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Norem J.,Argonne National Laboratory | Becker N.,Argonne National Laboratory | Kharitonov M.,Argonne National Laboratory | Klug J.A.,Argonne National Laboratory | And 5 more authors.
Proceedings - 25th Linear Accelerator Conference, LINAC 2010

An effort, centered at Argonne, has started to explore the use of Atomic Layer Deposition (ALD) to study and improve the performance of superconducting rf (SRF) accelerating structures. This effort has a number of parts: A survey the properties of ALD deposited films, a study of loss mechanisms of SRF structures, and a program of coating single cell cavities, to begin to optimize the performance of complete systems. Early results have included improving the performance of individual structures and, identification of magnetic oxides as a loss mechanism in SRF. We describe the program and summarize recent progress. Source

Pajerowski D.M.,NHMFL | Pajerowski D.M.,Center for Neutron Research | Gardner J.E.,University of Florida | Gardner J.E.,Intel Corporation | And 7 more authors.
Chemistry of Materials

Heterostructured thin films consisting of distinct layers of the Prussian blue analogues RbaCob[Fe(CN)6] c•mH2O (CoFe PBA) and RbjM k[Cr(CN)6]l•nH2O (MCr PBA, where M = Ni or Co) have been fabricated, and their photomagnetic properties have been investigated. The CoFe PBA is known to be photoactive, with light induced changes in the unit cell size and the spin states below ∼150 K and magnetic order below ∼20 K. The NiCr and CoCr PBAs do not have native photoeffects, but are known to have higher magnetic ordering temperatures (TC NiCr ∼ 70 K, TC CoCr ∼ 30 K), and a pressure dependence of the magnetization. The layered heterostructures are synthesized using aqueous chemistry and sequential adsorption techniques that allow for fine control of layer thickness. Some of the heterostructured films show photoinduced magnetization changes up to the ordering temperatures of the MCr PBA component, behavior that is not seen when the individual materials are measured separately. A variety of different layer arrangements and thicknesses has been investigated with the goal of identifying structures that optimize the photocontrol of the magnetic response in the MCr PBA lattices, which are in intimate contact with the photoactive CoFe PBA lattices. The new behavior is optimized when the constituent layers have thicknesses on the order of hundreds of nanometers. When layers are too thin, it is shown that mixing of ions at the interface between PBA components leads to mixed-metal phases. The concurrence of the maximum temperature of the large photomagnetic effect with the native ordering temperature of the MCr PBA lattice, as well as its magnetic field dependence, supports the interpretation that the photocontrol is the result of photoinduced structural changes in the CoFe PBA lattice coupling to the MCr PBA component of the heterostructure, inducing random magnetic anisotropy. © 2011 American Chemical Society. Source

Crawled News Article
Site: http://news.mit.edu/topic/mitenergy-rss.xml

In the pursuit of material platforms for the next generation of electronics, scientists are studying new compounds such as topological insulators (TIs), which support protected electron states on the surfaces of crystals that silicon-based technologies cannot. Dramatic new physical phenomena are being realized by combining this field of TIs with the subfield of spin-based electronics known as spintronics. The success within spintronics of realizing important magnetic technologies such as the spin valve have increased the expectations that new results in TIs might have near-term applications. However, combining these two research threads has relied on “shoehorning” magnetism by forcing magnetic atoms to partially occupy elemental positions in TIs or by applying a conventional magnetic field. Realizing an integrated material that is both intrinsically magnetic and has a topological character has proven more challenging. Recently a team of researchers based in the group of Joseph G. Checkelsky, assistant professor of physics at MIT, and collaborators at the NIST Center for Neutron Research (NCNR), Carnegie Mellon University, and the Beijing Institute of Technology have experimentally demonstrated a “hybrid material” solution to this problem. They studied a compound of three elements, gadolinium, platinum and bismuth, known together as a ternary compound. In their compound, gadolinium supplies the magnetic order while the platinum-bismuth components support a topological electronic structure. These two components acting in concert make a correlated material that is more than the sum of its parts, showing quantum mechanical corrections to electrical properties at an unprecedented scale. Their results were reported July 18 in Nature Physics. Proving this delicate interplay between the constituent elements of this compound required studying it from different perspectives. With experimental efforts led by research scientist Takehito Suzuki, the MIT group (including physics graduate student Aravind Devarakonda and physics undergraduate Yu-Ting Liu) synthesized single crystals and studied their electronic and magnetic properties. The team found these crystals at the same time were exotic magnets and exhibited signatures of electronic topology. The latter was observed through the so-called Berry phase corrections to electronic behavior, where they saw the largest such response reported to date in this type of magnet, which is known as an antiferromagnet. The Berry phase reflects the quantum mechanical nature of the charge-carrying electrons in metals and is influenced by magnetic order. They identified an antiferromagnetic transition temperature of 9.2 kelvins (-443 degrees Fahrenheit). At or below this temperature, magnetic moments of the gadolinium atoms align in an alternating pattern of spin up and spin down. Interestingly, for temperatures significantly higher than this they could observe remnants of this magnetic order in both magnetic and electronic properties, a possible hallmark of the underlying frustrated geometry of the crystal lattice. While these experiments were enticing, the team wanted to be sure that what they were observing originated from the topological properties that would connect this material to potentially ground-breaking types of future electronic devices.  Experimentally, this involved work at national facilities including the NCNR, where Suzuki worked with Robin Chisnell PhD ’14 and NIST Fellow Jeffrey W. Lynn. Using a triple axis spectrometer (BT-7), they studied the scattering of neutrons from carefully aligned single crystals in the low temperature antiferromagnetic phase including in different magnetic field conditions. These experiments provided the ability to map the behavior of the magnetic gadolinium spins to precisely know their orientation and response to temperature and magnetic field. In order to do these experiments, naturally occurring gadolinium could not be used due to its overwhelming neutron scattering cross-section, and the researchers used instead a costly isotope known as gadolinium -160. In general, growing single crystals of these compounds is challenging because of their high melting temperature; the growth process known as a “flux method” requires high-temperature centrifuging to remove the crystals from a bath of liquid bismuth.  “It’s a bit like growing rock candy sugar crystals, except that we use liquid bismuth instead of water,” says Checkelsky.  The process is well-known to solid state chemists, but can have a high failure rate. The team was able to obtain enough of the isotope for just two growth runs, both of which turned out to be successful. The subsequent experiments at NCNR were able to provide critical information about the gadolinium moments that shaped the team’s understanding of their results. The team also made use of the National High Magnetic Field Laboratory (NHMFL) based in Tallahassee, Florida. There, the MIT team brought the crystals to measure their response to extreme magnetic conditions involving magnetic fields in excess of 30 T (among the largest DC fields available in the world). The extreme conditions available at the NHMFL also allowed the group to broadly map the electronic and magnetic properties of the crystals to complete the picture of the magnetic order.  In particular, they were able to observe a previously unreported phase transition for the gadolinium spins near 25 T that appeared to finally “break” the antiferromagnetic state. The final aspect of the collaborative effort was with professors Di Xiao of Carnegie Mellon University and Wanxiang Feng of Beijing Institute of Technology, who provided first principles electronic structure calculations based on the experimental data taken at MIT, NCNR, and the NHMFL to determine the underlying electronic character of this new materials system. “The authors combine high-quality crystal growth, transport measurements, neutron spectroscopy, and theoretical calculations to establish the magnetic ordering and its profound effect on electrical properties in a topological material,” says Liang Fu, assistant professor of physics at MIT, who was not involved in this research. “This seminal work reveals surprising quantum phenomena arising from the interplay between electron topology and correlation. This type of correlated topological phenomena is long sought after, but has been difficult to find in real materials. By identifying the right material, Joe Checkelsky's group and collaborators have found a new, promising platform for fundamental research and potential spintronics applications.”

O'Shea F.H.,Radiabeam Technologies, LLC | Agustsson R.,Radiabeam Technologies, LLC | Chen Y.-C.,Radiabeam Technologies, LLC | Grandsaert T.,Radiabeam Technologies, LLC | And 5 more authors.
IPAC 2013: Proceedings of the 4th International Particle Accelerator Conference

The demand for high-brightness hard x-ray fluxes from next generation light sources has spurred the development of insertion devices with shorter periods and higher fields than is feasible with conventional materials and designs. RadiaBeam Technologies is currently developing a novel high peak field, ultrashort period undulator with praseodymium-iron-boron (PrFeB) permanent magnets and textured dysprosium (Tx Dy) ferromagnetic field concentrators. This device will offer an unparalleled solution for compact x-ray light sources, as well as for demanding applications at conventional synchrotron radiation sources. A 1.4T on-axis field has already been achieved in a 9mm period undulator (K∼1.2), demonstrating the feasibility of using Tx Dy poles in a hybrid undulator configuration with PrFeB magnets. Facets of the undulator design, optimization of the Tx Dy production and characterization process, and magnetic measurements of Tx Dy will be presented. Copyright © 2013 by JACoW- cc Creative Commons Attribution 3.0 (CC-BY-3.0). Source

Manson J.L.,Eastern Washington University | Lapidus S.H.,State University of New York at Stony Brook | Stephens P.W.,State University of New York at Stony Brook | Peterson P.K.,Eastern Washington University | And 19 more authors.
Inorganic Chemistry

[Ni(HF2)(pyz)2]X {pyz = pyrazine; X = PF 6 - (1), SbF6 - (2)} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN4F2 octahedra. At 298 K, 1 is monoclinic (C2/c) with unit cell parameters, a = 9.9481(3), b = 9.9421(3), c = 12.5953(4) Å, and β = 81.610(3)° while 2 is tetragonal (P4/nmm) with a = b = 9.9359(3) and c = 6.4471(2) Å and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the c-axis which are linked via μ-pyz bridges in the ab-plane to afford three-dimensional polymeric frameworks with PF6 - and SbF6 - counterions occupying the interior sites. A major difference between 1 and 2 is that the Ni-F-H bonds are bent (∼157°) in 1 but are linear in 2. Ligand field calculations (LFT) based on an angular overlap model (AOM), with comparison to the electronic absorption spectra, indicate greater π-donation of the HF2 - ligand in 1 owing to the bent Ni-F-H bonds. Magnetic susceptibility data for 1 and 2 exhibit broad maxima at 7.4 and 15 K, respectively, and λ-like peaks in dπT/dT at 6.2 and 12.2 K that are ascribed to transitions to long-range antiferromagnetic order (TN). Muon-spin relaxation and specific heat studies confirm these TN's. A comparative analysis of π vs T to various 1D Heisenberg/Ising models suggests moderate antiferromagnetic interactions, with the primary interaction strength determined to be 3.05/3.42 K (1) and 5.65/6.37 K (2). However, high critical fields of 19 and 37.4 T obtained from low temperature pulsed-field magnetization data indicate that a single exchange constant (J1D) alone is insufficient to explain the data and that residual terms in the spin Hamiltonian, which could include interchain magnetic couplings (J⊥), as mediated by Ni-pyz-Ni, and single-ion anisotropy (D), must be considered. While it is difficult to draw absolute conclusions regarding the magnitude (and sign) of J⊥ and D based solely on powder data, further support offered by related Ni(II)-pyz compounds and our LFT and density-functional theory (DFT) results lead us to a consistent quasi-1D magnetic description for 1 and 2. © 2011 American Chemical Society. Source

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