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Oxide shell reduction and magnetic property changes in core-shell Fe nanoclusters under ion irradiation
Ion irradiation effects are studied on the Fe-based core-shell nanocluster (NC) films with core as Fe and shell as Fe{sub 3}O{sub 4}/Fe{sub 3}N. These NC films were deposited on Si substrates to thickness of ∼0.5 μm using a NC deposition system. The films were irradiated at room temperature with 5.5 MeV Si{sup 2+} ions to ion fluences of 10{sup 15} and 10{sup 16} ions/cm{sup 2}. It is found that the irradiation induces grain growth, Fe valence reduction in the shell, and crystallization or growth of Fe{sub 3}N. The film retained its Fe-core and its ferromagnetic properties after irradiation. The nature and mechanism of oxide shell reduction and composition dependence after irradiation were studied by synthesizing additional NC films of Fe{sub 3}O{sub 4} and FeO + Fe{sub 3}N and irradiating them under the same conditions. The presence of nanocrystalline Fe is found to be a major factor for the oxide shell reduction. The surface morphologies of these films show dramatic changes in the microstructures due to cluster growth and agglomeration as a result of ion irradiation.
Chaotic magnetic fields in Vlasov-Maxwell equilibria
Stationary solutions of Vlasov-Maxwell equations are obtained by exploiting the invariants of single particle motion leading to linear or nonlinear functional relations between current and vector potential. For a specific combination of invariants, it is shown that Vlasov-Maxwell equilibria have an associated Hamiltonian that exhibits chaos.
Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl
Disordered CoFeCrAl and CoFeCrSi{sub 0.5}Al{sub 0.5} alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi{sub 0.5}Al{sub 0.5} is predicted to increase from 2.01 μ{sub B} to 2.50 μ{sub B} per formula unit, in good agreement with experiment.
Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks
The electromagnetic coupling of helical modes, even those having different toroidal mode numbers, modifies the distribution of toroidal angular momentum in tokamak discharges. This can have deleterious effects on other transport channels as well as on magnetohydrodynamic (MHD) stability and disruptivity. At low levels of externally injected momentum, the coupling of core-localized modes initiates a chain of events, whereby flattening of the core rotation profile inside successive rational surfaces leads to the onset of a large m/n = 2/1 tearing mode and locked-mode disruption. With increased torque from neutral beam injection, neoclassical tearing modes in the core may phase-lock to each other without locking to external fields or structures that are stationary in the laboratory frame. The dynamic processes observed in these cases are in general agreement with theory, and detailed diagnosis allows for momentum transport analysis to be performed, revealing a significant torque density that peaks near the 2/1 rational surface. However, as the coupled rational surfaces are brought closer together by reducing q{sub 95}, additional momentum transport in excess of that required to attain a phase-locked state is sometimes observed. Rather than maintaining zero differential rotation (as is predicted to be dynamically stable by single-fluid, resistive MHD theory), these discharges develop hollow toroidal plasma fluid rotation profiles with reversed plasma flow shear in the region between the m/n = 3/2 and 2/1 islands. The additional forces expressed in this state are not readily accounted for, and therefore, analysis of these data highlights the impact of mode coupling on torque balance and the challenges associated with predicting the rotation dynamics of a fusion reactor—a key issue for ITER.
Engineering magnetic anisotropy in two-dimensional magnetic materials
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Computational search for itinerant n -type and p -type magnetic semiconductors: Arsenopyrites as bipolar magnetic semiconductors
We report a computational search for three-dimensional bulk ferromagnetic semiconductors based on itinerant Stoner physics. Stoner-based ferromagnetic semiconductors require an unusual combination of high band edge density of states and moderate transport effective mass. Nonetheless, several potential materials are found. Here, we identify arsenopyrites, exemplified by FeAsS, as materials that, with suitable doping, exhibit ferromagnetic semiconducting behavior for both n- and p-type dopants. The existence of a material that becomes ferromagnetic for both p and n types may enable device concepts, for example, ferromagnetic p–n junction devices.
Spontaneous Magnetic Superdomain Wall Fluctuations in an Artificial Antiferromagnet.
Collective dynamics often play an important role in determining the stability of ground states for both naturally occurring materials and metamaterials. We studied the temperature dependent dynamics of antiferromagnetically ordered superdomains in a square artificial spin lattice using soft x-ray photon correlation spectroscopy. We observed an exponential slowing down of superdomain wall motion below the antiferromagnetic onset temperature, similar to the behavior of typical bulk antiferromagnets. Using a continuous time random walk model we show that these superdomain walls undergo low-temperature ballistic and high-temperature diffusive motions.
Superconducting Magnetic Energy Storage for Pulsed Power Magnet Applications.
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High Performance PrFeB Permanent Magnets through Novel Pulse Alignment and Compaction Method for Cryogenic Permanent Magnet Undulators (CPMU)
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Iso-nuclear tungsten dielectronic recombination rates for use in magnetically-confined fusion plasmas
Under the auspices of the IAEA Atomic and Molecular Data Center and the Korean Atomic Energy Research Institute, our assembled group of authors has reviewed the current state of dielectronic recombination (DR) rate coefficients for various ion stages of tungsten (W). Subsequent recommendations were based upon available experimental data, first-principle calculations carried out in support of this paper and from available recombination data within existing atomic databases. If a recommendation was possible, data were compiled, evaluated and fitted to a functional form with associated uncertainty information retained, where available. This paper also considers the variation of the W fractional abundance due to the underlying atomic data when employing different data sets.
Magnetic properties and phase stability of tetragonal Ce1-xSmxFe9Co2Ti 1:12 phase for permanent magnets
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Influence of the external pressure on the structure and magnetic properties of Nd 0.5 Sr 0.5 CoO 3 cobaltite
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Dicobalt (II) hydroxo-selenite: Hydrothermal synthesis, crystal structure and magnetic properties of Co2SeO3(OH)2
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The crystal structure, electronic, and magnetic properties of NaPd{sub 3}Ge{sub 2}
Highlights: • Correct crystal structure of NaPd{sub 3}Ge{sub 2} is solved by single crystal X-ray diffraction. • NaPd{sub 3}Ge{sub 2} is found to crystallize in the ErRh{sub 3}Si{sub 2} structure type. • NaPd{sub 3}Si{sub 2}, NaPd{sub 3}Ge{sub 2} and NaPd{sub 3}Sn{sub 2} are found to be metals down to 2 K. • All three compounds show a weak diamagnetic response down to 2 K. • Trend of p-block atom dimer breaking as function of size. - Abstract: Crystals of NaPd{sub 3}X{sub 2}, (X = Si, Ge, Sn) were synthesized and their crystal structures investigated by single crystal X-ray diffraction. The previously reported structures of NaPd{sub 3}Si{sub 2} and NaPd{sub 3}Sn{sub 2} were confirmed. NaPd{sub 3}Ge{sub 2} was found to have a different unit cell from that originally reported; therefore the structure was solved. It is an orthorhombically distorted version of the hexagonal CeCo{sub 3}Be{sub 2}-type structure, in space group Imma, with cell parameters a = 7.244(1) Å, b = 9.938(1) Å, c = 5.767(2) Å. The originally reported cell is explained through triple twinning of the true cell. The structure of NaPd{sub 3}Ge{sub 2} fits the trend of decreasing X–X dimerization as a function of increasing period from Si–Sn. All three compounds are metals showing weak diamagnetism with increasing resistivity from NaPd{sub 3}Si{sub 2}–NaPd{sub 3}Sn{sub 2}; no superconductivity is observed down to 2 K.
Controlling Magnetic Anisotropy in a Zero-Dimensional S = 1 Magnet Using Isotropic Cation Substitution
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