Electromagnetic fields in magnetic materials: A resource letter

This work is a collection of selected, significant references focussing upon magnetic materials and their interaction with electromagnetic fields. The major topics included are ferrites, plasmas (both atmospheric and laboratory), magnetic composites, magnetic dielectric fluids, ferroelectric liquid crystals. Sections discussing general magnetic materials and phenomena are also included.     

Computational materials design of filled tetrahedral compound magnetic semiconductors

Based on first-principles calculations within the density functional theory, materials design of filled tetrahedral compound magnetic semiconductors is proposed. By using the Korringa–Kohn–Rostoker coherent potential approximation, electronic structures of Mn-doped LiZnAs, LiZnP and LiZnN are calculated. First, by estimating free energy, phase diagrams of these systems are predicted. It is shown that these systems are phase separating systems and favor spinodal decomposition. However, by introducing Li vacancies, spinodal decomposition is strongly suppressed and Mn can be doped up to high concentration. Moreover, the introduced Li vacancies induce ferromagnetic interaction between Mn and thus we can expect high Curie temperature (T_C) in these systems. To see the chemical trend, electronic structure and T_C of Li(Zn, Cr)As are also calculated.     

Densification and magnetic properties of low-fire NiCuZn ferrites

The mixing of (Ni_0.38Cu_0.12Zn_0.50)Fe₂O₄ powders with Bi₂O₃ was performed using the solid-state mixing as well as wet chemical coating processes such as ammonia precipitation coating, urea precipitation coating, and solution coating. Ferrites prepared from the wet chemical coating processes could be densified at a lower sintering temperature without significant impact on the microstructural evolution compared with that prepared by solid-state mixing. In addition, samples prepared from the wet chemical coating process have a higher B_r and B_s and a lower H_c compared with that from solid-state mixing. Considering both the effects of sintering temperature and sintered density (>95% T.D.), ferrites with 1.5 wt% Bi₂O₃ addition by ammonia precipitation coating sintered at 900°C can provide the best permeability and quality factor (191 and 68.2, respectively) among all the cases studied.     

Thermoelectric power and magnetic properties of Cu doped NiZn ferrite for technological application

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2-xO₄ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. The thermoelectric power and the magnetic susceptibility were measured. The transition from the ferrimagnetic to the paramagnetic state is accompanied by an increase in the thermo EMF. NiZn ferrite shows n-type conductivity due to the presence of Fe²⁺ ions. The addition of Cu²⁺ ions creates lattice vacancies which give rise to p-type conductivity.

The Tawfik coefficient was determined for NiZn ferrite in the paramagnetic state. This coefficient was reduced by addition of Cu up to x < 0.5.     

Cosmological magnetic fields

Magnetic fields are observed not only in stars, but in galaxies, clusters, and even high redshift Lyman-α systems. In principle, these fields could play an important role in structure formation and also affect the anisotropies in the cosmic microwave background radiation (CMB). The study of cosmological magnetic fields aims not only to quantify these effects on large-scale structure and the CMB, but also to answer one of the outstanding puzzles of modern cosmology: when and how do magnetic fields originate? They are either primoridial, i.e. created before the onset of structure formation, or they are generated during the process of structure formation itself.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Single-dot spectroscopy in high magnetic fields

We report on photoluminescence measurements from a single InAs/GaAs quantum dot in magnetic fields up to 28 T. Mesa-patterned structure has been used to limit the number of investigated dots. Three pairs of Zeeman-split emission lines with the same effective g*-factor and diamagnetic shift have been observed. The attribution of the lines to recombination of a neutral exciton, a biexciton, and a charged exciton is discussed.     

Electrocrystallization under magnetic fields: experiment and model

We report some experimental results for quasi-two-dimensional electrocrystallization of copper under magnetic fields. Such results are theoretically investigated by large scale simulations of a DLA-like model in which random walkers can move along circular vortices enhanced by the Lorentz force. In addition, a sticking probability is used to take into account the complex reaction dynamics at the cathode surface. Our results indicate that the convective motion does not change the nature of the normal diffusive regime, but increases dramatically the diffusion constant by a factor of up to six. The characteristic features (morphology and scaling laws) of both random walks and growing electrodeposits under a perpendicular magnetic field are determined.     

Trapping energy of a magnetic monopole in magnetic materials

Analytical methods to investigate the interaction of magnetic monopoles with known magnetic media have been developed. Trapping energies of monopoles inside ferro-magnetic or super onducting materials of size greater than about 10⁻⁶ cm are found to be of the order of several kiloelectron volts. These are two to three orders of magnitude higher than in paramagnetic materials. Thus if stable magnetic monopoles exist at all in the universe, they are perhaps trapped in these magnetic materials. The effect of the finite size of the magnetic bodies is taken into account explicitly in our calculations of the trapping energy.     

Studies on multiferroic materials in high magnetic fields

In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative materials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr₂O₃ is introduced. Then drastic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite-type structure. In addition, high field studies on the magnetoelectric effects in BiFeO₃, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau-Ginzburg theory.     

Dipole magnetic anisotropy fields in rhombohedral antiferromagnetic materials

Results of calculations of the contribution of magnetic dipole interactions to the effective uniaxial anisotropy fields of antiferro- and ferromagnetism vectors in rhombohedral antiferromagnetic materials with theS ions are given as functions of the ratio of the hexagonal crystal cell parameters c_H/a_H. There is a strong dependence of the calculated curves on the lattice parameters of real compounds. From the dependences obtained the effective anisotropy fields are calculated for FeF₃, FeBO₃, and MnCO₃. L. V. Kirenskii Institute of Physics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 82–86, May, 1999.     

Causality study and numerical response of the magnetic permeability as a function of the frequency of ferrites using Kramers-Kronig relations

In this paper, the numerical treatment of magnetic loss of NiZn, MnZn, Ni₂Y, and NiZnCu ferrite and their composites, by using Krameres-Kronig relations, is investigated. The complex magnetic permeability spectra for ferromagnetic materials have been studied. Due to the principle of causality and time independence in the relation between magnetic induction B and magnetic field H, the real and the imaginary part of the complex magnetic permeability are mutually dependent, and the correlation is given by the Krameres-Kronig equations. Through them, it is possible to measure the real component of the complex magnetic permeability, assuming the real component is given, and by the Hilbert transform, the imaginary part of the magnetic permeability can be calculated. Magnetic circuit model has been studied theoretically, focusing on the model's poles in the complex plane to verify the principle of causality and the temporary independence.     

The effect of rotating fields in magnetic resonance

The phenomenon of magnetic resonance is studied by considering the transverse oscillatory field as superposition of two oppositely rotating fields. One of the rotating fields is taken as strong and the other relatively weak.     

Exact solution for the hydrogen atom in strong magnetic fields

The exact energy values and wavelengths of the Lyman a transition of the hydrogen atom in strong magnetic fields up to 2.35 × 105 T are obtained by using the pseudospectral method. They agree well with results calculated with the other method.     

Quark pair creation in color electric fields and effects of magnetic fields

The time evolution of a system where a uniform and classical SU(3) color electric field and quantum fields of quarks interact with each other is studied focusing on non-perturbative pair creation and its back reaction. We characterize a color direction of an electric field in a gauge invariant way, and investigate its dependence. Momentum distributions of created quarks show plasma oscillation as well as quantum effects such as the Pauli blocking and interference. Pressure of the system is also calculated, and we show that pair creation moderates degree of anisotropy of pressure. Furthermore, enhancement of pair creation and induction of chiral charge under a color magnetic field which is parallel to an electric field are discussed.     

Electromagnetic fields in general relativity: A constructive procedure

A new procedure for obtaining explicit solutions to Maxwell's equations in curved spaces is presented. The problem is reduced to solving one linear scalar wave equation. The formulation includes astrophysically important cosmological models, neutron star and black hole space-times.     

Calculation of electric fields induced near metal implants by magnetic resonance imaging switched-gradient magnetic fields

Electric (E) fields induced near metal implants by MRI switched-gradient magnetic fields are calculated by a new equivalent-circuit numerical technique. Induced E-field results are found for a metallic spinal-fusion implant consisting of two thin wires connected to the metallic case of a current generator as well as for its subsections: a bare U-shaped wire, an insulated U-shaped wire, a cut insulated wire, and a generator. The presence of the metallic implants perturbs the E field significantly. Near the ends of the bare U-shaped wire, the E field is 89.7 times larger than in the absence of the wire. The greatest E field concentration occurs near the ends of the cut insulated wire, where the E field is 196.7 times greater than in the absence of the wire. In all cases, the perturbation of the induced E field by the implanted wire is highly localized within a few diameters of the wire.     

Confined ferrofluid droplet in crossed magnetic fields

When a ferrofluid drop is trapped in a horizontal Hele-Shaw cell and subjected to a vertical magnetic field, a fingering instability results in the droplet evolving into a complex branched structure. This fingering instability depends on the magnetic field ramp rate but also depends critically on the initial state of the droplet. Small perturbations in the initial droplet can have a large influence on the resulting final pattern. By simultaneously applying a stabilizing (horizontal) azimuthal magnetic field, we gain more control over the mode selection mechanism. We perform a linear stability analysis that shows that any single mode can be selected by appropriately adjusting the strengths of the applied fields. This offers a unique and accurate mode selection mechanism for this confined magnetic fluid system. We present the results of numerical simulations that demonstrate that this mode selection mechanism is quite robust and “overpowers” any initial perturbations on the droplet. This provides a predictable way to obtain patterns with any desired number of fingers.     

Influence of environment and grain size on magnetic properties of nanocrystalline Mn–Zn ferrite

Nanocrystalline Mn_1−xZn_xFe₂O₄ (0.2?x?0.9) was prepared by mechanical alloying of the concerned oxide precursors and subsequent annealing in air and Ar atmosphere, respectively. Milling and annealing in air produces Zn-ferrites (ZnFe₂O₄) instead of Mn–Zn ferrites as MnO converts to higher oxides at higher oxygen partial pressure and fails to dissolve in the spinel phase. This is confirmed by careful quantitative X-ray diffraction analysis using Rietvelt profile matching and also by the non-saturating paramagnetic nature of the magnetization response with very low saturation level of these spinels milled and annealed in air. On the other hand, single-phase Mn–Zn ferrite results from the identical precursor oxide blend when milling and annealing are carried out under controlled (Ar) atmosphere. The average grain size of the as-milled and annealed powders, measured by Rietvelt refinement, varies between 6–8 and 14–18 nm, respectively. Further investigations performed with Mn_0.6Zn_0.4Fe₂O₄ reveal that a careful selection of annealing parameters may lead to an early superparamagnetic relaxation. Therefore, the blocking temperature can be significantly reduced through proper heat treatment schedule to ensure superparamagnetism and negligible hysteresis at low temperature.     

Mössbauer experiments in radio frequency magnetic fields: A method for investigations of nanostructured soft magnetic materials

We apply radio frequency (rf) effects, the sideband and the collapse effect, in the investigation of magnetic properties of nanostructured ferromagnetic alloys. We use the relative intensity of the sidebands in comparison to the central part of the spectrum to determine the relative samples' magnetostriction following successive preparation steps. Recent investigations of nanostructured soft ferromagnetic alloys in rf fields led to the discovery that the collapse in the Mössbauer spectra can become selective and partial. It means that the magnetisation reversal is not fast enough and varies differently in the various phases of the alloy. The application of rf magnetic fields then causes new kinds of rf forced relaxation‐type Mössbauer spectra. Experimental results and basic steps in the theoretical understanding are also presented.