Increase in temperature of a ferromagnetic substance in rf magnetic field

If one applies an rf magnetic field parallel to a strong static field, temperature of a ferromagnetic substance rises due to ferromagnetic resonance. This phenomenon finds immediate application in the field of hyperthermic oncology. In this work, we have found expression for the increase in temperature when a ferromagnetic material is placed under a static and a varying magnetic field of high frequency through spin wave approach. The numerical value of this increment of local temperature has been estimated for yttrium iron garnet (YIG). We also have examined the possibility of enhancing the temperature of a ferromagnetic material only by applying a strong static field.     

Design of small-scale gradient coils in magnetic resonance imaging by using the topology optimization method

A topology optimization method based on the solid isotropic material with penalization interpolation scheme is utilized for designing gradient coils for use in magnetic resonance microscopy. Unlike the popular stream function method, the proposed method has design variables that are the distribution of conductive material. A voltage-driven transverse gradient coil is proposed to be used as micro-scale magnetic resonance imaging(MRI) gradient coils, thus avoiding introducing a coil-winding pattern and simplifying the coil configuration. The proposed method avoids post-processing errors that occur when the continuous current density is approximated by discrete wires in the stream function approach. The feasibility and accuracy of the method are verified through designing the z-gradient and y-gradient coils on a cylindrical surface.Numerical design results show that the proposed method can provide a new coil layout in a compact design space.     

Failure to direct detect magnetic field dephasing corresponding to ERP generation

Functional magnetic resonance imaging (fMRI) has become the method of choice for mapping brain activity in human subjects and detects changes in regional blood oxygenation and volume associated with local changes in neuronal activity. While imaging based on blood oxygenation level dependent (BOLD) contrast has good spatial resolution and sensitivity, the hemodynamic signal develops relatively slowly and is only indirectly related to neuronal activity. An alternative approach termed magnetic source magnetic resonance imaging (msMRI) is based on the premise that neural activity may be mapped by magnetic resonance imaging (MRI) with greater temporal resolution by detecting the local magnetic field perturbations associated with local neuronal electric currents. We used a hybrid ms/BOLD MRI method to investigate whether msMRI could detect signal changes that occur simultaneously at the time of the production of well-defined event-related potentials, the P300 and N170, in regions that previously have been identified as generators of these electrical signals. Robust BOLD activations occurred after some seconds, but we were unable to detect any significant changes in the T2*-weighted signal in these locations that correlated temporally with the timings of the evoked response potentials (ERPs).     

Estimation of pore sizes in porous silicon by scanning electron microscopy and NMR cryoporometry

Certain possibilities of scanning electron microscopy and cryoporometry based on nuclear magnetic resonance (NMR) have been evaluated to determine pore sizes in porous silicon. The results obtained by these methods have been compared. NMR cryoporometry has been shown to be promising in the investigation of porous materials.     

Multimode magnetic responses in NIR and visible ranges

We demonstrate a nonsymmetrical composite metallic structure with realistic material parameters. This structure can realize double magnetic responses in near-infrared (NIR) range (0.92 μm) and in visible range (0.61 μm), which can be attributed to two different modes of magnetic coupling. The difference of the index dependence of two magnetic responses is due to the influence of metal on LC model at optical frequencies. The multimode magnetic responses may provide us a new way to design special nano-optical devices at high frequencies. We also show that magnetic response has larger sensitivity to the local index of dielectric than plasmon resonance and suggest a possible approach in detecting the local index of the dielectric surrounding metamaterials.     

Application of the semivariogram to MRI data: a useful approach for the characterization of natural porous rocks

Geostatistics is used to study the heterogeneities of reservoir cores, as well as to observe the orientation of layers inside the cores. This information obtained from geostatistics could be very useful for coreflood simulation programs. In this paper, it has been shown how semivariograms can be drawn using local porosities obtained from magnetic resonance imaging (MRI). These semivariograms are then used to determine the correlation lengths of the porosity in the cores and to qualitatively estimate the degree of heterogeneity in the core samples.     

Ferromagnetic resonance and magnetic force microscopy evidence for above room temperature ferromagnetism in Mn doped Si made by a solid state sintering process

Silicon doped with manganese made by a solid state sintering process starting with nanoparticles of Si and MnO is shown to be ferromagnetic above room temperature. The evidence for ferromagnetism is obtained from ferromagnetic resonance and magnetic force microscopy measurements. The magnetic force microscopy measurements show that the ferromagnetic resonance occurs in nanosized regions in the sample as has been suggested by Dietl. Raman and electric force microscopy measurements also suggest that then material is doped inhomogenously.     

Non-linear quasi-optical cell with polarized nuclear target (PNT) material

A resonance electromagnetic structure (a Fabry-Perot resonator) with a plane-parallel sample of the polarized nuclear target (PNT) material of a paramagnetic complex Na HMBA (Cr^V) is considered under condition of dynamic polarization of nuclear by millimeter waves. Non-linear effects observed at the electron spin resonance (ESR) in such structure have been studied. The results of the experiment with the quasi-optical resonance cell of the radiospectrometer designed for the investigation of the magnetic resonance in the millimeter wavelengths range are given. A hysteresis of the transfer coefficient of the cell in the ESR-line has been found while slow passing the ESR-line by retuning of the static magnetic field. The deformation of the form of non-stationary signals passed through the cell has been found.     

Shear thickening and migration in granular suspensions

We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using magnetic resonance imaging with macroscopic rheometry experiments. In steady state, we observe that the material is heterogeneous, and we find that the local rheology presents a continuous transition at low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with shear stresses proportional to the shear rate squared, as predicted by a scaling analysis. We show that the heterogeneity results from an unexpectedly fast migration of grains, which we attribute to the emergence of the Bagnoldian rheology. The migration process is observed to be accompanied by macroscopic transient discontinuous shear thickening, which is consequently not an intrinsic property of granular suspensions.     

固体核磁共振研究单斜磷酸钒锂的进展(英文)

在过去的二十年里，单斜型磷酸钒锂作为一种有前景的锂离子电池正极材料被广泛研究.固体核磁共振技术是一种研究原子局部环境和运动性，并能反映材料中长程/短程有序结构变化的有力表征手段，可以从多个角度满足磷酸钒锂材料的研究需求.本文从充放电机理、锂离子的迁移率和动力学、碳包覆、阳离子掺杂等方面简要介绍了固体核磁共振技术在单斜磷酸钒锂正极材料研究中的应用，同时涵盖了相关的理论计算工作.     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Homogenization of nanostructured media in magnetic field

Problem of homogenization of nanostructured media in magnetic field has been considered. Possibility of introduction of effective material parameters dielectric permittivity and magnetic permeability for three classes of media such as magnetic metal nanostructures, film metal–dielectric composite media and 3D-nanocomposites on the base of opal matrices has been investigated. It has been stated that the introduction of effective parameters far from magnetic resonance conditions is possible at millimeter waveband frequencies. Strict introduction of effective magnetic permeability of nanostructured media near magnetic resonance is not possible.     

Magnetic Resonance Microscopy of Heterogeneity in Polymer Electrolyte Membranes

Magnetic resonance microscopy of Nafion 117 is used to measure the spatial variation of solvent dynamics in protonated (acid form) membranes immersed in solvent. Spatial resolution allows determination of membrane material property heterogeneity via T ₂ and diffusion variations to be easily studied in the presence of strong free solvent signal. Variation of T ₂ and diffusion as a function of solvent methanol–water mole fraction is measured. Spatially averaged T ₂ and diffusion data are in general agreement with previous bulk nuclear magnetic resonance studies; however, significant disparities from sample to sample at fixed solvent concentration are present in the spatially averaged data. The variation is determined to be due to changes in solvent mobility within the membrane over scales of the order of 10 mm, indicating macroscale heterogeneity of the solvent saturated membrane morphology. Authors' address: Sarah L. Codd, Department of Mechanical and Industrial Engineering, Montana State University, Bozeman, MT 59717-3800, USA     

Time-Resolved EPR Spectra of Photoexcited Copper Porphyrin

Photoinduced spin-polarized transient electron paramagnetic resonance (EPR) spectra of copper 5,10,15,20-tetrakis(3-pyridyl)porphyrin (3PyNCu) in the frozen solution have been observed in the X-band. The time evolution and the temperature dependence of the spectra have been studied. The effect of molecular oxygen in the frozen solution on the polarization pattern has also been examined. The magnetic resonance parameters of the ground state of 3PyNCu have been obtained by comparing the experimental continuous-wave and echo-detected EPR spectra with the numerical computations. The magnetic resonance parameters of the excited states and the photoinduced polarizations have been investigated by time-resolved EPR (TREPR) spectroscopy and numerical analysis. The experimental spectra have been considered as a sum of the polarized spectra of the ground and excited states. Our analysis confirmed that the TREPR spectra consisted of two main patterns: the enhanced signal from the ground state and the multiplet contribution belonging to the excited quartet state.     

Local ferromagnetic resonance imaging with magnetic resonance force microscopy

We report nanoscale scanned probe ferromagnetic resonance force microscopy (FMRFM) imaging of individual ferromagnetic microstructures. This reveals the mechanism for high spatial resolution in FMRFM imaging: the strongly inhomogeneous local magnetic field of the cantilever mounted micromagnetic probe magnet used in FMRFM enables selective, local excitation of ferromagnetic resonance (FMR). This approach, demonstrated here in individual permalloy disks, is straightforwardly extended to excitation of localized FMR modes, and hence imaging in extended films.     

Field-variable magnetic domain characterization of individual 10 nm Fe_3O_4 nanoparticles

The local detection of magnetic domains of isolated 10 nm Fe_3O_4 magnetic nanoparticles(MNPs) has been achieved by field-variable magnetic force microscopy(MFM) with high spatial resolution.The domain configuration of an individual MNP shows a typical dipolar response.The magnetization reversal of MNP domains is governed by a coherent rotation mechanism, which is consistent with the theoretical results given by micromagnetic calculations.Present results suggest that the field-variable MFM has great potential in providing nanoscale magnetic information on magnetic nanostructures,such as nanoparticles, nanodots, skyrmions, and vortices, with high spatial resolution.This is crucial for the development and application of magnetic nanostructures and devices.     

Structural, static and dynamic magnetic properties of dextran coated γ-Fe(2)O(3) nanoparticles studied by (57)Fe NMR, M?ssbauer, TEM and magnetization measurements

The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), M?ssbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined M?ssbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed M?ssbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret M?ssbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.     

Heterogeneous in vivo MR images of soft tissue tumors: guide to gross specimen sampling.

Twelve soft tissue tumors preoperatively studied with magnetic resonance (MR) imaging contained areas of inhomogeneous intralesional signal intensity. The pathologist's selection of biopsy sites from gross specimens was based on specific MR images selected by a radiologist and depended on close cooperation between the radiologist and pathologist. The histopathology so sampled, further depended on the lesion's size, careful gross specimen orientation, discrete sampling, and a specially developed Grid Analysis technique. Our initial experience indicates that (1) histology may vary within different and often neighboring sites of a single soft tissue lesion; (2) that such variability is common in soft tissue tumors; (3) that histological variability within different sites of a single lesion influences signal heterogeneity; and (4) that although differing MR signal intensities per se are not tissue specific, MR may enhance the histopathological accuracy of pathologists who can be directed to particular gross specimen sites by magnetic resonance images. To our knowledge such a specific, detailed effort has not been made.     

Enhancing magnetic dipole emission with magnetic metamaterials

Magnetic dipole(MD) transitions are important for a range of technologies from quantum light sources and displays to lasers and bio-probes. However, the typical MD transitions are much weaker than their electric counterparts and are usually neglected in practical applications. Herein, we experimentally demonstrate that the MD transitions can be significantly enhanced by the well-developed magnetic metamaterials in the visible optical range. The magnetic metamaterials consist of silver nanostrips and a thick silver film, which are separated with an Eu~(3+):polymethyl methacrylate(PMMA) film. By controlling the thickness of the Eu~(3+):PMMA film, the magnetic resonance has been tuned to match the emission wavelength of MDs. Consequently,the intensity of MD emission has been significantly increased by around 30 times at the magnetic resonance wavelength, whereas the intensity of electric dipole emission is well-preserved. The corresponding numerical calculations reveal that the enhancement is directly generated by the magnetic resonance, which strongly increases the magnetic local density of states around the MD emitter and can efficiently radiate the MD emission into the far field. This is the first demonstration, to the best of our knowledge, that MD transitions can be improved by an additional degree of magnetic freedom, and we believe this research shall pave a new route towards bright magnetic emitters and their potential applications.     

Magnetic resonances spectroscopy of nanosize particles La0.7Sr0.3MnO3

Using a co-precipitation method, perovskite-type manganese oxide La_0.7Sr_0.3MnO₃ nanoparticles (NPs) with particle size 12 nm were prepared. Detailed studies of both ⁵⁵Mn nuclear magnetic resonance and superparamagnetic resonance spectrum, completed by magnetic measurements, have been performed to obtain microscopic information on the local magnetic structure of the NP. Our results on nuclear dynamics provide direct evidence of formation of a magnetically dead layer, of the thickness ≈2 nm, at the particle surface. Temperature dependences of the magnetic resonance spectra have been measured to obtain information about complex magnetic properties of La_0.7Sr_0.3MnO₃ fine-particle ensembles. In particular, electron paramagnetic resonance spectrum at 300 K shows a relatively narrow sharp line, but as the temperature decreases to 5 K, the apparent resonance field decreases and the line width considerably increases. The low-temperature blocking of the NPs magnetic moments has been clearly observed in the electron paramagnetic resonances. The blocking temperature depends on the measuring frequency and for the ensemble of 12 nm NPs at 9.244 GHz has been evaluated as 110 K.