Homogeneous Magnetization Research Articles

Influence of demagnetizing effects on the magnetization curves of finite-size rectangular slabs

According to some recent studies, the magnetoresistance curves of ferromagnetic strip-shaped samples can significantly differ depending on whether the in-plane external applied magnetic field H is oriented in parallel to either the long or the short edge of the strip. To address this problem, in the present work magnetization curves M(H) were measured for similarly shaped samples with both magnetic field orientations used in the magnetoresistance measurements. It was found that the M(H) curves strongly depend on the saturation magnetization and shape of the samples as well as on the magnetic field orientations. For some samples with sufficiently large saturation magnetization, the effective demagnetizing factors could be deduced from the measured M(H) curves. By considering the investigated samples as a ferromagnetic slab, and approximating them with a general ellipsoid, the demagnetizing factors were calculated from known formulae and compared to the experimental values. A fairly good matching was observed, although the latter data were systematically slightly larger, certainly due to the not completely homogeneous magnetization within the rectangular slab as opposed to the case of a general ellipsoid. The differences in the M(H) curves for the two orientations of the magnetic field could be completely attributed to demagnetizing effects.

Design requirements for human UHF magnets from the perspective of MRI scientists

Abstract The highest magnetic field strength for human-sized magnetic resonance imaging (MRI) currently lies at 11.7 tesla. Given the opportunities for enhanced sensitivity and improved data quality at higher static magnetic fields, several initiatives around the world are pursuing the implementation of further human MRI systems at or above 11.7 tesla. In general, members of the MR research community are not experts on magnet technology. However, the magnet is the technological heart of any MR system, and the MRI community is challenging the magnet research and design community to fulfill the current engineering gap in implementing large-bore, highly homogeneous and stabile magnets at field strengths that go beyond the performance capability of niobium-titanium. In this article, we present an overview of magnet design for such systems from the perspective of MR scientists. The underlying motivation and need for higher magnetic fields are briefly introduced, and system design considerations for the magnet as well as for the MRI subsystems such as the gradients, the shimming arrangement, and the radiofrequency hardware are presented. Finally, important limitations to higher magnetic fields from physiological considerations are described, operating under the assumption that any engineering or economic barriers to realizing such systems will be overcome.

Exact and fractional solution of MHD generalized Couette hybrid nanofluid flow with Mittag–Leffler and power law kernel

This study investigates the complex behavior of Jeffrey nanofluid flow in a porous oscillating microchannel under the influence of magnetohydrodynamic (MHD) effects. The research explores how magnetic field distortions lead to diverse accumulation patterns of nanofluid particles, a phenomenon attributed to homogeneous magnetization in fluid dynamics. Nanoparticles ranging from 0.25 % to 0.5 % (low and inexpensive concentrations) are remarkably consistent for best results in most machining procedures. Different concentrations of various nanomaterial's is utilized (φ1 = φ2 = 0.01, 0.02, 0.03, 0.04) to make the simple nanfluid and hybrid nanofluid suspensions. By employing fractal-fractional derivatives governed by power law, a mathematical model developed to describe the time-varying, compressible MHD flow of Jeffrey nanofluid. The model incorporates the effects of heat transfer, pressure, and magnetic fields on the fluid dynamics. A novel fractional approach utilizing the Laplace transform is applied to solve the fractal MHD hybrid-fluid model integrated into a porous medium. The study reveals velocity flow decrease with increasing Reynolds numbers but increase with channel inclination. Additionally, both the Darcy number and magnetic field orientation enhance heat transfer rates. In addition, the velocity profile enhanced by the hybrid nanofluid suspension as compared to simple nanofluid flow. The research validates its findings by demonstrating the convergence of fractional and numerical solution methods. Furthermore, the study compares the performance of different hybrid nanofluids, concluding that water-based (H2O + GO + MoS2) hybrid fluids exhibit slightly superior characteristics compared to (CMC + GO + MoS2) hybrid nanofluids.

Free-breathing abdominal chemical exchange saturation transfer imaging using water presaturation and respiratory gating at 3.0 T.

Free-breathing abdominal chemical exchange saturation transfer (CEST) has great potential for clinical application, but its technical implementation remains challenging. This study aimed to propose and evaluate a free-breathing abdominal CEST sequence. The proposed sequence employed respiratory gating (ResGat) to synchronize the data acquisition with respiratory motion and performed a water presaturation module before the CEST saturation to abolish the influence of respiration-induced repetition time variation. In vivo experiments were performed to compare different respiratory motion-control strategies and B0 offset correction methods, and to evaluate the effectiveness and necessity of the quasi-steady-state (QUASS) approach for correcting the influence of the water presaturation module on CEST signal. ResGat with a target expiratory phase of 0.5 resulted in a higher structural similarity index and a lower coefficient of variation on consecutively acquired CEST S0 images than breath-holding (BH) and respiratory triggering (all p < 0.05). B0 maps derived from the abdominal CEST dataset itself were more stable for B0 correction, compared with the separately acquired B0 maps by a dual-echo time scan and B0 maps derived from the water saturation shift referencing approach. Compared with BH, ResGat yielded more homogeneous magnetization transfer ratio asymmetry maps at 3.5 ppm (standard deviation: 3.96% vs. 3.19%, p = 0.036) and a lower mean squared difference between scan and rescan (27.52‱ vs. 16.82‱, p = 0.004). The QUASS approach could correct the water presaturation-induced CEST signal change, but its necessity for in vivo scanning needs further verification. The proposed free-breathing abdominal CEST sequence using ResGat had an acquisition efficiency of approximately four times that using BH. In conclusion, the proposed free-breathing abdominal CEST sequence using ResGat and water presaturation has a higher acquisition efficiency and image quality than abdominal CEST using BH.

Thermal evolution of low-temperature magnetic texture modulation in fept thin films by direct visualization

Patterns of ferroic domains and domain walls are being intensively studied to implement new logic schemes. Any technological application of such objects depends on a detailed understanding of them. Using low-temperature magnetic force measurements (10–300 K), the evolution of ferromagnetic stripes on equiatomic FePt thin films is thoroughly analyzed. Since FePt is known to develop a transition from in-plane homogeneous magnetization to stripe domains upon varying its thickness, multiple samples are studied demonstrating the well-established reduction upon thickness decrease and a non-trivial dependence on temperature. Moreover, the room-temperature uniform distribution of the pattern evolves into a distorted one upon temperature cycling. Finally, dissimilar stripe patterns are obtained upon reducing and increasing temperature indicating the states are dependent on the history of applied stimuli rather than the parametric conditions.

Magnitoopticheskaya kerr-spektroskopiya nanokompozitov

Magnetooptical spectroscopy is an effective method for studying the magnetic microstructure of homogeneous and heterogeneous magnets. This review is devoted to analysis of numerous factors affecting the intensity and spectral dependence of a magnetooptical signal of the equatorial Kerr effect in nanocomposites “ferromagnetic metal–dielectric” in the visible and near infrared spectral regions. Examples of the influence of the metal concentration, nanoparticle size and shape, the substrate, the material of the dielectric, the amorphization of grains, the deposition method, and other factors on the magnetooptical spectrum are considered. The differences in the magnetooptical spectra for the superparamagnetic, superferromagnetic, and ferromagnetic states are demonstrated. It is noted that in the presence of fractions with different field dependences of the magnetization in a nanocomposite, the magnetooptical signal is not proportional to the total magnetization. Examples of enhancement and sign inversion of the magnetooptical signal in nanocomposites are considered. The possibility of the description of magnetooptical spectra using the methods of the effective medium (the Bruggeman method and the Maxwell–Garnett symmetrized approximation) is discussed.

High-homogeneity double-focusing stigmatic analyzer magnet for electromagnetic isotope separator

An electromagnetic Isotope Separator (EMIS) is used for separating isotopes by magnetic dispersion method. A dipole-bending magnet performs the fundamental function of separation and is an essential building block of EMIS. Unlike mass spectrometers and ion implantation systems, the EMIS prefers ribbon beams for effective mass separation and collection. The bending magnet described in this paper has an inbuilt hexapole and a tunable quadrupole doublet. This magnet separates the charged particles based on the mass-to-charge ratio, performs chromatic correction, and focuses the beam in both axes. All sector magnets focus the beam in the radial direction, the introduction of edge angle results in the vertical focusing of the beam at the expense of radial focusing. The exit pole of the magnet is rotatable for adjusting the focal point. The entry pole is curved for ensuring stigmatism correction in the beam and works like a hexapole. High magnetic field homogeneity is a key requirement for this magnet. This enhances the mass resolution. As per beam simulations, magnetic field non-homogeneity shall not exceed 200 ppm in good field region (GFR) of 200 mm (radial) and 60 mm(vertical). The paper discusses the design, production, installation, testing, and commissioning of a double-focusing Stigmator dipole magnet. A method for achieving low non-homogeneity by using non-magnetic cavities and compensation coils is described and implemented. Very high homogeneity magnets can be designed using this technique. The paper also describes an efficient method for the Magnetic qualification of bending magnets. Based on the design, a double-focusing stigmatic dipole magnet is produced, characterized, and installed.

Influence of the Shape of a Spin-Tunnel Element on the Dependence of Its Magnetoresistance

A theoretical and experimental study of the dependence of the magnetoresistance for two spin-tunnel junctions (STJs) of ellipsoidal shape has been made. The one-sided homogeneous magnetization reversal mode of an ellipsoidal STJ with different aspect ratios has been experimentally selected. Despite the reverseinhomogeneous remagnetization, this selection has allowed for the calculation of the magnetic parameters of these elements by developing the Stoner-Wohlfarth theory.

Coercivity Mechanism and Magnetization Reversal Process in Nanocomposite Pr2Fe14B/α‐Fe Alloy with Phosphorous Addition

An investigation is conducted for the coercivity mechanism in as‐spun Pr2Fe14B/α‐Fe ribbons containing different phosphorus (P) prepared at a wheel speed of 30 ms−1. With the increase of P addition, the volume of the amorphous phase in the ribbons is increased, and the grain size of Pr2Fe14B is decreased. The hysteresis loops of the ribbons with 0.1 at% P exhibit a homogeneous magnetization reversal process similar to single‐phase magnet, demonstrating that soft magnetic grains are fully coupled with the hard ones. The highest coercivity of 12.0 kOe (300 K)–38.0 kOe (100 K) is achieved in the ribbon with 0.3 at% P‐addition, which shows weak exchange coupling interaction between grains, and the high coercivity comes mainly from the decoupled hard phase. The values of the microstructure parameter in all compositions are less than 0.3 and imply that the main coercivity mechanism is the mixture of the nucleation of reverse domain and domain wall pinning. The minor hysteresis loops and initial magnetization curves show the dominant domain wall pinning in the ribbon with 0–0.1 at% P, nucleation and pinning in the ribbon with 0.3 at% P, and dominant domain nucleation in the ribbon with 0.5 at% P.

A pulsed current-mode class-D low-voltage high-bandwidth power amplifier for portable NMR systems

Low-field NMR has seen growing interest in recent years, especially for portable applications. The lower homogeneity magnets used for portable applications require short RF pulses to ensure enough transmit bandwidth to excite the sample volume and also support short echo periods. Furthermore, the preferred use of a high-Q coil to improve signal-to-noise ratio (SNR) prolongs the pulse transients. Thus, at such low Larmor frequencies, the excitation pulse transients become comparable or longer than the pulse length, such that the transmit bandwidth begins to limit measurement SNR. This paper describes the design of a pulsed current-mode class-D power (PCMCD) transmitter that addresses this issue by generating high power in a tuned sample coil while maintaining short transients, thus resulting in high output bandwidth. The transmitter also uses a charge recycling mechanism to maximize power efficiency for RF train excitation, which also results in faster pulse repetition rate and reduces allowable echo time. Experimental results from a small form-factor PCMCD transmitter are presented. This design generates a peak RF power of 240 W into a 9.16 μH coil at 4 MHz while operating off a single 12 V power supply. NMR measurement results using the transmitter are also described, showing minimum achievable echo time of 70 μs and 25 μs depending on the transmitter mode of operation.

Spin–Spin Interaction in ε-VOPO4 through Doping Light Elements

Nonmagnetic transition-metal oxides are usually magnetized by doping magnetic metallic elements. Here, we propose an alternative route to achieving magnetization by doping nonmagnetic H and Li atoms. Using vanadium oxide (VOPO4) as an example, we find that the doping of H and Li can effectively localize the excess electrons on V-d orbitals. As such, nonmagnetic ε-VOPO4 is predicted to possess short-range spin–spin interaction. When the doped H2 molecules are bonded with the axial oxygen, we observe a notable charge transfer from oxygen to vanadium. The transferred electrons are localized on two adjacent VO6 octahedrons and induce the spin–spin interaction. We further show that H2 dopants in ε-VOPO4 tend to be dispersed with a homogeneous magnetization. Our results suggest that H2-doped ε-VOPO4 can be utilized in the field of spintronics.

Micromagnetic Modes of Anisotropy of Magnetic Susceptibility in Natural Magnetite Particles

AbstractAnisotropy of magnetic susceptibility (AMS) is commonly used to assess sedimentation, deformation, tectonics, rock fabric, and texture. Using focused‐ion beam nanotomography, we develop a micromagnetic method to investigate the AMS of individual magnetite inclusions in silicates across the transition between single‐domain (SD) to multidomain behavior. We calculate individual AMS tensors by modeling the magnetization response of a particle to weak applied fields in three orthogonal directions. The main AMS mode of elongated SD particles is not a homogeneous magnetization rotation, but focused alignment of spins at their edges and tips. In single‐vortex particles, vortex displacement is the dominant AMS mode, which focuses the largest magnetization changes in a planar region containing the vortex core, and perpendicular to the direction of vortex motion. In multi‐vortex structures a combined motion of all vortex centers can lead to high degrees of anisotropy when some motion patterns are energetically much easier to achieve than others.

Magnetization reversal and ground states in thin truncated conical nanodisks: Analytical and micromagnetic modelling approach

Simulation of magnetization reversal of soft permalloy thin truncated conical disks is done using micromagnetic modelling. The thickness of the disks are fixed at 20nm. Base radius (R) of the conical disk is varied between 50−100nm while the radius of upper surface (r) is varied such that the ratio σ=r/R lies between 0 to 1. As the conical disks are gradually tapered by reducing r, the remanent states change from vortex state to shifted vortex state to buckled magnetization state. In buckled state, the magnetic moments are essentially parallel except at the corners where they curl away from the applied field direction. The extent of curling away of magnetization in buckled state reduce as base radius R is decreased. Thus at smaller base radius, the buckled states converge to homogeneous magnetization state or so called ferromagnetic state. Analytical expressions for the total energy of these three remanent states are calculated using simple models. Analytical calculations reveal that vortex configuration is the ground state for a wide range of geometrical parameters and shifted vortex state is a metastable state which appears as remanent state in the in-plane magnetization reversal due to a possible existence of energy barrier. Other observations from micromagnetic simulations are well corroborated by the results of analytical calculations.

Effect of Multilayered Structure on the Static and Dynamic Properties of Magnetic Nanospheres.

The development of flexible and lightweight electromagnetic interference (EMI)-shielding materials and microwave absorbers requires precise control and optimization of core–shell constituents within composite materials. Here, a theoretical model is proposed to predict the static and dynamic properties of multilayered core–shell particles comprised of exchange-coupled layers, as in the case of a spherical iron core coupled to an oxide shell across a spacer layer. The theory of exchange resonance in homogeneous spheres is shown to be a limiting special case of this more general theory. Nucleation of magnetization reversal occurs through either quasi-uniform or curling magnetization processes in core–shell particles, where a purely homogeneous magnetization configuration is forbidden by the multilayered morphology. The energy is minimized through mixing of modes for specific interface conditions, leading to many inhomogeneous solutions, which grow as 2n with increasing layers, where n represents the number of magnetic layers. The analytical predictions are confirmed using numerical simulations.

Characterization of transient rheological behavior of soft materials using ferrofluid droplets

Physical material properties, such as elasticity, viscosity, or viscoelasticity, can be characterized by using rheometers or stick-type solenoid electromagnets. In this work, we developed a magnet measurement setup based on a Helmholtz arrangement of electromagnets. While applying homogeneous magnet fields to ferrofluid droplets inside a soft material of interest, the deformations of the ellipsoidal deformed droplets were measured. Kelvin-Voigt models and corresponding analytical descriptions were used to calculate the values of viscosity and Young’s modulus of materials under test. For calibration purposes of the developed setup, glycerin/water mixtures and methylcellulose/water solutions were characterized as viscous and polyacrylamide gels as elastic materials, respectively. In addition, the interfacial tensions were calculated with respect to the magnetic Bond number from the droplet deformations. For the first time, the transient rheological behavior of viscoelastic material was measured using the method of ferrofluid droplet deformation. When polyacrylamide gel with a shear modulus of 230 Pa was evacuated for less than 40 min during preparation, it showed a strong time-depending viscoelastic behavior several minutes after starting the measurements. Here, Young’s modulus increased up to the value of elastic behavior, whereas the values for viscosity decreased to a baseline. The developed setup can favorably be used in future applications to investigate local and also time-dependent rheological properties of soft materials.

Persistent current effects in highly homogeneous superconducting magnets

Many factors affect the magnetic field quality of superconducting magnet, among which the persistent current originating from the inherent attribute of superconducting material exists for a long time and has a greater influence on the magnetic field quality, and its influence on the magnetic field of highly homogeneous magnets has not been reported yet. Based on the critical state model, the magnetization curve of a single superconducting wire, as well as the variation of the higher-order magnetic field components with current of a typical pre-cycle are simulated for a 7 T highly homogeneous magnet. For the validation, the magnetization curves of a single superconducting wire are measured, and the superconducting magnet is subjected to a cool-down test. Compared to zonal harmonics, more obvious hysteresis phenomena for tesseral harmonic components are observed and validated, which confirmed the effectiveness of this methodology.

Точные формулы напряженности магнитного поля внутри и вне однородного эллипсоидального магнетика во внешнем однородном в области магнетика поле

The direct problem of magnetostatics is considered - the calculation of the resulting magnetic field strength inside and outside a homogeneous magnet in the form of an arbitrary ellipsoid placed in the area of uniformity of the external magnetic field. The final formulas for calculating the field strength containing elementary functions or incomplete elliptic integrals are given. From them, the strength formulas for the "limiting" cases of a magnet of the specified shape are obtained – a plate of finite thickness, an infinite circular cylinder and a cylinder with an elliptical cross section, a ball. A comparison with a number of known formulas for these cases is made.

Spin-wave resonances in a diffusion-sensitive epitaxial film YIG

It is shown that in a tangentially magnetized film of iron yttrium garnet exist two types of resonances --- a homogeneous ferrimagnetic resonance in the region of the film's homogeneous magnetization (outside the diffusion layer), and inhomogeneous spin-wave resonance within the thickness of the diffusion layer. A method of calculating the magnetization distribution over the film thickness using measured frequencies of the spin-wave and ferrimagnetic resonances. The method of mathematical modeling has established that the magnetic inhomogeneity of the transition layer layer introduces significant distortions of the oscillation epure at the frequency of the spin-wave resonance, and the nature of the distortions significantly depends on the magnitude of the magnetizing field. The results of the conducted can be useful for nondestructive control of the layered structure of YIG epitaxial films. Keywords: yttrium iron garnet epitaxial films, exchange spin waves, spinwave electronics, magnonics.

Спин-волновые резонансы в диффузионном слое эпитаксиальной пленки ЖИГ

It is shown that in a tangentially magnetized film of iron yttrium garnet exist two types of resonances - a homogeneous ferrimagnetic resonance in the region of the film's homogeneous magnetization (outside the diffusion layer), and inhomogeneous spin-wave resonance within the thickness of the diffusion layer. A method of calculating the magnetization distribution over the film thickness using measured frequencies of the spin-wave and ferrimagnetic resonances. The method of mathematical modeling has established that the magnetic inhomogeneity of the transition layer layer introduces significant distortions of the oscillation epure at the frequency of the spin-wave resonance, and the nature of the distortions significantly depends on the magnitude of the magnetizing field. The results of the conducted can be useful for nondestructive control of the layered structure of YIG epitaxial films.

Improved (BH)max and squareness in diffusion-processed hot-deformed Nd-Fe-B magnets by self-diffusion process under pressure

To recover the remanence (or maximum energy product ((BH)max)) and squareness with high coercivity in diffusion-processed hot-deformed Nd-Fe-B magnets, new efforts have been made in this work. Annealing at a selected temperature of 800 ℃ under pressure was applied to the hot-deformed magnets diffused by the Nd70Cu30 or Nd70Cu15Ga15 alloy. After this process, the thick powder boundary phase dissolved in the matrix and diffused into the intergranular regions. The c-axis misalignment of Nd2Fe14B grains near the diffusion surface was significantly improved, leading to increased remanence by 0.10–0.18 T and (BH)max by 47–85 kJ/m3 in the two diffusion-processed magnets. The distribution of the powder boundary phase, the proportions of the intergranular Nd-rich phase, the c-axis alignment of grains and the grain sizes from the diffusion surface to the center of the magnets tend to be relatively uniform. The squareness factor of the demagnetization curve increased from 0.89 to 0.95 for the Nd70Cu30-diffused magnet and from 0.85 to 0.93 for the Nd70Cu15Ga15-diffused magnet after the process, returning to the level of the as-deformed magnet. Microstructure analysis and micromagnetic simulations indicate that the improved squareness factor is due to the homogeneous magnetization reversal process caused by the homogeneous microstructure.