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  • richardmitnick 5:08 pm on December 17, 2014 Permalink | Reply
    Tags: , , , Magnetics,   

    From LBL: “Switching to Spintronics” 

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    Berkeley Lab

    December 17, 2014
    Lynn Yarris (510) 486-5375

    In a development that holds promise for future magnetic memory and logic devices, researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Cornell University successfully used an electric field to reverse the magnetization direction in a multiferroic spintronic device at room temperature. This demonstration, which runs counter to conventional scientific wisdom, points a new way towards spintronics. and smaller, faster and cheaper ways of storing and processing data.

    Conceptual illustration of how magnetism is reversed (see compass) by the application of an electric field (blue dots) applied across gold capacitors. Blurring of compass needles under electric field represents two-step process. (Image courtesy of John Heron, Cornell)

    “Our work shows that 180-degree magnetization switching in the multiferroic bismuth ferrite can be achieved at room temperature with an external electric field when the kinetics of the switching involves a two-step process,” says Ramamoorthy Ramesh, Berkeley Lab’s Associate Laboratory Director for Energy Technologies, who led this research. “We exploited this multi-step switching process to demonstrate energy-efficient control of a spintronic device.”

    Ramesh, who also holds the Purnendu Chatterjee Endowed Chair in Energy Technologies at the University of California (UC) Berkeley, is the senior author of a paper describing this research in Nature. The paper is titled Deterministic switching of ferromagnetism at room temperature using an electric field. John Heron, now with Cornell University, is the lead and corresponding author. (See below for full list of co-authors).

    Ramamoorthy Ramesh is Berkeley Lab’s Associate Laboratory Director for Energy Technologies, a UC Berkeley professor, and a leading authority on multiferroics. (Photo by Roy Kaltschmidt)

    Multiferroics are materials in which unique combinations of electric and magnetic properties can simultaneously coexist. They are viewed as potential cornerstones in future data storage and processing devices because their magnetism can be controlled by an electric field rather than an electric current, a distinct advantage as Heron explains.

    “The electrical currents that today’s memory and logic devices rely on to generate a magnetic field are the primary source of power consumption and heating in these devices,” he says. “This has triggered significant interest in multiferroics for their potential to reduce energy consumption while also adding functionality to devices.”

    Nature, however, has imposed thermodynamic barriers and material symmetry constrains that theorists believed would prevent the reversal of magnetization in a multiferroic by an applied electric field. Earlier work by Ramesh and his group with bismuth ferrite, the only known thermodynamically stable room-temperature multiferroic, in which an electric field was used as on/off switch for magnetism, suggested that the kinetics of the switching process might be a way to overcome these barriers, something not considered in prior theoretical work.

    “Having made devices and done on/off switching with in-plane electric fields in the past, it was a natural extension to study what happens when an out-of-plane electric field is applied,” Ramesh says.

    Ramesh, Heron and their co-authors set up a theoretical study in which an out-of-plane electric field – meaning it ran perpendicular to the orientation of the sample – was applied to bismuth ferrite films. They discovered a two-step switching process that relies on ferroelectric polarization and the rotation of the oxygen octahedral.

    John Heron is the lead author of a Nature paper describing the switching of ferromagnetism at room temperature using an electric field.

    “The two-step switching process is key as it allows the octahedral rotation to couple to the polarization,” Heron says. “The oxygen octahedral rotation is also critical because it is the mechanism responsible for the ferromagnetism in bismuth ferrite. Rotation of the oxygen octahedral also allows us to couple bismuth ferrite to a good ferromagnet such as cobalt-iron for use in a spintronic device.”

    To demonstrate the potential technological applicability of their technique, Ramesh, Heron and their co-authors used heterostructures of bismuth ferrite and cobalt iron to fabricate a spin-valve, a spintronic device consisting of a non-magnetic material sandwiched between two ferromagnets whose electrical resistance can be readily changed. X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images showed a clear correlation between magnetization switching and the switching from high-to-low electrical resistance in the spin-valve. The XMCD-PEEM measurements were completed at PEEM-3, an aberration corrected photoemission electron microscope at beamline 11.0.1 of Berkeley Lab’s Advanced Light Source.

    LBL Advanced Light Source
    LBL ALS interior

    “We also demonstrated that using an out-of-plane electric field to control the spin-valve consumed energy at a rate of about one order of magnitude lower than switching the device using a spin-polarized current,” Ramesh says.

    In addition to Ramesh and Heron, other co-authors of the Nature paper were James Bosse, Qing He, Ya Gao, Morgan Trassin, Linghan Ye, James Clarkson, Chen Wang, Jian Liu, Sayeef Salahuddin, Dan Ralph, Darrell Schlom, Jorge Iniguez and Bryan Huey.

    See the full article here.

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  • richardmitnick 9:35 am on June 20, 2014 Permalink | Reply
    Tags: , Magnetics,   

    from physicsworld.com: “Electrons’ magnetic interactions isolated at long last” 


    Jun 19, 2014
    Tushna Commissariat

    A measurement of the extremely weak magnetic interaction between two single electrons has been carried out by an international team of physicists. Using experimental techniques first developed for quantum-information and ion-trapping technologies, the team made its measurement despite the presence of magnetic noise, which is a million times stronger than the signal it was seeking. Apart from measuring magnetism at the shortest length scale thus far, the researchers say that their technique could be applied to other measurement scenarios where noise is a dominant factor, such as for quantum-error corrections.

    Since the 1920s, researchers have known that the electron possesses an intrinsic angular momentum and an associated magnetic moment – known as its spin magnetic moment. Essentially, each electron acts like a tiny, indivisible magnetic dipole that is affected by magnetic fields. Although researchers have accurately measured the magnetic field of an individual electron, the magnetic interactions between two electrons have proved much more difficult to observe. When two electrons are separated by a very small distance (atomic-scale separations), the magnetic interactions are at their strongest and should be easy to measure. However, in this scenario Pauli’s exclusion principle and Coulomb electrical repulsion dominate the interactions between the electrons, drowning out the magnetic interaction. While these two effects weaken as the electrons move further apart, so does the magnetic interaction, which is then almost completely obscured by ambient magnetic noise.

    Isolated interactions

    One way of coping with this noise is to completely isolate the electrons from the environment – a technique that is often employed in quantum-information processing. This is the concept that Shlomi Kotler, from the Weizmann Institute of Science, Israel, and colleagues adopted to make their exquisite measurements. Indeed, Kotler says that the team’s measurement was performed at a scale “which is quite exotic – two microns. It is the size of an E. coli bacteria”, meaning that “the most dominant process is not of force between the electrons but of noise”. He further explains that the main tool used by the team to make its measurement was that of “decoherence-free subspaces” – a quantum-computing technique where a system is completely decoupled from its environment to protect information.

    Kotler likens the difficulty of this measurement to trying to measure the size of a pea floating in the ocean, with huge waves of noise moving it erratically, across a distance of kilometres. While it would initially seem impossible to make the measurement (thanks to the pea’s constant movement), the trick would be to float alongside the pea. In that case, a wave would have the same effect on the pea and the observer, so that the effect of the waves would be inconsequential. In the researchers’ experiment, one electron is trying to sense the magnetic field of the other. “But that field is riding on top of magnetic noise in the lab, which is a million times bigger,” says Kolter. “The only way to make this measurement possible is to place the two electrons on an equal footing with respect to the ambient magnetic noise. This way, magnetic noise becomes irrelevant.”

    Photograph of the experimental set-up, including the trap Ions inside: the experimental set-up. No image credit.

    To do so, the team uses two strontium (88Sr+ ) ions, in a vacuum chamber at a fixed distance of 2 μm from one another, held using a Paul ion trap. Each ion has a single ground-state, spin-1/2 valence electron and no nuclear spin. Using lasers tuned to the atomic transition of the ions, the team manipulated the electrons and prepared them in an initial state where the north pole of one electron is facing the north pole of the other. Like a regular bar magnet, the like poles repel each other and would rotate, thereby interacting. But as the magnets in this case are electrons, quantum effects come into play and the electrons become entangled in what Kolter describes as “both a north–north and south–south facing state”.
    Elongated entanglement

    Even more surprising is that this naturally created entanglement lasts for 15 s – a surprisingly long time for a system to remain in a coherent, quantum state. After that time, the researchers use laser pulses to detect “whether their north poles are facing or anti-facing each other”. By varying the separation between the two ions, they were able to measure the strength of the magnetic interaction as a function of distance – confirming the expected inverse-cubic (1/d3) dependence of the interaction.

    Kolter told physicsworld.com that while the result itself was not surprising – current theories say that magnetism behaves similarly at all scales – it was how long the electrons were entangled for that was unexpected. “Our main surprise was the coherence – the fact that the electrons behaved quantum-mechanically for a ‘human-scale duration’ (15 s and more) and that the tiny forces of magnetism are still strong enough to entangle the two particles over this time. In many respects this is unprecedented.” Conventionally, quantum mechanics is thought to work for tiny systems at short time scales. The team’s system was rather large – 2 µm and so, almost macroscopic – and it still preserved the quantum-mechanical property of entanglement for an extremely long time.

    Kolter points out that, nearly 20 years ago, quantum-computation experiments adapted advanced spectroscopic tools to generate entanglement between massive particles, and spectroscopy has been a driving force in experimental quantum computing ever since. Now, Kolter’s research has turned this around by using quantum-computing tools to “do a very sensitive spectroscopy experiment. We believe that this trend will continue to be fruitful in the near future”, he says. Beyond validating the behaviour of the magnetic force at the micron scale, the team’s system could be used to set a bound on “anomalous spin forces” that might come into play beyond Standard Model physics. But it could also be generalized and applied to other scenarios, such as for quantum-error correction protocols.

    The research was published in Nature.

    See the full article here.

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  • richardmitnick 9:59 am on March 7, 2014 Permalink | Reply
    Tags: , , Magnetics, ,   

    From UC Berkeley: “Colored diamonds are a superconductor’s best friend” 

    UC Berkeley

    March 6, 2014
    Robert Sanders

    Flawed but colorful diamonds are among the most sensitive detectors of magnetic fields known today, allowing physicists to explore the minuscule magnetic fields in metals, exotic materials and even human tissue.

    Dmitry Budker and Ron Folman build ‘atom chips’ to probe the minuscule magnetic properties of high-temperature superconductors. Robert Sanders photo.

    University of California, Berkeley, physicist Dmitry Budker and his colleagues at Ben-Gurion University of the Negev in Israel and UCLA have now shown that these diamond sensors can measure the tiny magnetic fields in high-temperature superconductors, providing a new tool to probe these much ballyhooed but poorly understood materials.

    “Diamond sensors will give us measurements that will be useful in understanding the physics of high temperature superconductors, which, despite the fact that their discoverers won a 1987 Nobel Prize, are still not understood,” said Budker, a professor of physics and faculty scientist at Lawrence Berkeley National Laboratory.

    High-temperature superconductors are exotic mixes of materials like yttrium or bismuth that, when chilled to around 180 degrees Fahrenheit above absolute zero (-280ºF), lose all resistance to electricity, whereas low-temperature superconductors must be chilled to several degrees above absolute zero. When discovered 28 years ago, scientists predicted we would soon have room-temperature superconductors for lossless electrical transmission or magnetically levitated trains.

    It never happened.

    “The new probe may shed light on high-temperature superconductors and help theoreticians crack this open question,” said coauthor Ron Folman of Ben-Gurion University of the Negev, who is currently a Miller Visiting Professor at UC Berkeley. “With the help of this new sensor, we may be able to take a step forward.”

    Budker, Folman and their colleagues report their success in an article posted online Feb. 18 in the journal Physical Review B.

    Flawed but colorful

    Colorful diamonds, ranging from yellow and orange to purple, have been prized for millennia. Their color derives from flaws in the gem’s carbon structure: some of the carbon atoms have been replaced by an element, such as boron, that emits or absorbs a specific color of light.

    Once scientists learned how to create synthetic diamonds, they found that they could selectively alter a diamond’s optical properties by injecting impurities. In this experiment, Budker, Folman and their colleagues bombarded a synthetic diamond with nitrogen atoms to knock out carbon atoms, leaving holes in some places and nitrogen atoms in others. They then heated the crystal to force the holes, called vacancies, to move around and pair with nitrogen atoms, resulting in diamonds with so-called nitrogen-vacancy centers. For the negatively charged centers, the amount of light they re-emit when excited with light becomes very sensitive to magnetic fields, allowing them to be used as sensors that are read out by laser spectroscopy.

    Folman noted that color centers in diamonds have the unique property of exhibiting quantum behavior, whereas most other solids at room temperature do not.

    “This is quite surprising, and is part of the reason that these new sensors have such a high potential,” Folman said.

    Applications in homeland security?

    Technology visionaries are thinking about using nitrogen-vacancy centers to probe for cracks in metals, such as bridge structures or jet engine blades, for homeland security applications, as sensitive rotation sensors, and perhaps even as building blocks for quantum computers.

    The crystal lattice of a pure diamond is pure carbon (black balls), but when a nitrogen atom replaces one carbon and an adjacent carbon is kicked out, the ‘nitrogen-vacancy center’ becomes a sensitive magnetic field sensor.

    Budker, who works on sensitive magnetic field detectors, and Folman, who builds ‘atom chips’ to probe and manipulate atoms, focused in this work on using these magnetometers to study new materials.

    “These diamond sensors combine high sensitivity with the potential for high spatial resolution, and since they operate at higher temperatures than their competitors – superconducting quantum interference device, or SQUID, magnetometers – they turn out to be good for studying high temperature superconductors,” Budker said. “Although several techniques already exist for magnetic probing of superconducting materials, there is a need for new methods which will offer better performance.”

    The team used their diamond sensor to measure properties of a thin layer of yttrium barium copper oxide (YBCO), one of the two most popular types of high-temperatures superconductor. The Ben-Gurion group integrated the diamond sensor with the superconductor on one chip and used it to detect the transition from normal conductivity to superconductivity, when the material expels all magnetic fields. The sensor also detected tiny magnetic vortices, which appear and disappear as the material becomes superconducting and may be a key to understanding how these materials become superconducting at high temperatures.

    “Now that we have proved it is possible to probe high-temperatures superconductors, we plan to build more sensitive and higher-resolution sensors on a chip to study the structure of an individual magnetic vortex,” Folman said. “We hope to discover something new that cannot be seen with other technologies.”

    Researchers, including Budker and Folman, are attempting to solve other mysteries through magnetic sensing. For example, they are investigating networks of nerve cells by detecting the magnetic field each nerve cell pulse emits. In another project, they aim at detecting strange never-before-observed entities called axions through their effect on magnetic sensors.

    Coauthors include Amir Waxman, Yechezkel Schlussel and David Groswasser of Ben-Gurion University of the Negev, UC Berkeley Ph.D. graduate Victor Acosta, who is now at Google [x] in Mountain View, Calif., and former UC Berkeley post-doc Louis Bouchard, now a UCLA assistant professor of chemistry and biochemistry.

    The work was supported by the NATO Science for Peace program, AFOSR/DARPA QuASAR program, the National Science Foundation and UC Berkeley’s Miller Institute for Basic Research in Science.

    See the full article here.

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