Noncollinear Magnetic Field Research Articles

PARAMETRIC FREQUENCY GENERATOR ON NONCOLLINEAR MAGNETIC FIELDS

Purpose. The article is aimed to study a parametric frequency generator on noncollinear magnetic fields, which performs the functions of a stabilized power source and means of protection against the effects of powerful impulse noises in the form of switching overvoltages and lightning discharges. Methodology. To carry out experimental studies, a prototype model of a parametric generator was fabricated. A number of studies were carried out to determine the optimal parameters of the pump circuits and the circuit of the device, in which the best energy characteristics and maximum output power are achieved with stable generation in the system. Findings. The authors proved the importance to develop a means of protecting signaling equipment, centralization and blocking from the effects of powerful impulse noises. The developed construction of the device completely eliminates the transformation of energy, it means the penetration of noise from the input to the output. As a result of the experimental studies, the optimal parameters of the generator circuits, which provide its best energy characteristics, were determined. Originality. The device of the parametric generator, which has energy-consuming elements in its design, has been improved, this significantly improves the energy characteristics and properties. In order to improve the reliability of automatic blocking systems, the methods for protecting railway automation equipment and telemechanics from the effects of high-power impulse noises and high levels of traction currents were further developed. Practical value. Based on the obtained results, the device under investigation can be introduced into production and used in railway automation systems to organize a stabilized secondary power source for railway automation track equipment, as well as to protect equipment from the effects of powerful impulse noises created by lightning discharges and lightning or switching overvoltages. The results of work can also be used in re-equipment of the hauls with new railway automation systems and in the preparation of design documentation for track circuits, station systems and automatic blocking systems. The results are used during the studying of the subjects: «Special Measurements at Railway Transport» and «Automatic Devices Power Supply»; the organization of scientific workshops, training courses etc.

Magnetic coupling in 3D-hierarchical MnO2 microsphere

The development of new routes for the synthesis of the magnetic nanostructures with low cost, simple and rapid growth has inspired as alternative materials in a wide range of applications. In this report, 3D-hierarchical MnO2 microspheres with an ultrathin nanosheet structure have been synthesized by microwave heating method. X-ray diffraction analysis exhibit the microcrystalline nature of microspheres and (211) is the major pattern with the highest intensity. High purity and surface adsorbed oxygen species have been detected by valance state of microspheres. The magnetic properties of microsphere α-MnO2 investigated experimentally and combined with density functional theory (DFT) calculations, suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interaction in their nanostructures. The observed FM state is arising from noncollinear magnetic field that tilts two AFM Mn spins consequently, which make a distortion through intermediate oxygen ion in the crystal structure. While, the canted angle between two Mn sublattices lead them to a weak FM state. Similarly, the superexchange interaction is also responsible for magnetic ordering in the adjacent O ion, that can help to exchange magnetic information by direct electron hops of Mn–Mn chain. This is further confirmed from our DFT calculations. They may be found the strong potential for alternative materials other than partially oxidized metal nanoparticles.

Density-functional theory for systems with noncollinear spin: Orbital-dependent exchange-correlation functionals and their application to the Hubbard dimer

A new class of orbital-dependent exchange-correlation (xc) potentials for applications in noncollinear spin-density-functional theory is developed. Starting from the optimized effective potential (OEP) formalism for the exact exchange potential - generalized to the noncollinear case - correlation effects are added via a self-consistent procedure inspired by the Singwi-Tosi-Land-Sjolander (STLS) method. The orbital-dependent xc potentials are applied to the Hubbard dimer in uniform and noncollinear magnetic fields and compared to exact diagonalization and to the Bethe-ansatz local spin-density approximation. The STLS gives the overall best performance for total energies, densities and magnetizations, particularly in the weakly to moderately correlated regime.

Diffusion tensor magnetic resonance imaging strategies for color mapping of human brain anatomy

A color mapping of fiber tract orientation using diffusion tensor imaging (DTI) can be prominent in clinical practice. The goal of this paper is to perform a comparative study of visualized diffusion anisotropy in the human brain anatomical entities using three different color-mapping techniques based on diffusion-weighted imaging (DWI) and DTI.The first technique is based on calculating a color map from DWIs measured in three perpendicular directions. The second technique is based on eigenvalues derived from the diffusion tensor. The last technique is based on three eigenvectors corresponding to sorted eigenvalues derived from the diffusion tensor. All magnetic resonance imaging measurements were achieved using a 1.5 Tesla Siemens Vision whole body imaging system. A single-shot DW echoplanar imaging sequence used a Stejskal-Tanner approach. Trapezoidal diffusion gradients are used. The slice orientation was transverse. The basic measurement yielded a set of 13 images. Each series consists of a single image without diffusion weighting, besides two DWIs for each of the next six noncollinear magnetic field gradient directions.The three types of color maps were calculated consequently using the DWI obtained and the DTI. Indeed, we established an excellent similarity between the image data in the color maps and the fiber directions of known anatomical structures (e.g., corpus callosum and gray matter).In the meantime, rotationally invariant quantities such as the eigenvectors of the diffusion tensor reflected better, the real orientation found in the studied tissue.

Diffusion tensor magnetic resonance imaging strategies for color mapping of human brain anatomy

A color mapping of fiber tract orientation using diffusion tensor imaging (DTI) can be prominent in clinical practice. The goal of this paper is to perform a comparative study of visualized diffusion anisotropy in the human brain anatomical entities using three different color-mapping techniques based on diffusion-weighted imaging (DWI) and DTI. The first technique is based on calculating a color map from DWIs measured in three perpendicular directions. The second technique is based on eigenvalues derived from the diffusion tensor. The last technique is based on three eigenvectors corresponding to sorted eigenvalues derived from the diffusion tensor. All magnetic resonance imaging measurements were achieved using a 1.5 Tesla Siemens Vision whole body imaging system. A single-shot DW echoplanar imaging sequence used a Stejskal-Tanner approach. Trapezoidal diffusion gradients are used. The slice orientation was transverse. The basic measurement yielded a set of 13 images. Each series consists of a single image without diffusion weighting, besides two DWIs for each of the next six noncollinear magnetic field gradient directions. The three types of color maps were calculated consequently using the DWI obtained and the DTI. Indeed, we established an excellent similarity between the image data in the color maps and the fiber directions of known anatomical structures (e.g., corpus callosum and gray matter). In the meantime, rotationally invariant quantities such as the eigenvectors of the diffusion tensor reflected better, the real orientation found in the studied tissue.

Isotope-Identifying neutron reflectometry

The possibilities of an isotope-indentifying study of layered structures in different regimes of a neutron wave field are considered. The detection of specularly reflected neutrons and secondary radiation (caused by neutron capture) in the form of charged particles, γ quanta, and nuclear fission fragments, as well as neutrons spin-flipped in a noncollinear magnetic field and on nuclei of elements with spin, makes it possible to implement isotope-indentifying neutron reflectometry.

Scattering by magnetic nanocylinders and the method of distorted waves in noncollinear fields

In the perturbation theory framework, we compute the cross section of scattering by a magnetic nanocylinder and a helicoid arbitrarily oriented in an external magnetic field. We are the first to obtain the matrix Green’s function for two media with an interface and noncollinear magnetic fields on the two sides of the interface. We show how to compute scattering by magnetic inclusions in one of the media.

Spin-polarized electron tunneling through an aluminum particle in a noncollinear magnetic field

We present measurements of spin-polarized electron tunneling through a nanometer-scale Al particle in contact with two ferromagnets as a function of the direction of the applied magnetic field at 4.2 K. We find that if the magnetizations of the ferromagnets are aligned parallel, the tunnel current has a weak dependence on magnetic-field direction ${I}_{\ensuremath{\uparrow}\ensuremath{\uparrow}}\ensuremath{\approx}\text{const}$, while if the magnetizations of the ferromagnets are aligned antiparallel, the current has significant dependence on magnetic-field direction ${I}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}(\ensuremath{\alpha})\ensuremath{\approx}{I}_{\ensuremath{\uparrow}\ensuremath{\uparrow},0}\text{ }{\text{sin}}^{2}(\ensuremath{\alpha})+{I}_{\ensuremath{\uparrow}\ensuremath{\downarrow},0}\text{ }{\text{cos}}^{2}(\ensuremath{\alpha})$, where $\ensuremath{\alpha}$ is the angle between the magnetic field and the magnetizations. Those dependencies are in agreement with the model of spin accumulation by incoherent electron transport via Zeeman-split energy levels of the particle. They demonstrate that the electron spin in the particle at finite current accumulates along the direction of the magnetic field rather than the magnetization direction. In zero magnetic field, the spin-accumulation direction is set by the field of the environment making it possible to study inhomogeneous spin dephasing. A lower limit of 8 ns is found for the inhomogeneous spin-dephasing time.

Magnetic Field Effects on Quantum-Dot Spin Valves

We study the magnetic field effects on the spin-polarized transport of the quantum dot (QD) spin valve in the sequential tunneling regime. A set of generalized master equation is derived. Based on that, we discuss the collinear and noncollinear magnetic field effects, respectively. In the collinear magnetic field case, we find that the Zeeman splitting can induce a negative differential conductance (NDC), which is quite different from the one found in previous studies. It has a critical polarization in the parallel arrangement and will disappear in the antiparallel configuration. In the noncollinear magnetic field case, the current shows two plateaus and their angular dependence is analyzed. Although sometimes the two current plateaus have similar angular dependence, their mechanisms are different. Our formalism is also suitable for calculating the transport in magnetic molecules, in which the spin splitting is induced not by a magnetic field but by the intrinsic magnetization.

Potential in spin-density-functional theory of noncollinear magnetism determined by the many-electron ground state

Since spin-density-functional theory was first proposed, but also recently, examples were constructed to show that a spin potential may share its ground state with other spin potentials. In fact, for collinear magnetic fields and systems with fixed magnetization, the mapping between potentials and ground states is invertible, provided the magnetization is not saturated and that spin potentials are determined within a spin constant. We complete the proof that the mapping is invertible also for noncollinear magnetic fields and systems with more than one electron. We then discuss the noncollinear exchange and correlation energy functional in the local-spin-density approximation and suggest improvements.

Noncollinear magnetoconductance of a quantum dot

We study theoretically the linear conductance of a quantum dot connected to ferromagnetic leads. The dot level is split due to a noncollinear magnetic field or intrinsic magnetization. The system is studied in the noninteracting approximation, where an exact solution is given, and, furthermore, with Coulomb correlations in the weak tunneling limit. For the noninteracting case, we find an antiresonance for a particular direction of the applied field, noncollinear to the parallel magnetization directions of the leads. The antiresonance is destroyed by the correlations, giving rise to an interaction induced enhancement of the conductance. The angular dependence of the conductance is thus distinctly different for the interacting and noninteracting cases when the magnetizations of the leads are parallel. However, for antiparallel lead magnetizations, the interactions do not alter the angle dependence significantly.