Showing papers on "Single domain published in 2022"

Ferrimagnetic Large Single Domain Iron Oxide Nanoparticles for Hyperthermia Applications

Abstract: This paper describes the preparation and obtained magnetic properties of large single domain iron oxide nanoparticles. Such ferrimagnetic particles are particularly interesting for diagnostic and therapeutic applications in medicine or (bio)technology. The particles were prepared by a modified oxidation method of non-magnetic precursors following the green rust synthesis and characterized regarding their structural and magnetic properties. For increasing preparation temperatures (5 to 85 °C), an increasing particle size in the range of 30 to 60 nm is observed. Magnetic measurements confirm a single domain ferrimagnetic behavior with a mean saturation magnetization of ca. 90 Am2/kg and a size-dependent coercivity in the range of 6 to 15 kA/m. The samples show a specific absorption rate (SAR) of up to 600 W/g, which is promising for magnetic hyperthermia application. For particle preparation temperatures above 45 °C, a non-magnetic impurity phase occurs besides the magnetic iron oxides that results in a reduced net saturation magnetization.

Advanced characterization of magnetization dynamics in iron oxide magnetic nanoparticle tracers.

Abstract: Characterization of the magnetization dynamics of single-domain magnetic nanoparticles (MNPs) is important for magnetic particle imaging (MPI), magnetic resonance imaging (MRI), and emerging medical diagnostic/therapeutic technologies. Depending on particle size and temperature, nanoparticle magnetization relaxation time constants span from nanoseconds to seconds. In solution, relaxation occurs via coupled Brownian and Néel relaxation mechanisms. Even though their coexistence complicates analysis, the presence of two timescales presents opportunities for more direct control of magnetization behavior if the two processes can be understood, isolated, and tuned. Using high frequency coils and sample temperature tunability, we demonstrate unambiguous determination of the specific relaxation processes for iron oxide nanoparticles using both time and frequency domain techniques. Furthermore, we study the evolution of the fast dynamics at ≈ 10 nanosecond timescales, for magnetic field amplitudes relevant to MPI.

Field-Free Magnetization Switching Induced by Bulk Spin–Orbit Torque in a (111)-Oriented CoPt Single Layer with In-Plane Remanent Magnetization

Abstract: Spin–orbit torque (SOT)-induced perpendicular magnetization switching is one of the key solutions for the next generation of magnetic memory and spin logic applications. Recently, the bulk SOT effect in a single magnetic layer with a vertical composition gradient has attracted a lot of attention because it can break through the interfacial nature of the SOT effect in a traditional bilayer structure. However, the dependency of the external in-plane magnetic field or the additional pinning layer for deterministic switching hinders the further application of this technology. Here, for the first time, we implement field-free magnetization switching induced by bulk SOT in a single (111)-oriented CoPt magnetic layer with in-plane remanent magnetization. The initialized longitudinal in-plane remanent magnetization can substitute the external magnetic field to break the inversion symmetry and realize continuous field-free perpendicular magnetization switching. Furthermore, the in-plane remanent magnetization can be manipulated by the SOT effective field induced by lateral current pulses, leading to a tunable switching chirality. A multi-domain micromagnetic model is established to describe in depth the experimental observations and clarify the relationship between switching amplitude and easy magnetization cone angle. Our work provides an alternative solution to realize field-free perpendicular magnetization switching in a single magnetic layer, which can promote the development of emerging high-density and low-power SOT-based devices.

Observation of Spin-Glass-like Behavior over a Wide Temperature Range in Single-Domain Nickel-Substituted Cobalt Ferrite Nanoparticles

Abstract: In this study, single-domain NixCo1−xFe2O4 ferrite nanoparticles with 0≤x≤1 were hydrothermally prepared and characterized using X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometry. According to the Rietveld refinement results, all of the prepared nanoparticles were single phase with spinel-type structures. Increasing the Ni content increased the average crystallite size and X-ray density while decreasing the lattice constant. According to the TEM observations, the nanoparticles were spherical in shape. The formation of a single-phase spinel structure with two lattices centered at tetrahedral and octahedral sites was confirmed by the observation of two absorption bands in all FT-IR spectra. Magnetization data showed that the prepared nanoparticles of all compositions were ferrimagnetic across the entire temperature range of 300 K to 10 K. Magnetic properties such as saturation magnetization, remanent magnetization, coercivity, magnetic anisotropy, and magnetic moments per unit cell were found to decrease with increasing Ni content. The big difference in Hc of the x = 0, 0.25, 0.5, 0.75 ferrites between 300 K and 10 K suggested that these ferrite nanoparticles are truly single-domain nanoparticles. The small value of Hc of the NiFe2O4(x=1) ferrite and its very weak temperature dependence suggested that this sample is in a multi-domain regime. The ZFC–FC curves revealed the existence of spin-glass-like behavior in these ferrite nanoparticles over the entire temperature range.

Real picture of magnetic domain dynamics along the magnetic hysteresis curve inside an advanced permanent magnet

Abstract: Abstract In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet.

Structure and magnetization of a magnetoactive ferrocomposite.

Abstract: This work is devoted to the theoretical study of the structural and magnetic properties of an ensemble of single-domain interacting magnetic nanoparticles immobilized in a non-magnetic medium. This model is typical for describing magnetically active soft materials, "smart" polymer ferrocomposites, which have been applied in science-intensive industrial and biomedical technologies. It is assumed that the ferrocomposite is obtained by solidification of the carrier medium in a ferrofluid under an external magnetic field, the intensity of which is determined by the Langevin parameter αp; after the solidification of the carrier liquid, the nanoparticles retain the spatial distribution and orientation of their easy magnetization axes. The features of the orientational texture formed in the sample are analyzed depending on the intensity of the magnetic field αp and interparticle dipole-dipole interactions. The magnetization of a textured ferrocomposite in the magnetic field α is also investigated. Our results show that in the case of a co-directional arrangement of the considered fields and if α < αp, the ferrocomposites are magnetized much more efficiently than ferrofluids due to their texture. In the fields α > αp, the ferrocomposite is magnetized less efficiently than the ferrofluid due to the internal magnetic anisotropy of the nanoparticles. The analytical expressions presented here make it possible to predict the magnetization of a ferrocomposite depending on its internal structure and synthesis conditions, which is the theoretical basis for the synthesis of ferrocomposites with a predetermined magnetic response in a given magnetic field.

Magnetic Domain Structure Observation for Initial Magnetization and Demagnetization Processes of a Nd-Fe-B Hot-Deformed Magnet Using Soft X-ray Magnetic Circular Dichroism Microscopy

Abstract: A large step on the initial magnetization curve has been widely observed in Nd-Fe-B hot-deformed magnets. This behavior has been considered as that the first- and latter-half parts of the initial magnetization curve correspond to the magnetization reversals of multi- and single-domain grains, respectively. In this study, the detailed magnetic domain structure of a Nd-Fe-B hot-deformed magnet has been studied for the initial magnetization and demagnetization processes by using a soft X-ray magnetic circular dichroism (XMCD) microscopy. The observed magnetization reversal behavior for the initial magnetization process is quite different from that previously considered. The multi-domain states move from grains to grains through the gradual displacement of domain wall passing through many grains, and then the massive domain wall displacement takes place. The latter part of domain wall displacement is similar with that found in the demagnetization process near the coercivity. Moreover, we found that there are several strong pinning sites which are identical for both the initial magnetization and the demagnetization processes.

Investigation of the structural, morphological, and magnetic properties of small crystalline Co–Cu ferrite nanoparticles in the single-domain regime

Abstract: Single-domain Co0.5Cu0.5Fe2O4 ferrite nanoparticles with a crystallite size of 23 nm were hydrothermally prepared and characterized using x-ray diffraction, transmission electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and vibrating sample magnetometry. According to the Rietveld refinement results, the prepared nanoparticles were single-phase with spinel type structures. The transmission electron microscope measurements demonstrated that the nanoparticles were spherical in shape. The FT-IR spectrum showed two principle absorption bands, confirming the characteristic features of cubic spinel ferrites. Magnetization data revealed that the prepared nanoparticles were ferrimagnetic from room temperature to 10 K, with well-defined saturation magnetization, coercivity, and remanence magnetization. The remanence magnetization and coercivity were found to increase with decreasing temperature. The value of room temperature squareness ratio (Mr/Ms) of 0.42 was found to be somewhat similar to that expected (0.5) for a system of noninteracting single-domain nanoparticles, suggesting that the prepared nanoparticles are in a single-domain regime. The temperature dependence of coercivity was found to have slight deviations from Kneller’s law, possibly due to interactions between nanoparticles. The zero field cooled–field cooled curves indicated that below 150 K, the nanoparticles were ferrimagnetic dressed with spin-glass behavior, resulting from interactions between the ferrimagnetic nanoparticles and/or random freezing of surface spins.

Enhanced piezoelectric properties and domain morphology under alternating current electric field poled in [001]-oriented PIN-PMN-PT single crystal

Abstract: The piezoelectric and dielectric properties of PMN-PT single crystals were significantly enhanced by alternating current electric field poling (ACP). In this work, to investigate the mechanisms of piezoelectric performance enhancement in relaxor ferroelectric single crystals by ACP, a [001]-oriented PIN-PMN-PT single crystal with a diameter of 3 in was grown by the modified Bridgman method, and a series of single crystal samples within 100 mm of the height of the crystal boule were prepared. Compared with their direct current electric field poling (DCP) counterparts, the electrical properties of single crystal samples at different heights by ACP were regularly enhanced. The piezoelectric coefficient d33 and the dielectric constant ɛ33 of the rhombohedral samples both increased by nearly 20%. Based on the results of polarized light microscopy, the domain wall in [001]-poled ACP samples could not be observed along the [001] direction. The brightness of polarized light propagating along the [010] orientation was enhanced after ACP. The domain images of {100} in the DCP and ACP samples were observed by piezoelectric force microscopy. The results showed that the domain size of ACP samples increased. The existence of layered domains can be clearly found in the scanning electron microscopy results of the stress fracture surface of the ACP sample. According to the analysis, the improvement of the piezoelectric performance in ACP samples comes from the elimination of the 71° domain walls. This work demonstrates that certain ACPs can regulate the domain structure and steadily improve the piezoelectric properties of R-phase PIN-PMN-PT single crystals.

Drift of suspended single-domain nanoparticles in a harmonically oscillating gradient magnetic field

Abstract: We study the nonlinear dynamics of single-domain ferromagnetic nanoparticles in a viscous liquid induced by a harmonically oscillating gradient magnetic field in the absence and presence of a static uniform magnetic field. Under some physically reasonable assumptions, we derive a coupled set of stiff ordinary differential equations for the magnetization angle and particle coordinate describing the rotational and translational motions of nanoparticles. Analytical solutions of these equations are determined for nanoparticles near and far from the coordinate origin, and their correctness is confirmed numerically. We show that if a uniform magnetic field is absent, the magnetization angle and particle coordinate of each nanoparticle are periodic functions of time. In contrast, the presence of a uniform magnetic field makes these functions aperiodic. In this case, we perform a detailed analysis of the nanoparticle dynamics and predict the appearance of the drift motion (directed transport) of nanoparticles. We calculate both analytically and numerically the drift velocity, study its dependence on time and model parameters, analyze the physical origin of the drift phenomenon and discuss its potential biomedical applications.

Mapping Magnetic Signals of Individual Magnetite Grains to Their Internal Magnetic Configurations Using Micromagnetic Models

Abstract: Micromagnetic tomography (MMT) is a technique that combines X-ray micro computed tomography and scanning magnetometry data to obtain information about the magnetic potential of individual grains embedded in a sample. Recovering magnetic signals of individual grains in natural and synthetic samples provides a new pathway to study the remanent magnetization that carries information about the ancient geomagnetic field and is the basis of all paleomagnetic studies. MMT infers the magnetic potential of individual grains by numerical inversion of surface magnetic measurements using spherical harmonic expansions. The magnetic potential of individual particles in principle is uniquely determined by MMT, not only by the dipole approximation, but also more complex, higher order, multipole moments. Here, we show that such complex magnetic information together with both particle shape and mineral properties severely constrains the internal magnetization structure of an individual grain. To this end, we apply a three dimensional micromagnetic model to predict the multipole signal from magnetization states of different local energy minima. We show that for certain grains it is even possible to uniquely infer the magnetic configuration from the inverted magnetic multipole moments. This result is crucial to discriminate single-domain particles from grains in more complex configurations such as multi-domain or vortex states. As a consequence, our investigation proves that by MMT it is feasible to select statistical ensembles of magnetic grains based on their magnetization states, which opens new possibilities to identify and characterize stable paleomagnetic recorders in natural samples.

Tailoring Magnetic Domains and Magnetization Switching in CoFe Nanolayer Patterns with Their Thickness and Aspect Ratio on GaAs (001) Substrate

Abstract: The thickness and aspect ratio dependence of magnetic domain formation in CoFe nanolayer patterns on GaAs (001) substrates are investigated by means of a direct approach using magnetic force microscopy at room temperature. Magnetic force microscope observations under as‐deposition condition show that magnetic domain formation in the patterns depends strongly on the aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates. A single magnetic domain is more easily formed in the patterns with a higher aspect ratio, with a thinner nanolayer thickness, and along the ⟨110⟩ direction of the substrates. The magnetic fields are next applied in the direction parallel to the ⟨1–10⟩ and ⟨110⟩ orientations of the substrates to characterize a magnetization switching behavior in the patterns. The aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates strongly affect the magnetic fields needed for magnetization switching in the patterns. A higher magnetic field is required for a higher aspect ratio and a thinner nanolayer thickness of the patterns. All the direct observations confirm that the magnetic domain structures and magnetization switching are tuned by controlling aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates.

Quantum Stoner-Wohlfarth model of two-dimensional single domain magnets

Abstract: The Stoner-Wohlfarth (SW) model is a classical model for magnetic hysteresis of single-domain particles. For two-dimensional magnets at finite temperature, the SW model must be extended to include intrinsic strong spin fluctuations. We predict several fundamentally different hysteresis properties between 2D and 3D magnets. The magnetization switching diagram known as the Asteroid figure in the conventional SW model becomes highly temperature dependent and asymmetric with respect to the transverse and longitudinal magnetic fields. Our results provide new insights for 2D magnetic materials based spintronics applications.

Effect of thickness on the magnetic properties of single domain GdBCO bulk superconductor

Abstract: To study the influence of thickness on the magnetic properties of REBCO (RE = Y, Gd, Sm, Nd, etc.) bulk superconductors, a single domain gadolinium barium copper oxide (GdBCO) bulk superconductor fabricated by the RE + 011 top seeded infiltration growth (RE + 011 TSIG) method was continuously sliced along the bottom to obtain samples of different thickness. The levitation force and attractive force of these samples were tested at 77 K in the zero-field-cooled (ZFC) state. It was found that as the sample thickness decreases, the levitation force decreases gradually whereas the attractive force increases. This is related to the varied ability to resist the penetration of magnetic field occasioned by varying sample thickness, which were deeply revealed combined with the characteristics of the non-ideal type-II superconductor. Further, the levitation force exhibits a trend of slow initial change followed by rapid change, which may be attributed to the growth of the sample. Measurement of the trapped field shows that a similar distribution of trapped field at the top and bottom surfaces can be achieved by removing some materials from the bottom of the bulk. These results provide a reference for meeting the actual requirements of REBCO bulks of different thicknesses and greatly contribute to practical design and application. Keywords: single domain GdBCO bulk superconductor; thickness; levitation force; attractive force

Low-Frequency Dynamic Magnetic Susceptibility of Antiferromagnetic Nanoparticles with Superparamagnetic Properties

Abstract: As is known, the multi-sublattice structure of antiferromagnets (AFMs) entails that, under size diminution to the nanoscale, compensation of the sublattice magnetizations becomes incomplete. Due to that, the nanoparticles acquire small, but finite permanent magnetic moments. An AC field applied to such particles induces their magnetic response, the measurement of which is well within the sensitivity range of the experimental technique. Given the small size of the particles, their magnetodynamics is strongly affected by thermal fluctuations, so that their response bears a considerable superparamagnetic contribution. This specific feature is well-known, but usually is accounted for at the estimation accuracy level. Herein, a kinetic model is proposed to account for the magnetic relaxation of AFM nanoparticles, i.e., the processes that take place in the frequency domain well below the magnetic resonance band. Assuming that the particles possess uniaxial magnetic anisotropy, the expressions for the principal components of the both linear static and dynamic susceptibilities are derived, yielding simple analytical expressions, including those for the case of a random distribution of the particle axes.

Anisotropy of magnetic susceptibility and hysteresis parameters for natural magnetite particle assemblages: Micromagnetic analysis of focused-ion-beam nanotomography data with MERRILL

Abstract: &lt;p&gt;Three-dimensional shapes of 68 magnetite grains in pyroxene and 234 magnetite grains in plagioclase, were obtained by &amp;#8220;slice-and-view&amp;#8221; focused-ion-beam nanotomography (FIB-nt) on mineral separates from the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction (EBSD) determined the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. For each particle, hysteresis loops in 20 equidistributed field directions were calculated by the finite-element micromagnetic code MERRILL. For each direction, the averages over the particle ensemble were compared to corresponding hysteresis loops measured with a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. FIB-nt combined with micromagnetic modelling allows to explore the mechanisms controlling the magnetic anisotropy for each individual particle and to analyze the combined effect for bulk magnetic properties. This combination is of interest for anyone who interprets magnetic anisotropy because it helps understanding how domain states and crystal alignment in natural samples influence the measured anisotropy. Our results demonstrate that natural particle shapes, their orientations and domain states control the anisotropy of magnetic remanence, coercivity and susceptibility in natural particles. We can show for our specific examples, how the connection between mineral texture and magnetic anisotropy depends on specific domain states. Our data explain why natural magnetite particles at the transition between single-domain and single or multiple vortex states do not always follow the relation between axis orientation and&amp;#160; magnetic anisotropy which is theoretically expected for simple particle shapes.&lt;/p&gt;

Micromagnetic modelling and single particle multipole expansions from Micromagnetic Tomography

Abstract: &lt;p&gt;Micromagnetic Tomography is a technique that combines X-ray micro tomography and scanning magnetometry data to obtain magnetic information of individual grains embedded in a sample. Recovering magnetic signals of individual grains in rock samples and synthetic samples provides a new pathway to study the rock-magnetic properties of remanent magnetizations that are crucial to paleomagnetic studies. This is possible by numerically inverting the surface magnetic signal for the magnetic potential of individual magnetic grains via their spherical harmonic expansion [1]. Resulting magnetic moment solutions are uniquely determined as dipole and higher order multipole moments, which has been proved in [2]. Furthermore, the higher order multipole signals in the magnetic particles are an indication that the grains carry complex magnetic orderings, such as multi-domain or vortex configurations [3]. In this work we show that the magnetic moment information can be used to constrain the internal magnetic configuration of individual grains using micromagnetic modelling. We first review the multipole expansion method used in Micromagnetic Tomography [3]. Further, we show three dimensional micromagnetic modelling results to predict the multipole signal of magnetic particles in different local energy minimum magnetization states. We show that for certain grains it is possible to uniquely infer the magnetic configuration from the inverted magnetic multipole moments. This result is crucial to discriminate single-domain particles from grains in more complex configurations. Our investigation proves the feasibility to select statistical ensembles of magnetic grains based on their magnetization states, which opens new possibilities to characterize stable paleomagnetic recorders in natural samples.&amp;#160;&lt;/p&gt;&lt;p&gt;[1] L. V. de Groot, K. Fabian, A. B&amp;#233;guin, M. E. Kosters, D. Cort&amp;#233;s-Ortu&amp;#241;o, R. R. Fu, C. M. L. Jansen, R. J. Harrison, T. van Leeuwen, A. Barnhoorn. Micromagnetic tomography for paleomagnetism and rock-magnetism. Journal of Geophysical Research: Solid Earth, 126:e2021JB022364, 2021.&lt;br&gt;[2] K. Fabian and L. V. de Groot. A uniqueness theorem for tomography-assisted potential-field inversion. Geophysical Journal International, 216(2):760&amp;#8211;766, 2018.&lt;br&gt;[3] D. Cort&amp;#233;s&amp;#8208;Ortu&amp;#241;o, K. Fabian and L. V. De Groot. Single particle multipole expansions from Micromagnetic Tomography. Geochemistry, Geophysics, Geosystems, 22:e2021GC009663, 2021.&lt;/p&gt;