promise for ultracompact data storage and processing, but may also open up entire new areas of study in the emerging field of quantum topology.

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    Advanced Light Source images of a Cu2SeO3 sample show five sets of dual-peak skyrmion structures, highlighted by the white ovals. The dual peaks represent the two skyrmion sub-lattices that rotate with respect to each other. All peaks fall on an arc (dotted line) representing the constant amplitude of the skyrmion wave vector. No image credit

    “Using resonant x-ray scattering, we were able to gather unique element-specific, orbital-sensitive electronic and magnetic structural information not available by any other method,” says Sujoy Roy, a physicist who oversees research at ALS Beamline 12.0.2 where the study was carried out, and the corresponding author of a paper describing this research in Physical Review Letters titled Coupled Skyrmion Sublattices in Cu2OSeO3.

    A skyrmion is an atom-sized whirlwind of magnetism, in which the spins of charged particles form a vortex. In this image the color scale – red for longer and blue for shorter vectors – shows that the magnetization is highest at the center of the skyrmion. (Image by Matthew Langner)

    “We found the unexpected existence of two distinct skyrmion sub-lattices that rotate with respect to each other, creating a moiré-like pattern,” Roy says. “Compared to materials with a simpler magnetic structure, the sub-lattices provide for an extra degree of freedom to minimize the free energy. This leads to magnetic excitations that can’t exist in materials with a single magnetic lattice structure.”

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    A skyrmion is an atom-sized whirlwind of magnetism, in which the spins of charged particles form a vortex. In this image the color scale – red for longer and blue for shorter vectors – shows that the magnetization is highest at the center of the skyrmion. (Image by Matthew Langner)

    Although skyrmions act like baryons, they are actually magnetic vortices – discrete swirls of magnetism – formed from the spins of charged particles. Spin is a quantum property in which the charged particles act as if they were bar magnets rotating about an axis and pointing in either an “up” or “down” direction. The discovery of skyrmions – named for Tony Skyrme, a British physicist who first theorized their existence – in manganese silicide generated much excitement in the materials sciences world because their exotic hedgehog-like spin texture is topologically protected – meaning it can’t be perturbed. Add to this the discovery that skyrmions can be moved coherently over macroscopic distances with a tiny electrical current and you have a strong spintronic candidate.

    “A major breakthrough came with the discovery of skyrmions in copper selenite because its magnetic properties can be controlled with an electric field,” says Roy. “To achieve this control, however, we must understand how different electron orbitals stabilize the skyrmionic phase. Until our study, the copper selenite skyrmions had only been observed with neutron scattering and transmission electron microscopy, techniques that are insensitive to electron orbitals.”

    ALS Beamline 12.0.2 is an undulator beamline with experimental facilities optimized for coherent x-ray scattering studies of magnetic materials. The collaboration, which included researchers from Berkeley Lab’s Materials Sciences Division and Japan’s RIKEN Institute, used these facilities to first identify the magnetic vortex. Then, at a certain applied electric field and temperature, they saw x-ray signals due to the formation of a skyrmion lattice.

    “We were able to show that although the skyrmions act like magnetic particles, their origin in copper selenite is electronic,” says Matthew Langner, lead author of the Physical Review Letters paper. “We also found that temperature can be used to move the skyrmions in copper selenite in either a clockwise or counter-clockwise direction.”

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    From left, Matthew Langner, Stephan Kevan, Sujoy Roy, Robert Schoenlein and Xiaowen Shi were part of an international team of researchers that used the Advanced Light Source to provide new information on the quasiparticles known as skyrmions. (Photo by Roy Kaltschmidt)

    Controlling the movement of skyrmions in a multiferroic compound suggests these magnetic vortices could be used to read and write data. Skyrmions are considered especially promising for the holographic information storage concept known as magnetic race-track memory.

    “The skyrmion is topologically distinct from the other ground-state magnetic structures, meaning it can be moved around the sample without losing its shape,” Langner says. “The combination of this stability and the low magnetic and electric fields required for manipulating the skyrmions is what makes them potentially useful for spintronic applications.”

    In addition to device applications, the collaboration’s findings show that is now possible to use x-rays to study spectroscopic and electronic aspects of the skyrmion, and to study skyrmion dynamics on the time-scale of fundamental interactions.

    Co-authors of the Physical Review Letters paper, in addition to Roy and Langner, are Shrawan Mishra, Jason Lee, Xiaowen Shi, Muhammad Hossain, Yi-De Chuang, Shinichiro Seki, Yoshinori Tokura, Stephen Kevan and Robert Schoenlein.

    This research was supported by the U.S. Department of Energy’s Office of Science.

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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  • richardmitnick 3:09 pm on January 16, 2014 Permalink | Reply
    Tags: , , , , , Spintronics, superconductors   

    From Berkeley Lab: “Natural 3D Counterpart to Graphene Discovered” 


    Berkeley Lab

    Researchers at Berkeley Lab’s Advanced Light Source Find New Form of Quantum Matter

    January 16, 2014
    Lynn Yarris (510) 486-5375 lcyarris@lbl.gov

    The discovery of what is essentially a 3D version of graphene – the 2D sheets of carbon through which electrons race at many times the speed at which they move through silicon – promises exciting new things to come for the high-tech industry, including much faster transistors and far more compact hard drives. A collaboration of researchers at the U.S Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered that sodium bismuthate can exist as a form of quantum matter called a three-dimensional topological Dirac semi-metal (3DTDS). This is the first experimental confirmation of 3D Dirac fermions in the interior or bulk of a material, a novel state that was only recently proposed by theorists.

    graph
    A topological Dirac semi-metal state is realized at the critical point in the phase transition from a normal insulator to a topological insulator. The + and – signs denote the even and odd parity of the energy bands.

    “A 3DTDS is a natural three-dimensional counterpart to graphene with similar or even better electron mobility and velocity,” says Yulin Chen, a physicist with Berkeley Lab’s Advanced Light Source (ALS) when he initiated the study that led to this discovery, and now with the University of Oxford. “Because of its 3D Dirac fermions in the bulk, a 3DTDS also features intriguing non-saturating linear magnetoresistance that can be orders of magnitude higher than the materials now used in hard drives, and it opens the door to more efficient optical sensors.”

    Chen is the corresponding author of a paper in Science reporting the discovery. The paper is titled Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi. Co-authors were Zhongkai Liu, Bo Zhou, Yi Zhang, Zhijun Wang, Hongming Weng, Dharmalingam Prabhakaran, Sung-Kwan Mo, Zhi-Xun Shen, Zhong Fang, Xi Dai and Zahid Hussain.

    See the full article and all of the excitment here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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  • richardmitnick 1:59 pm on December 17, 2012 Permalink | Reply
    Tags: , , , , Solid State Physics, Spintronics   

    From Berkeley Lab: “New Insight into an Intriguing State of Magnetism” 


    Berkeley Lab

    December 17, 2012
    Paul Preuss

    Magnonics is an exciting extension of spintronics, promising novel ways of computing and storing magnetic data. What determines a material’s magnetic state is how electron spins are arranged (not everyday spin, but quantized angular momentum). If most of the spins point in the same direction, the material is ferromagnetic, like a refrigerator magnet. If half the spins point one way and half the opposite, the material is antiferromagnetic, with no everyday magnetism.

    There are other kinds of magnetism. In materials where the electrons are “itinerant” – moving rapidly through the crystal lattice like a gas, so that their spins become strongly coupled to their motions – certain crystalline structures can cause the spins to precess collectively to the right or left in a helix, producing a state called helimagnetism.

    A team of scientists from Berkeley Lab’s Materials Sciences Division, UC
    Berkeley’s Department of Physics, and the Technical University of Munich, led by Jake Koralek and Dennis Meier, has studied magnons in a material that becomes helimagnetic below about 30 kelvin: iron silicide doped with cobalt. They investigated how helimagnons evolve as the temperature increases, destroying the magnetic order, as well as how the magnetic phases are affected by an external magnetic field.”

    There is way too much information on helimagnetism in this article for me to pick out important items, they are all interlinked. See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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