Structure Of Magnetic Materials Research Articles

Comparison of conventional and Lorentz transmission electron microscopy in magnetic imaging of permanent magnets

Magnetization structures in magnetic materials are usually imaged in dedicated Lorentz transmission electron microscopes. Compared to conventional transmission electron microscopes, the magnetic field of the objective lens at the sample is removed by replacing the objective lens with a Lorentz lens below the sample. While this modification is critical for soft-magnetic materials whose magnetic state is affected by the strong magnetic field of the objective lens, we propose that this is not necessary for permanent magnets such as Sm–Co and Nd–Fe–B. Conventional and Lorentz microscopes are compared for imaging divergent and convergent domain walls in a Sm(Co,Fe,Cu,Zr)7.7 magnet. Both techniques provide an almost identical resolution and accuracy in the measurement of the domain-wall width parameter using focal-series imaging of divergent domain walls. It is further demonstrated that both techniques can be utilized to analyze the intensity profile of convergent domain walls. From this, the product of sample thickness and magnetic induction is extracted. These results illustrate that conventional microscopes can be used to image the magnetic state of permanent magnets with a resolution comparable to dedicated Lorentz microscopes, which make magnetic imaging experiments significantly more accessible to a wider scientific community.

High-Pressure Neutron Diffraction Study of the Crystal and Magnetic Structure of Materials at the Pulsed Reactor IBR-2: Current Opportunities and Prospects

The experimental capabilities and results of recent studies of the crystal and magnetic structure of functional materials on the diffractometers DN-12 and DN-6 of upgraded high-flux pulsed reactor IBR-2 in wide ranges of high pressures and temperatures are reviewed. The combination of high neutron flux on a sample, wide-aperture multidetector systems, and focusing devices has made it possible to implement diffraction experiments on samples with record-low volumes under ultrahigh pressures (up to 35 GPa) in the temperature range of 5–300 K. The results of recent studies of the structural and magnetic phase transitions in ferroelectrics, multiferroics, low-dimensional magnets, and other materials are presented.

Configurational entropy of skyrmions and half-skyrmions in planar magnetic elements

This works deals with the presence of localized planar structures in magnetic materials that admit integer or half-integer topological charge. We study models in which the internal disposition of magnetization is driven by a single parameter, that controls the topological charge density of the magnetic structure. In particular, we focus mainly on the configurational entropy of these skyrmions and half-skyrmions and show how to increase or diminish their informational capabilities.

In situneutron powder diffraction studies

AbstractNeutron powder diffraction provides insight into the crystal and magnetic structures of materials. This insight can be achieved on pure materials under ambient conditions, but for greater understanding of the relationship between structure and function, crystallographic information during a process can prove powerful for rational material design strategies. These processes can involve temperature, applied magnetic or electric fields, applied pressure, gas dosing or devices such as electrochemical cells. These experiments are often referred to as a variable process orin situoroperandodepending on the conditions applied to the materials or devices. Sample environments to undertake such experiments are accessible at a range of neutron scattering facilities around world. This work provides details on the theory behind diffraction, a snapshot of neutron diffractometers that are suited to such experiments and recent studies exploring the power of neutron powder diffraction in elucidating crystal and magnetic structures under various external conditions.

Configurational entropy for skyrmion-like magnetic structures

In this work we explore the relationship between two ideas recently introduced in the literature. The first one deals with a quantity related to the informational contents of solutions of spatially localized structures, and the second consists of obtaining analytical solutions to describe skyrmion-like structures in magnetic materials. In particular, we use the topological charge density to extract information on the configurational entropy of the magnetic structure.

Spherical neutron polarimetry under high pressure for a multiferroic delafossite ferrite

The analysis of three-dimensional neutron spin polarization vectors, using a technique referred to as spherical neutron polarimetry (SNP), is a very powerful means of determining complex magnetic structures in magnetic materials. However, the requirement to maintain neutrons in a highly polarized state has made it difficult to use this technique in conjunction with extreme experimental conditions. We have developed a high pressure cell made completely of nonmagnetic materials and having no effect on neutron polarizations. Herein, we report the first SNP analyses under high pressure up to 4.0 GPa in the magnetoelectric multiferroic delafossite CuFeO2. This study also determined the complex spiral magnetic structures in these pressure-induced phases, by measuring the full neutron polarization matrix. The results presented herein demonstrate that the SNP measurements are feasible under high pressure conditions, and that this method is a useful approach to study pressure-induced physical phenomena.

Physical Mechanisms of Reverse Magnetization Jumps and Improvement of the Functional Characteristics of Ferromagnetics

The physical mechanism of reverse magnetization jumps, which are induced by the minimization of the total energy of a growing magnetic phase in the process of magnetization of a ferromagnetic, is revealed. The ways toward enhancing the magnetic induction and suppressing the magnetostriction and magnetic losses in Fe–3% Si alloys through adjustment of the parameters of magnetization jumps are outlined. The effective refinement of the structure of magnetic materials leads to a considerable improvement in the magnetic characteristics.

Design of Multi-Nanoparticles Technique for Enhancing Magnetic Characterization of Power Transformers Cores

The structure of magnetic materials is an essential parameter for specifying magnetic characterization of the transformer core. This paper presents enhancing magnetic characterization of transformer cores by using new nanotechnology techniques. The effective magnetic parameters of new magnetic nanocomposites materials for the transformer cores (single-phase and three-phase) have been predicted based on recent theoretical approaches. The new design, the effects of variant types and concentrations of magnetic multi-nanoparticles on magnetization loss of transformers cores were studied with respect to traditional transformer cores. Optimal types and concentrations of nanoparticles were defined for controlling of reluctance and magnetization loss of transformer cores using multi-nanoparticles technique. A comparative study depicted the industrial features for using multi-nanoparticles against separate nanoparticles in transformers cores.

MERRILL: Micromagnetic Earth Related Robust Interpreted Language Laboratory.

Complex magnetic domain structures and the energy barriers between them are responsible for pseudo‐single‐domain phenomena in rock magnetism and contribute significantly to the magnetic remanence of paleomagnetic samples. This article introduces MERRILL, an open source software package for three‐dimensional micromagnetics optimized and designed for the calculation of such complex structures. MERRILL has a simple scripting user interface that requires little computational knowledge to use but provides research strength algorithms to model complex, inhomogeneous domain structures in magnetic materials. It uses a finite element/boundary element numerical method, optimally suited for calculating magnetization structures of local energy minima (LEM) in irregular grain geometries that are of interest to the rock and paleomagnetic community. MERRILL is able to simulate the magnetic characteristics of LEM states in both single grains, and small assemblies of interacting grains, including saddle‐point paths between nearby LEMs. Here the numerical model is briefly described, and an overview of the scripting language and available commands is provided. The open source nature of the code encourages future development of the model by the scientific community.

Laser-Induced Intersite Spin Transfer.

Laser pulses induce spin-selective charge flow that we show to generate dramatic changes in the magnetic structure of materials, including a switching of magnetic order from antiferromagnetic (AFM) to transient ferromagnetic (FM) in multisub-lattice systems. The microscopic mechanism underpinning this ultrafast switching of magnetic order is dominated by spin-selective charge transfer from one magnetic sublattice to another. Because this spin modulation is purely optical in nature (i.e., not mediated indirectly via the spin-orbit interaction) this is one of the fastest means of manipulating spin by light. We further demonstrate this mechanism to be universally applicable to AFM, FM, and ferri-magnets in both multilayer and bulk geometry and provide three rules that encapsulate early-time magnetization dynamics of multisub-lattice systems.

Electron Accumulation and Emergent Magnetism in LaMnO_{3}/SrTiO_{3} Heterostructures.

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO_{3}/SrTiO_{3} (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO_{3}. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO_{3}, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO_{3} films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

Direct Visualization of Local Electromagnetic Field Structures by Scanning Transmission Electron Microscopy.

The functional properties of materials and devices are critically determined by the electromagnetic field structures formed inside them, especially at nanointerface and surface regions, because such structures are strongly associated with the dynamics of electrons, holes and ions. To understand the fundamental origin of many exotic properties in modern materials and devices, it is essential to directly characterize local electromagnetic field structures at such defect regions, even down to atomic dimensions. In recent years, rapid progress in the development of high-speed area detectors for aberration-corrected scanning transmission electron microscopy (STEM) with sub-angstrom spatial resolution has opened new possibilities to directly image such electromagnetic field structures at very high-resolution. In this Account, we give an overview of our recent development of differential phase contrast (DPC) microscopy for aberration-corrected STEM and its application to many materials problems. In recent years, we have developed segmented-type STEM detectors which divide the detector plane into 16 segments and enable simultaneous imaging of 16 STEM images which are sensitive to the positions and angles of transmitted/scattered electrons on the detector plane. These detectors also have atomic-resolution imaging capability. Using these segmented-type STEM detectors, we show DPC STEM imaging to be a very powerful tool for directly imaging local electromagnetic field structures in materials and devices in real space. For example, DPC STEM can clearly visualize the local electric field variation due to the abrupt potential change across a p-n junction in a GaAs semiconductor, which cannot be observed by normal in-focus bright-field or annular type dark-field STEM imaging modes. DPC STEM is also very effective for imaging magnetic field structures in magnetic materials, such as magnetic domains and skyrmions. Moreover, real-time imaging of electromagnetic field structures can now be realized through very fast data acquisition, processing, and reconstruction algorithms. If we use DPC STEM for atomic-resolution imaging using a sub-angstrom size electron probe, it has been shown that we can directly observe the atomic electric field inside atoms within crystals and even inside single atoms, the field between the atomic nucleus and the surrounding electron cloud, which possesses information about the atomic species, local chemical bonding and charge redistribution between bonded atoms. This possibility may open an alternative way for directly visualizing atoms and nanostructures, that is, seeing atoms as an entity of electromagnetic fields that reflect the intra- and interatomic electronic structures. In this Account, the current status of aberration-corrected DPC STEM is highlighted, along with some applications in real material and device studies.

Topological excitations in magnetic materials

In this work we propose a new route to describe topological excitations in magnetic systems through a single real scalar field. We show here that spherically symmetric structures in two spatial dimensions, which map helical excitations in magnetic materials, admit this formulation and can be used to model skyrmion-like structures in magnetic materials.

Small-angle electron scattering of magnetic fine structures

Magnetic structures in magnetic artificial lattices and chiral magnetic orders in chiral magnets have been quantitatively analyzed in the reciprocal space by means of small-angle electron scattering (SAES) method. Lorentz deflection due to magnetic moments and Bragg diffraction due to periodicity are simultaneously recorded at an angle of the order of or less than 1 × 10(-6) rad, using a camera length of more than 100 m. The present SAES method, together with TEM real-space imaging methods such as in-situ Lorentz microscopy, is very powerful in analyzing magnetic fine structures in magnetic materials. Indeed, the existence of both a chiral helimagnetic structure and a chiral magnetic soliton lattice in a chiral magnet CrNb3S6 has been successfully verified for the first time using the present complementary methods.

Preparation, Characterization, and Application of Magnetic Fe-SBA-15 Mesoporous Silica Molecular Sieves

Magnetic Fe-SBA-15 mesoporous silica molecular sieves were prepared, characterized, and used for magnetic separation. Wet impregnation, drying, and calcination steps led to iron inclusion within the mesopores. Iron oxide was reduced to the metal form with hydrogen, and the magnetic Fe-SBA-15 was obtained. Fourier-transform infrared spectroscopy confirmed the preparation process from the oxide to metal forms. The structure of magnetic materials was confirmed by Mössbauer spectra. Powder X-ray diffraction data indicated that the structure of Fe-SBA-15 retained the host SBA-15 structure. Brunauer-Emmett-Teller analysis revealed a decrease in surface area and pore size, indicating Fe-SBA-15 coating on the inner surfaces. Scanning electron micrographs confirmed the decrease in size for modified SBA-15 particles. From scanning electron micrographs, it was found that the size of the modified SBA-15 particles decreased. Transmission electron micrographs also confirmed that modified SBA-15 retained the structure of the parent SBA-15 silica. Fe-SBA-15 exhibited strong magnetic properties, with a magnetization value of 8.8 emu g−1. The iron content in Fe-SBA-15 was determined by atom adsorption spectroscopy. Fe-SBA-15 was successfully used for the magnetic separation of three aromatic compounds in water. Our results suggest wide applicability of Fe-SBA-15 magnetic materials for the rapid and efficient separation of various compounds.

Gas flow sputtering: Versatile process for the growth of nanopillars, nanoparticles, and epitaxial thin films

Gas flow sputtering is a sputter-deposition method that enables soft and high-rate deposition even for oxides or nitrides. It involves sputtering at a high pressure of around 100 Pa and hollow cathode discharge in a tubular or parallel plate target with forced Ar flow. Depending on the sputtering conditions, various structures of magnetic materials are obtained, and some examples are shown in this paper. Co–Pt and Fe nanopillars are fabricated using a tubular target with a large inner diameter (6–40 mm). Fe nanoparticles with diameters ranging from a few nanometers to 150 nm are fabricated using a tubular target with a small inner diameter (5 mm). Magnetite epitaxial thin films are fabricated on MgO and GaAs substrates by substrate heating.

A terahertz spectrometer with a pulsed high magnetic field

A terahertz (THz) spectrometer based on backward-wave tubes and combined with a pulsed-magnetic-field generator with a pulse duration of ∼100 μs and an amplitude of up to 50 T is described. This spectrometer is indented for studying magnetic resonances of various magnets in the THz frequency range (30 GHz-1.5 THz) in the Faraday (longitudinal field) and Voigt (transverse field) geometries. Its operating principle is based on measurements of the change in the transmission of polarized THz radiation by studied samples exposed to an applied pulsed magnetic field. Its advantage over the magnetic-resonance technique, in which radiation guides with uncontrolled radiation polarization are used, is the possibility of performing precise polarization measurements necessary for determining the conditions for excitation of resonance magnetic modes and yielding important information on the magnetic structure of materials. The results of polarization measurements of test samples are presented. The degree of radiation polarization is 99.99%; the dynamic measurement range is 25 dB; the measurement accuracy is no worse than 1%; the time resolution of the recording system is 1s; the magnetic-field inhomogeneities at the solenoid’s center at a base of ±2 mm are no worse than 1 (experimental) and 5% (calculated) for the axial and radial components, respectively; and the maximum stored energy is 40.5 kJ.

Development and Application of Magneto‐Optical Microscope Using Polarization‐Modulation Technique

AbstractMagneto‐optical (MO) effect is commonly used not only for an observation of magnetic domain structures, but also for a research of electrical structures in magnetic materials. In addition, distribution of a magnetic flux diverging from a magnetized sample and/or distribution of a current in the sample can be visualized with the help of an MO‐indicator film placed on the sample. In order to obtain quantitative information of the flux distribution we have developed an MO microscope using a polarization‐modulation technique. Utilizing the MO microscope with a magnetic garnet film as an indicator we have obtained a quantitative image of current distribution in a superconducting film. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Direction-Free Magnetic Field Application System

To observe the dynamical behavior of superconducting vortices and the motion of domain structure in magnetic materials by transmission electron microscopy (TEM), it is important that the direction of an external magnetic field applied to the specimen can be adjusted appropriately as well as the electro-optical axis, the crystalline orientation and morphological orientation of the specimens. Direction-free magnetic field application system that has three coil-pairs for generating and controlling the external magnetic field in any direction was developed to achieve the purpose. The system also has twenty-four coils to compensate the deflected electron beam by the external magnetic field applied. The system has been installed in the 1-MV and 300-kV field emission electron microscopes to utilize application research works. [DOI: 10.1380/ejssnt.2008.29]

Magnetic field analysis of laminated core by using homogenization method

This paper describes an approach to efficient modeling of the microscopic structure of magnetic materials in the nonlinear magnetostatic field analysis of a laminated core. When the ordinary finite element method is applied, large memory and computational cost are required due to the huge number of elements generated by detailed modeling of a laminated core. In order to overcome the above difficulties, we introduce the homogenization method, which is widely used in continuum mechanics, into the magnetic field computation and expand the method into the nonlinear analysis. Furthermore, the specialized homogenization method for a laminated core is carried out to verify the above homogenization method in the electromagnetism. Some numerical results are also presented to demonstrate the effectiveness of the proposed method.