Modern magnetic memories, such as hard drives installed in almost every computer, can store a very large amount of information thanks to very tiny, nanoscale magnetic sensors used for memory readout. The operation of these magnetic sensors, called the spin-valves, is based on the effect of giant magnetoresistance (GMR), for which its inventors Albert Fert and Peter Gruenberg were awarded a Nobel Prize in Physics in 2007. The GMR effect is based on the idea of electrical conduction in ferromagnetic metals, proposed by Sir Nevill F. Mott as early as in 1936. In Mott's picture, the conduction electrons in ferromagnetic metals experience scattering depending on their microscopic magnetic moment - the spin. That is, the electrons with one spin orientation scatter less and are therefore more conductive than the electrons with the opposite spin orientation. This spin-asymmetry in electron conduction is greatly amplified when the thin films of ferromagnetic and nonmagnetic metals are combined together to form a spin-valve in which electrical resistivity becomes very sensitive to the magnetic field, leading to a GMR effect. 
Difference in conduction by electrons with opposite spins in ferromagnetic metals can be precisely resolved using terahertz waves. (Image: MPI-P) Even though the Mott spin-dependent conductivity is at the heart of magnetic memories and many other technologies, its direct observation has been a long time challenge. Indeed the fundamental parameters of Mott conduction - spin-dependent electron scattering time and spin-dependent electron density - can be directly and unambiguously determined only if the conductivity of the metal is measured on the same ultrafast timescale at which the electron scattering occurs, that is sub-100 femtosecond (1 fs = 10-15 s, i.e. one millionth of one billionth of a second). For many decades, such an extremely fast timescale of experimental measurement precluded the observation of magnetotransport in metals on the fundamental level. In a collaborative work carried out by the research groups at the Max Planck Institute for Polymer Research (MPI-P) and the Johannes Gutenberg University (JGU), with the contribution of Sensitec GmbH and the Fritz Haber Institute of the Max Planck Society, the scientists managed to break the speed barrier for fundamental magnetotransport measurements by using a method called ultrafast terahertz spectroscopy (1 THz = 1012 Hz, i.e. one thousand billion oscillations per second). "By studying the interaction of THz electromagnetic waves - which oscillate about as fast as the electrons in metal scatter their momentum - with a spin-valve, we could directly measure for the first time the fundamental parameters of Mott conduction", explains Dmitry Turchinovich, project leader at the MPI-P. "In particular, we found that the traditional measurements performed on the slower timescales significantly underestimate the spin-asymmetry in electron scattering which is responsible for the magnetic sensor operation". The results of the research team: Zuanming Jin, Alexander Tkach, Frederick Casper, Victor Spetter, Hubert Grimm, Andy Thomas, Tobias Kampfrath, and Mischa Bonn, led by Dmitry Turchinovich (MPI-P) and Mathias Klaeui (JGU) have recently been published in ("Accessing the fundamentals of magnetotransport in metals with terahertz probes"). This work adds a new and powerful tool, ultrafast THz spectroscopy, to the studies in spintronics, opening up a new research field - terahertz spintronics.
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New nanocatalyst does more with less platinum
Platinum is a highly reactive and in-demand catalyst across the chemical and energy industries, but a team of University of Wisconsin-Madison and Georgia Institute of Technology scientists could reduce the world’s dependence on this scarce and expensive metal. In a paper published July 2, 2015 in the journal ("Palladium–platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction"), Paul A. Elfers and Vilas Distinguished Achievement Professor of Chemical and Biological Engineering Manos Mavrikakis and his research group describe a new catalyst that combines platinum with the less expensive metal palladium. This not only reduces the need for platinum but actually proves significantly more catalytically active than pure platinum in the oxygen reduction reaction, a chemical process key to fuel cell energy applications. The palladium-platinum combination also proves more durable, compounding the advantage of getting more reactivity with less material. Just as importantly, the paper offers a way forward for chemical engineers to design still more new catalysts for a broad range of applications by fine-tuning materials on the atomic scale. 
The particle's icosahedral structure makes for enhanced performance in the oxygen reduction reaction, a crucial process in the chemical and energy industries. The discovery was set in motion last year, when researchers at Georgia Tech developed nanoparticles consisting mostly of palladium, with a relatively small amount of platinum worked into the surface. Early on they realized that these particles showed significantly greater activity than pure platinum, as measured by dividing the current produced by the oxygen reduction reaction by the mass of platinum used. But to understand the why—the chemistry driving this apparent advantage—the Georgia Tech researchers turned to the UW-Madison team of Mavrikakis, graduate student Luke Roling and postdoctoral researcher Jeffrey Herron. Roling says that from a chemist’s point of view, the new catalyst’s high performance at first seemed counterintuitive. But the Mavrikakis group, by applying its strengths in modeling and computational analysis, discovered that the advantage lays in the nanoparticle’s icosahedral, or 20-faceted, structure. Moving forward, researchers looking for new catalysts can experiment with similar structures and perhaps find even more reactive materials. For Mavrikakis, the results vindicate years of research—on both the theoretical and synthesis sides of catalysis—that has focused on the importance of how a particular substance’s reactivity changes depending on whether it’s structured as an icosahedron, an octahedron or another shape. “This is speaking to the precise arrangement of atoms on the surface of a nanoparticle,” Mavrikakis says. “That can make an enormous difference in how fast the reaction takes place. Theory has been instrumental for about 10 years now to demonstrate the importance of being able to tailor-make specific facets of the same material.” As rapidly improving technology to synthesize new tailor-made materials aligns with quantum mechanics, this research could make all manner of catalysis-driven processes more efficient and less expensive. “The goal here is to try to minimize the amount of platinum that you use, and eventually find a complete replacement of platinum,” Mavrikakis says. “If we can move away from platinum, many of these applications have the potential to become more robust financially."
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3D atomic map gives clues to extending catalyst life
Despite decades of industrial use, the exact chemical transformations occurring within zeolites, a common material used in the conversion of oil to gasoline, remain poorly understood. Now scientists have found a way to locate—with atomic precision—spots within the material where chemical reactions take place, and how these spots shut down. Called active sites, the spots help rip apart and rearrange molecules as they pass through nanometer-sized channels, like an assembly line in a factory. A process called steaming causes these active sites to cluster, effectively shutting down the factory, the scientists reported in ("Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography"). This knowledge could help devise how to keep the factory running longer, so to speak, and improve catalysts that help produce fuel, biofuel and other chemicals. The team included scientists from the Department of Energy’s Pacific Northwest National Laboratory, petroleum refining technology company UOP LLC and Utrecht University. To make this discovery, they reconstructed the first 3-D atomic map of an industrially relevant zeolite material to track down its key element, aluminum. 
Green dots in this image represent aluminum atoms within a zeolite crystal. When aluminum atoms bunch up, zeolites lose their ability to convert oil to gasoline and other chemicals. An international team of scientists created the first 3-D atomic map of the material in order to find out how to extend the catalyst’s life. When things get steamy, structure changes Zeolites are minerals made up of aluminum, silicon and oxygen atoms arranged in a three-dimensional crystalline structure. Though they look like white powder to the naked eye, zeolites have a sponge-like network of molecule-size pores. Aluminum atoms along these pores act like workers on an assembly line—they create active sites that give zeolites their catalytic properties. Industry uses about a dozen synthetic zeolites as catalysts to process petroleum and chemicals. One major conversion process, called fluid catalytic cracking, depends on zeolites to produce the majority of the world’s gasoline. To awaken active sites within zeolites, industry pretreats the material with heat and water, a process called steaming. But too much steaming somehow switches the sites off. Changing the conditions of steaming could extend the catalyst’s life, thus producing fuel more efficiently. Scientists have long suspected that steaming causes aluminum to move around within the material, thus changing its properties. But until now aluminum has evaded detailed analysis. Strip away the atoms Most studies of zeolite structure rely on electron microscopy, which can’t easily distinguish aluminum from silicon because of their similar masses. Worse, the instrument’s intense electron beam tends to damage the material, changing its inherent structure before it’s seen. Instead, the team of scientists turned to a characterization technique that had never before been successfully applied to zeolites. Called atom probe tomography, it works by zapping a sample with a pulsing laser, providing just enough energy to knock off one atom at a time. Time-of-flight mass spectrometers analyze each atom—at a rate of about 1,000 atoms per second. Unlike an electron microscope, this technique can distinguish aluminum from silicon. Though atom probe tomography has been around for 50 years, it was originally designed to look at conductive materials, such as metals. Less conductive zeolites presented a problem. PNNL materials scientist Danny Perea and his colleagues overcame this hurdle by adapting a Local Electrode Atom Probe at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility accessible to scientists around the world. Most attempts to image the material ended prematurely, when electromagnetic forces within the instrument vaporized the entire sample. The key to success was to find the right conditions to prepare a sample and then to coat it with a layer of metal to help provide conductivity and strength to withstand analysis. After hours of blasting tens-of-millions of atoms, the scientists could reconstruct an atomic map of a sample about a thousand times smaller than the width of a human hair. These maps hold clues as to why the catalyst fails. 
These coffin-shaped growths make up one variety of porous materials called zeolites. An international team of scientists discovered that when aluminum atoms in the material cluster in the overlapping intersections of these sub-units, zeolites lose their ability to convert oil to gasoline and other chemicals. A place to cluster The images confirmed what scientists have long suspected: Steaming causes aluminum atoms to cluster. Like workers crowded around one spot on the assembly line, this clustering effectively shuts down the catalytic factory. The scientists even pinpointed the place where aluminum likes to cluster. Zeolite crystals often grow in overlapping sub-units, forming something like a 3-D Venn diagram. Scientists call the edge between two sub-units a grain boundary, and that’s where the aluminum clustered. The scientists suspect that open space along grain boundaries attracted the aluminum. With the guidance of these atomic maps, industry could one day modify how it steams zeolites to produce a more efficient, longer lasting catalyst. The research team will next examine other industrially important zeolites at different stages of steaming to provide a more detailed map of this transformation.
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