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Projection-operator methods for classical transport in magnetized plasmas. Part 2. Nonlinear response and the Burnett equations [Projection-operator methods for classical transport in magnetized plasmas. Part 1. Nonlinear response and the Burnett equations]
The time-independent projection-operator formalism for the derivation of Burnett equations is extended and considered in the context of multispecies and magnetized plasmas. The procedure provides specific formulas for transport coefficients in terms of two-time correlation functions involving both two and three phase-space points. It is shown how to calculate those correlation functions in the limit of weak coupling. The results are used to demonstrate, with the aid of a particular non-trivial example, that the Chapman–Enskog methodology employed by Catto & Simakov (CS) (Phys. Plasmas, vol. 11, 2004, pp. 90–102) to calculate the contributions to the parallel viscosity driven by temperature gradients is consistent with formulas previously derived from the two-time formalism by Brey (J. Chem. Phys., vol. 79, 1983, pp. 4585–4598). Here, the work serves to unify previous work on plasma kinetic theory with formalism usually applied to turbulence. Additional contributions include discussions of (i) Braginskii-order interspecies momentum exchange from the point of view of two-time correlations; and (ii) a simple stochastic model, unrelated to many-body theory, that exhibits Burnett effects. Insights from that model emphasize the role of non-Gaussian statistics in the evaluation of Burnett transport coefficients, including the effects calculated by CS that stem from the nonlinear collision operator. Together, Parts 1 and 2 of this series provide an introduction to projection-operator methods that should be broadly useful in theoretical plasma physics.
BaFe 4 O 7 and K 0.22 Ba 0.89 Fe 4 O 7 : Canted Antiferromagnetic Diferrites with Exceptionally High Magnetic Ordering Temperatures [BaFe 4 O 7 and K 0.22 Ba 0.89 Fe 4 O 7 – New Canted Antiferromagnetic Diferrites with Exceptionally High Magnetic Ordering Temperatures]
Here, complex iron containing oxides, typically referred to as ferrites, have been extensively studied for their electronic and magnetic properties, including magnetotransport, ferromagnetism, ferroelectricity, and a combination of the latter two, known as multiferroism. The search for materials possessing simultaneous ferromagnetic and ferroelectric behavior is of significant interest both for the advancement of fundamental science and for applications in random access memory (RAM). Despite the great technological potential of multiferroics, one or both of the ferroic ordering temperatures tend to fall below room temperature, such as in the compositions TbMnO 3 and Ni 3 V 2 O 8 , thus eliminating their practical utility for RAM applications. The scarcity of multiferroics possessing room temperature ferroic orderings continues to drive exploratory research in this field. For this reason, ferrites have garnered significant interest due to their often high ferroic ordering temperatures, for example in BiFeO 3 . Among all ferrites, hexaferrites are the most studied prototypical examples and constitute a specific subclass of ferrites that follows the stoichiometry Ba O– n Fe 2 O 3 , in which the n value dictates the nomenclature, i.e., ( n = 6) is a hexaferrite or ( n = 1) is a monoferrite. This versatile class of materials encompasses a wide range of related structures best described by Pullar, serving as the motivation for this work.
Ab Initio Study of Vibronic and Magnetic 5f-to-5f and Dipole-Allowed 5f-to-6d and Charge-Transfer Transitions in [UX 6 ] n – (X = Cl, Br; n = 1, 2)
The absorption spectra of the octahedral [UX 6 ] n– (X = Cl, Br; n = 1, 2) complexes in the near-infrared (NIR) and UV–vis spectral regions were studied theoretically, using a relativistic restricted active space second-order perturbation theory (RASPT2) wavefunction framework, with the spin–orbit (SO) coupling treated by state interaction, in conjunction with Kohn–Sham density functional theory calculations for determining the vibrational normal modes. The electric-dipole-allowed ligand-to-metal charge-transfer (LMCT) and 5f-to-6d transitions, and the electric-dipole-forbidden 5f-to-5f ligand field (LF) transitions, are thereby obtained within the same theoretical framework. For the 5f-to-5f LF transitions, the observed absorption intensity is mostly due to vibronic coupling with low-energy electric-dipole-allowed transitions, but in some cases, the magnetic dipole intensity of the purely electronic transition has comparable intensity to the vibronic transitions. Here, experimental LF spectra of 5f 2 open-shell systems have been reported decades back, but ab initio calculations of their vibronic intensity have not yet been reported in the literature. Although the LF spectra for the 5f 2 systems can be assigned in detail, based on the calculations, the spectra are very complex and the underlying electronic states are strongly multiconfigurational. Therefore, the usefulness of the LF spectra beyond serving as a “fingerprint” of the LF and the metal oxidation state appears to be limited.
Zero-field edge plasmons in a magnetic topological insulator [Zero-field edge magnetoplasmons in a magnetic topological insulator]
Incorporating ferromagnetic dopants into three-dimensional topological insulator thin films has recently led to the realisation of the quantum anomalous Hall effect. These materials are of great interest since they may support electrical currents that flow without resistance, even at zero magnetic field. To date, the quantum anomalous Hall effect has been investigated using low-frequency transport measurements. However, transport results can be difficult to interpret due to the presence of parallel conductive paths, or because additional non-chiral edge channels may exist. Here we move beyond transport measurements by probing the microwave response of a magnetised disk of Cr-(Bi,Sb) 2 Te 3 . We identify features associated with chiral edge plasmons, a signature that robust edge channels are intrinsic to this material system. Finally, our results provide a measure of the velocity of edge excitations without contacting the sample, and pave the way for an on-chip circuit element of practical importance: the zero-field microwave circulator.
Correlation of dielectric, electrical and magnetic properties near the magnetic phase transition temperature of cobalt zinc ferrite
CZFO exhibits low loss tangent, high dielectric constant, large magnetization and magnetodielectric coupling above RT.
Monte Carlo simulation of magnetic domain structure and magnetic properties near the morphotropic phase boundary
The magnetization properties changing with the anisotropy energy barrier (a) and temperature (b) under a constant field.
High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium( iii ) metallocenium single-molecule magnets
Subtle changes in ligand substitution result in substantial changes in molecular structure and magnetic properties in a series of dysprosium( iii ) metallocenium salts.
Exploring room-temperature transport of single-molecule magnet-based molecular spintronics devices using the magnetic tunnel junction as a device platform
A device architecture utilizing a single-molecule magnet (SMM) as a device element between two ferromagnetic electrodes may open vast opportunities to create novel molecular spintronics devices.
Electron transfer induced magnetic ordering of metal-cyanide magnets
Solution synthesis of molecular cyanide magnets via FeSe template is promoted by thermal activation, where TCNE molecules substitute Se atoms in two-dimensional FeSe framework.
Relaxation dynamics in see-saw shaped Dy( iii ) single-molecule magnets
Unusual see-saw shaped Dy( iii ) single-molecule magnets, [K(DME) n ][L Ar Dy(X) 2 ] (L Ar = {C 6 H 4 [(2,6- i PrC 6 H 3 )NC 6 H 4 ] 2 } 2− ), X = Cl ( 1 ) and X = I ( 2 ) were synthesized and display high effective energy barriers ( U eff = 1278–1334 K) in zero field.
Clean measurements of the nucleon axial-vector and free-neutron magnetic form factors
We discuss the feasibility of a weak charged current experiment using a low energy electron beam. A first goal is to measure the Q{sup 2} dependence of the axial-vector form factor g{sub a}(Q{sup 2}). It can be measured model-independently and as robustly as for electromagnetic form factors from typical electron scattering experiments, in contrast to the methods used so far to measure g{sub a}(Q{sup 2}). If g{sub a}(Q{sup 2}) follows a dipole form, the axial mass can be extracted with a better accuracy than the world data altogether. The most important detection equipment would be a segmented neutron detector with good momentum and angular resolution that is symmetric about the beam direction, and covers a moderate angular range. A high intensity beam (100 uA) is necessary. Beam polarization is highly desirable as it provides a clean measurement of the backgrounds. Beam energies between 70 and 110 MeV are ideal. This range would provide a Q{sup 2} mapping of g{sub a} between 0.01 <Q{sup 2}< 0.04 GeV{sup 2}. 60 days of beam can yield 14 data points with a subpercent statistical and point to point uncorrelated uncertainties on each point. Such an experiment may also allow to measure the free-neutron magnetic form factor G{sub M}{sup n}. The experiment employs the usual techniques of electron-nucleon scattering and presents no special difficulty. Higher energy extensions are possible. They could yield measurements of g{sub a}(Q{sup 2}) up to Q{sup 2}=3 GeV{sup 2} and the possibility to access other form factors, such as the almost unknown pseudoscalar form factor g{sub P}. However, the experiments become much more challenging as soon as beam energies pass the pion production threshold.
Resolving the controversy of a possible relationship between perpendicular magnetic anisotropy and the magnetic damping parameter
We use broadband ferromagnetic resonance spectroscopy to systematically measure the Landau-Lifshitz damping parameter, perpendicular anisotropy, and the orbital moment asymmetry in Co{sub 90}Fe{sub 10}/Ni multilayers. No relationship is found between perpendicular magnetic anisotropy and the damping parameter in this material. However, inadequate accounting for inhomogeneous linewidth broadening, spin-pumping, and two-magnon scattering could give rise to an apparent relationship between anisotropy and damping. In contrast, the orbital-moment asymmetry and the perpendicular anisotropy are linearly proportional to each other. These results demonstrate a fundamental mechanism by which perpendicular anisotropy can be varied independently of the damping parameter.
Layer resolved magnetic domain imaging of epitaxial heterostructures in large applied magnetic fields
In this work, we use X-ray Excited Luminescence Microscopy to investigate the elemental and layer resolved magnetic reversal in an interlayer exchange coupled (IEC) epitaxial Fe/Cr wedge/Co heterostructure. The transition from strongly coupled parallel Co-Fe reversal for Cr thickness t(Cr) < 0.34 nm to weakly coupled layer independent reversal for t(Cr) > 1.5 nm is punctuated at 0.34 < t(Cr) < 1.5 nm by a combination of IEC guided domain wall motion and stationary zig zag domain walls. Domain walls nucleated at switching field minima are guided by IEC spatial gradients and collapse at switching field maxima.
Effective Hamiltonians for correlated narrow energy band systems and magnetic insulators: Role of spin-orbit interactions in metal-insulator transitions and magnetic phase transitions
Using a second-quantized many-electron Hamiltonian, we obtain (a) an effective Hamiltonian suitable for materials whose electronic properties are governed by a set of strongly correlated bands in a narrow energy range and (b) an effective spin-only Hamiltonian for magnetic materials. The present Hamiltonians faithfully include phonon and spin-related interactions as well as the external fields to study the electromagnetic response properties of complex materials and they, in appropriate limits, reduce to the model Hamiltonians due to Hubbard and Heisenberg. With the Hamiltonian for narrow-band strongly correlated materials, we show that the spin-orbit interaction provides a mechanism for metal-insulator transition, which is distinct from the Mott-Hubbard (driven by the electron correlation) and the Anderson mechanism (driven by the disorder). Next, with the spin-only Hamiltonian, we demonstrate the spin-orbit interaction to be a reason for the existence of antiferromagnetic phase in materials which are characterized by a positive isotropic spin-exchange energy. This is distinct from the Néel-VanVleck-Anderson paradigm which posits a negative spin-exchange for the existence of antiferromagnetism. We also find that the Néel temperature increases as the absolute value of the spin-orbit coupling increases.
Tuning the magnetoresistance symmetry of Pt on magnetic insulators with temperature and magnetic doping
We present a comparison study of the temperature dependence of the intriguing magnetoresistance (MR) in Pt/YIG (yttrium iron garnet), Pt/YIG BB (the YIG substrate has been bombarded with Ar + ), and Pt/SiO 2 (with different Fe doping levels). With decreasing temperature, the MRs in Pt/YIG and Pt/YIG BB change symmetry from R z = R x > R y at room temperature to R x > R z > R y at low temperature. A similar behavior in both Pt/YIG and Pt/YIG BB implies that the underlying physics is due to magnetic scattering, instead of the pure spin current across the interface. By changing the Fe doping level in the SiO 2 substrate, we can further systematically modulate the symmetry of MR in Pt/SiO 2 (Fe doped). The doping level dependent symmetry can also qualitatively explain the controversy over the MRs of Pt/YIG and similar structures at low temperature.
Axial magnetic field injection in magnetized liner inertial fusion
MagLIF is a fusion concept using a Z-pinch implosion to reach thermonuclear fusion. In current experiments, the implosion is driven by the Z-machine using 19 MA of electrical current with a rise time of 100 ns. MagLIF requires an initial axial magnetic field of 30 T to reduce heat losses to the liner wall during compression and to confine alpha particles during fusion burn. This field is generated well before the current ramp starts and needs to penetrate the transmission lines of the pulsed-power generator, as well as the liner itself. Consequently, the axial field rise time must exceed hundreds of microseconds. Any coil capable of being submitted to such a field for that length of time is inevitably bulky. The space required to fit the coil near the liner, increases the inductance of the load. In turn, the total current delivered to the load decreases since the voltage is limited by driver design. Yet, the large amount of current provided by the Z-machine can be used to produce the required 30 T field by tilting the return current posts surrounding the liner, eliminating the need for a separate coil. However, the problem now is the field penetration time, across the liner wall. This paper discusses why skin effect arguments do not hold in the presence of resistivity gradients. Numerical simulations show that fields larger than 30 T can diffuse across the liner wall in less than 60 ns, demonstrating that external coils can be replaced by return current posts with optimal helicity.
Evaluation of magnetic field error in ExtendedEBIS
The upgrade of the EBIS, called ExtendedEBIS, is now in progress in Brookhaven National Laboratory. Two 5T-superconducting solenoids have been placed in series with 200 mm distance from each other for higher trap capacity and production of polarized 3He ions. Since the two superconducting solenoids are used, the field error is expected to be larger. In this research, the field error is estimated based on simulation. It was found that the magnetic field line can be 17 mm off from the center axis at the entrance of the collector, and this offset can be adjusted by three types of correction coils. Additionally, mismatch of the spin direction of polarized 3He due to the misalignment was estimated to be small.