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The purpose of this work is to experimentally study, using the inverse spin Hall effect (ISHE), the detection of focused beams of magnetostatic surface waves (MSSW) in integrated YIG (3.9 µm) / Pt (4 nm) thin-film microstructures, where the focusing effect was ensured by the curvilinear shape of the exciting antenna. Make a comparison with the case of detecting MSSWs excited by a rectilinear antenna. Methods. Experiments were carried out using the delay line structures based on the YIG/Pt. The amplitude-frequency characteristics of the YIG/Pt structure and the frequency dependence of the EMF (V(f)) induced in platinum were studied. Results. It was shown that at frequencies f near the long-wavelength limit of the MSSW spectrum, the magnitude of the EMF V(f) generated by a focused MSSW can be several times higher than the values of V(f) in the case of MSSW excitation by a common (straight) antenna. In this case, in the short-wavelength part of the spectrum, on the contrary, the magnitude of the EMF generated by the focused MSSW beam turns out to be noticeably smaller. This behavior is associated with chromatic aberration of the focusing antenna for the MSSW, which manifests itself in the frequency dependence of the focal length of the antenna, which is confirmed by the results of micromagnetic modeling. It is shown that the drop in the EMF signal generated by a focused MSSW beam in the short-wavelength part of the spectrum is associated with the focus reaching the area of the YIG not covered with the Pt film. In this case, the increase in V(f) in the long-wavelength region of the MSSW spectrum is explained by an increase in the linear power density of the MSSW and the formation of caustics under the Pt film. Conclusion. Obtained results can be used for the development of highly sensitive spin wave detectors and the creation of spin logic devices.

Magnetotactic bacteria produce chains of nanoscopic iron minerals used for navigation, which can be preserved over geological timescales in the form of magnetofossils. Micrometer-sized magnetite crystals with unusual shapes suggesting a biologically controlled mineralization have been found in the geological record and termed giant magnetofossils. The biological origin and function of giant magnetofossils remains unclear, due to the lack of modern analogues to giant magnetofossils. Using distinctive Ptychographic nanotomography data of Precambrian (1.88 Ga) rocks, we recovered the morphology of micrometric cuboid grains of iron oxides embedded in an organic filamentous fossil to construct synthetic magnetosomes. Their morphology is different from that of previously found giant magnetofossils, but their occurrence in filamentous microfossils and micromagnetic simulations support the hypothesis that they could have functioned as a navigation aid, akin to modern magnetosomes. [ABSTRACT FROM AUTHOR]

A study of spin wave spectra in a two-layer structure of iron–yttrium garnet (YIG) with different magnitudes of the saturation magnetizations of the layers has been carried out. Different modes of spin wave propagation (reciprocal, nonreciprocal, single-wave) depending on the type of structure and width of the central waveguide are investigated. The classification of spin wave spectra is carried out, and the class of guided, outgoing, and edge spin modes is identified. In particular, it is shown that in a system of planar magnetic comb-type LS-type microwaveguide tubes with periodic boundary conditions, two non-contiguous frequency regions of existence of guided modes of the central waveguide are observed for a width w of the central waveguide. Two adjacent frequency regions exist in the system of planar magnetic comb-type HS-type microwaveguide tubes at any values of the width of the central waveguide: in the high-frequency region, the mode with outflowing modes of the structure is realized, while in the low-frequency region, the mode with guided modes of the central waveguide is realized. It is shown that in systems of both types in the region of strongly inhomogeneous magnetic fields there can exist modes of boundary waves having a mutual character of propagation. The results obtained can be used to expand and clarify the physics of wave processes in complicated magnetic structures. [ABSTRACT FROM AUTHOR]

Purpose. Investigation of the joint manifestation of the effects of anisotropic signal propagation, coupling, and nonlinear power dependence of the medium parameters in a lattice of laterally and vertically coupled spin-wave (SW) microwaveguides. Consideration of the case of the influence of the rotation of the magnetization angle and the change of the lateral gap between microwaveguides located on the same substrate on the transverse profile of the spin-wave beam and the spatial localization of the SW amplitude. Methods. The method of micromagnetic modeling based on the numerical solution of the Landau–Lifshitz–Hilbert equation shows the possibility of controlling the direction of propagation of SW in an ensemble of laterally and vertically coupled iron yttrium garnet (YIG) microwaveguides by changing the magnetization angle. By the method of numerical integration of the system of coupled discrete nonlinear Schrodinger equations, the possibility of changing the transverse profile of the spin-wave beam by changing the level of the initial signal amplitude is shown. Results. The spatial distributions of the components of the dynamic magnetization of the SW excited in two microwaveguides located on the same substrate obtained in micromagnetic simulations indicate a change in the character of localization of the SW power in the output sections of the microwaveguides. At variation of the lattice magnetization angle, a shift of the threshold power value is observed, at which a characteristic curbing of the transverse width of the spin-wave beam in the nonlinear mode appears. Conclusion. When excitation of surface magnetostatic SW in a lattice of laterally and vertically coupled microwaveguides, a transformation of the transverse profile of the wave is observed at a deviation of the magnetization angle of the structure by 15º , which is manifested in the change of the SW length and its localization in each of the microwaveguides. The combined effects of dipole coupling, gyrotropy, and nonlinearity of the medium make it possible to control the value of the threshold power of the SW, at which the mode of diffractionless propagation of the spin-wave beam is realized in a single layer of the structure.

Numerical simulation of the process of remagnetization of small pseudo–single-domain magnetite particles (Тc = 580°С) was performed. The particles are cylindrical in shape, with a height h of 60–350 nm and a height-to-diameter ratio of 1.29. This geometry enables preferential anisotropy of the shape, causing the magnetic moment of the particle to align along the cylinder’s axis in a stable state. As the size increases, the domain structure shifts from the single-domain state (60 nm) to the flower mode (h = 70–85 nm), and then to a vortex structure. Particles in the range of h = 75–250 nm are remagnetized through a vortex state, with the axis aligned along their diameter. In the range of h = 300–350 nm, at the top of the potential barrier, the domain structure transforms from a single vortex to a multi-vortex configuration. The blocking temperatures Tb of the particles vary from 520 to 580°C, while the dependence Тb(h) is non-monotonic and manifests a “pit” at h = 90–140 nm. At the same time, at h = 300–350 nm, Tb values differ from by Тс no more than 1°C. At h = 100 nm, the ratio of magnetic energy in the external field B of the order of the earth to thermal energy at T = Tb reaches 1. This suggests a strong nonlinearity of the TRM(B) dependence even in such small fields and particle sizes. The results obtained highlight the need to revise the existing micromagnetic models by taking into account the specific shape and deficiency of the crystal structure of particles in order to bring them in line with the properties of actual ferrimagnets present in rocks.

Here we present the results of the study of the joint manifestation of the effects of anisotropic spin waves (SW) propagation in a system of laterally and vertically coupled ferrimagnetic microwaveguides when both the magnetization angle and air gap between waveguide are changed. The micromagnetic modeling method based on the numerical solution of the Landau–Lifshitz–Hilbert equation was used to provide the possibility of controlling the direction of SW propagation in a system of laterally and vertically coupled iron-yttrium garnet (YIG) microwaveguides by changing the magnetization angle and direction of the equilibrium of magnetization direction. The spatial distributions of the dynamic magnetization out-of-plane component of the SW excited in two microwaveguides located on the same substrate obtained in micromagnetic simulations indicate a change in the nature of the SW power localization in the output sections of the microwaveguides. The variation of the magnetization angle of the array leads to the variation of the transverse width of the spin-wave beam and localization of the amplitude maximum in each of the microwaveguides within the array. The joint manifestation of dipole coupling effects in each separately taken layer of the structure realizes the regime of non-diffraction propagation of the spin-wave beam. [ABSTRACT FROM AUTHOR]

Based on computer simulation and the analysis of technological, experimental, and theoretical results, the technological requirements for the formation of electronic devices based on magnetic vortices and skyrmions are formulated. The main types of interactions determined by technological factors are determined. The design features of electronic devices based on magnetic vortices and skyrmions are studied. Various technological approaches to the fabrication of structures with different magnetic anisotropy properties are studied. [ABSTRACT FROM AUTHOR]

Artificial glasses containing nanoscale inclusions of iron oxides, including magnetite and hematite, were obtained via the method of the high-temperature melting of rocks. The main factors influencing the magnetic properties of glasses are the composition of the initial charge and the conditions of cooling of the melt. The data of magnetic granulometry and frequency-field dependencies of magnetic susceptibility showed the presence of a sufficiently large superparamagnetic fraction in the samples. Coordinated theoretical modeling using two independent models that take into consideration possible the chemical inhomogeneity of particles and magnetostatic interaction between them made it possible to calculate hysteresis characteristics corresponding to the experiment and to estimate ferrimagnetic concentrations in the samples, including the superparamagnetic fraction. [ABSTRACT FROM AUTHOR]

Abstract—The evolution of the domain structure (DS) of cubic submicron- and micron-sized magnetite particles has been studied in detail during "cooling" of specimens from the Curie temperature Тс to room temperature Тr followed by their "reheating" to Тс in order to determine the degree of irreversibility of DS changes during heat treatment and their possible effects on the thermoremanent magnetization (TRM) properties. It is shown that typical magnetic configurations in particles up to 2 μm in size have flower or vortex shapes with one or two vortices. A model of the formation of thermoremanent magnetization (TRM) in submicron-sized pseudo-single-domain particles (PSD) is proposed based on an expansion of Neel's single-domain thermofluctuation model of TRM acquisition. According to the model, the equality of blocking and deblocking temperatures is retained for submicron PSD grains. Consequently, for this magnetic fraction, the Thellier laws of pTRM additivity and independence must also be valid. [ABSTRACT FROM AUTHOR]

• Autoresonance is a promising nonlinear effect that allows controlling the eigen oscillations of a system by automatically adjusting the frequency of these oscillations for the changing frequency of weak external influence. This phenomenon is applied in various fields of physics, from plasma excitation and particle acceleration to the description of the orbits of celestial bodies. However, it has never been applied in magnonics due to limitations in theoretical models, which do not allow determining the parameters necessary for its experimental observation in magnets. The highlights of the paper include the following: - Presented research is a bridge from existing models of isotropic one-dimensional magnets to real objects widely used in microwave devices – thin films of yttrium iron garnet (YIG). - A micromagnetic model of a magnetized-to-saturation yttrium iron garnet (YIG) film has been developed, in which highly-amplitude magnetic oscillations are excited by a weak, linearly polarized alternating field of about 1 Oe, capable of completely demagnetizing the film due to the autoresonance effect. The model takes into account boundary conditions, demagnetization factors, magnetic crystalline anisotropy of the film and the damping coefficient. - In addition to determining the parameters necessary for the experimental observation of the effect, the influence of magnetic crystalline anisotropy and damping on the autoresonance excitation of magnetic oscillations in YIG films has been discovered. Micromagnetic modeling of non-linear autoresonance magnetization oscillations in thin films of yttrium iron garnet (YIG) with specified growth directions is conducted. It is found that in the case of rapid frequency modulation (sweep rate of the order of 1016 Hz/sec) of 1Oe excitation magnetic field, the maximum precession angle of magnetization can achieve up to 160°. For the first time, the influence of demagnetization fields, magneto-crystalline anisotropy, and Gilbert damping on autoresonance phenomena in YIG films is numerically calculated. It is shown that demagnetization fields and damping have a weak influence on parameters of autoresonance. Simultaneously, damping provides a shorter phase-locking time between the excitation field and intrinsic magnetization oscillations in the film, favoring high amplitude of magnetization oscillations. The magneto-crystalline anisotropy leads to a reduction of the threshold sweep rate of the pumping field for YIG films with [100] direction, as well as the emergence of parametric instability for [210] films. The results of the work are aimed to be applied for the experimental observation of autoresonance phenomena in thin yttrium iron garnet films. [ABSTRACT FROM AUTHOR]

Abstract—The magnetic properties of Fe3O4–Fe3 –xTixO4 composites synthesized by various methods were simulated based on the model of an ensemble of magnetostatically interacting particles. The results are in good agreement with the hysteresis characteristics of the samples calculated earlier in the framework of the model of chemically heterogeneous two-phase particles. It is shown that the used approach is also applicable to the samples consisting mainly of superparamagnetic particles in which the saturation remanent magnetization is provided by the particles blocked due to the magnetostatic interaction. [ABSTRACT FROM AUTHOR]

Recent micromagnetic simulations have found that particles in the transitional zone between the single domain (SD) and single vortex (SV) zone are prone to thermal and external magnetic field instabilities that could adversely affect the accuracy of interpretations of paleomagnetic recordings. In this study, we attempt to evaluate the internal magnetization characteristics of these magnetically unstable (MU) particles and the influence on paleomagnetic observations by simulating the magnetic behavior of 68–104 nm truncated octahedral magnetite particles using the MERRILL modeling software. We found that: (a) The size region of the "MU zone" for grains of truncated octahedron shape is different from cubic octahedrons and spheres, indicating that the zone may be controlled by the geometry and shape of particles; (b) The MU zone has a range of 79–97 nm region, which is dominated by a hard‐axis aligned single vortex (HSV); and (c) MU particles are unstable as a function of temperature and have low coercive fields. Finally, the numerical fitting of hysteresis parameters for experimental data suggests that the influence of such MU particles in samples should not be ignored, especially for samples with fine‐grained magnetic minerals as the primary magnetic recording carriers. This research has extended our understanding of the behavior of the "MU zone" and its significance on paleomagnetic records. Plain Language Summary: Magnetic minerals in geological samples can record geomagnetic information over billions of years, which makes it possible to study the evolution of the Earth. However, this ability of magnetic minerals is greatly affected by particle size and the range we call these "magnetically unstable (MU) zone." Recently, micromagnetic simulations found that the particles in the "MU zone" can be easily affected by slight thermal disturbances, which means a detailed investigation of the "MU zone" is very important to improve the accuracy of geological information interpretations. This study presents a detailed evolution of the MU zone with grain size and temperature, and will deepen our understanding of the magnetic characteristics of the MU zone and its significance on paleomagnetic records. Key Points: The magnetically unstable (MU) zone for equant truncated octahedra is ∼20 nm wide and is geometry and shape dependentThe unstable grains are characterized by low coercivities owing to their multiple possible states with small intervening energy barriersThe domain states of MU grains vary with temperature, and their influence on geological samples should not be ignored [ABSTRACT FROM AUTHOR]

Anisotropy 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. Plain Language Summary: Many rocks have physical properties that vary according to the direction in which these are measured. Such rocks are called anisotropic. A fast and simple measurement which can uncover such anisotropy is the magnetization response to applied fields in different directions. This anisotropy of magnetic susceptibility (AMS), or AMS, is used in numerous applications to detect hidden information about geological processes. However, the interpretation of AMS data requires additional knowledge about the small magnetic mineral grains in the rock, because very small grains behave exactly opposite (inverse) to very large grains (normal). Here, we use hundreds of slices of electron microscopy images to reconstruct the exact shape of hundreds of such tiny magnetite grains. Then we run computer models to predict how these grains respond to applied magnetic fields. These data reveal how magnetic grains react to magnetic fields, and help us to understand better why small and large grains behave differently. Now we have a much better insight into the physical processes that are measured during AMS determinations made in laboratories all over the world. Key Points: Micromagnetics and nanotomography elucidate the physical anisotropy of magnetic susceptibility (AMS) modes of natural magnetite particlesThe data explain the transition from inverse to normal AMS fabric with increasing grain sizeA systematic characterization of AMS modes in single‐domain and pseudo‐single domain magnetite particles is established [ABSTRACT FROM AUTHOR]

The effect of nanoparticles (NPs) on microwave magnetoresistance in MgO/CoFeB(free)/Ta/CoFeB(fixed)/MgO)/Ta platform and the effect of individual NP on free CoFeB ferromagnetic layer magnetization have experimentally been revealed and analyzed. The area of the platform where magnetization is inverted under scattering field of NPs has been observed. Micromagnetic modeling of dipole magnetic interaction confirms the existence of such areas in “NP-platform” system. The increase in critical field of the platform and the change of resistance under scattering field on NPs have experimentally been revealed. [ABSTRACT FROM AUTHOR]

Subject. A change in the spectrum of spin waves (SW) in a magnetic cross is investigated when two signals pass through it: a pump signal and a probe signal. Objective. Detection of specific features in formation of the spectra of the output signals of SW in the multiport structure based on a yttrium iron garnet (YIG) film in the case of excitation of two magnetostatic surface waves (MSSW) simultaneously by the input antenna, where the first, with power higher than the first-order parametric instability threshold is the pump, and the second one is a probe. Methods. The experiments were performed for a cross structure from YIG film in the form of two orthogonal waveguides with the SW wire antennas placed at the ends of the waveguides, where one of the antennas on the transversely magnetized waveguide was considered as the input. Result. It was found that by choosing the probing signal frequency, one can significantly (by 10 dB) change the relative signal levels for the satellite waves at the output antennas, which are secondary MSSWs with some new frequencies and appear in the output signals spectrum as a result of the thresholdless processes of merging of parametric spin waves generated by MSSW pumping. In this case the secondary MSSWs frequencies can differ at the output antennas located on orthogonal waveguides. Discussion. The discovered effect is associated with the nonreciprocal nature of propagation of both the pumping wave and the waves generated at parametric instability condition in the structure.

Modeling of the hysteresis characteristics of Fe3O4–Fe3–xTixO4 composites was performed using the method of magnetic rectangles. Their magnetic properties are well explained within the model of an ensemble of magnetostatically interacting two-phase chemically inhomogeneous particles. It is shown that accounting for the contribution of the superparamagnetic fraction makes it possible to obtain agreement between the theoretical and experimental magnetic characteristics of the samples. The model allows calculating the magnetization reversal fields and magnetizations of the studied composites theoretically and also to show that the main contribution to the remanent magnetization is made by two phase particles. In addition, a possibility of using this model for samples containing superparamagnetic particles is demonstrated.

Using numerical micromagnetic modeling, we have investigated the development of domain structure and magnetization reversal in multilayer thin-film structures. The permalloy (Ni80Fe20) magnetic layers had the inplane uniaxial and perpendicular magnetic anisotropy. We found that as the thickness of nonmagnetic interlayers decreases, the in-plane configuration of magnetic moments in the permalloy layers transforms from a single domain state to stripe domains, which is caused by the increase of magnetostatic interaction between layers. In structures with "thick" interlayers, even weak magnetostatic interaction enforces the neighboring single domain permalloy layers to have opposite orientations of magnetic moments. The saturation field of such samples increases linearly with the number of layers. By analyzing the dynamic characteristics of multilayers, we determined the optimum number of layers ensuring the maximum conversion efficiency of wideband microwave microstrip sensors of weak magnetic fields. [ABSTRACT FROM AUTHOR]

A phenomenological model of the formation of exchange bias field and coercivity of polycrystalline ferromagnet/antiferromagnet composites is constructed by an example of the Fe19Ni81/Fe50Mn50 system. Based on the experimental data on temperature variations of the hysteresis properties, the estimation of distributions of crystallite blocking temperature and crystallite magnetic anisotropy constant for the antiferromagnetic layer is performed. The obtained results are used in order to perform the micromagnetic simulation of the temperature dependences of the coercivity and exchange bias field of ferromagnetic layer. It is shown that the micromagnetic model can be used successfully for the analysis and prediction of hysteresis properties of film with the exchange bias. [ABSTRACT FROM AUTHOR]

Isothermal remanent magnetization (IRM) curves are used widely in rock magnetic and environmental magnetic studies to characterize magnetic properties of natural samples. Previous theoretical and experimental investigations indicate that magnetostatic interactions can strongly affect the IRM behavior. In order to quantitatively investigate this effect, we modeled IRM curves and hysteresis properties for magnetite assemblages with variable morphologies (grain size, elongation, and shape) using the finite‐element micromagnetic modeling code MERRILL. We also conducted an analytical study of IRM behavior for noninteracting single domain grains utilizing the Stoner–Wohlfarth model. It is found that multiple Gaussian components do not necessarily reflect multiple magnetic mineral populations. For example, magnetostatic interactions can produce coercivity distributions that resemble results of some samples containing a detrital and extracellular (D + EX) and a biogenic soft or hard (BS/BH) coercivity components. Simulated IRM curves for different morphologies of interacting flattened grains are similar, indicating that the effect of interactions can overshadow the effect of grain morphologies. Moreover, our micromagnetic and analytical calculations consistently indicate that the IRM curves for noninteracting fine‐grained assemblages are mostly determined by grain shape (elongated or flattened) and elongation distributions of the magnetic mineral assemblages. Our simulations present possible outcome of IRM curves for magnetic mineral assemblages with controlled grain size, shape and microstructures, which provides important theoretical constraints on characterizing complex magnetic mineral components in natural samples through IRM curves and hysteresis. Plain Language Summary: Tiny magnetic mineral grains, often in the nanometer and micrometer size, in geological samples retain critical information about past environmental changes and geomagnetic field variations. Knowing the properties of magnetic minerals is crucial to decode magnetic mineral records. Decomposing isothermal remanent magnetization (IRM) curve into multiple magnetic mineral parts is a powerful technique, but is often complicated by the common presence of magnetic interactions where magnetic mineral grains are clumped to each other. This process is similar to attracting and repelling of magnets, but occurs at very small, microscopic scale. Here we have applied computer‐modeling to calculate how neighboring magnetic grains affect the IRM curves. Our simulation results provide new physical constraints on analyzing IRM data, which is important to make robust environmental and geophysical interpretations. Key Points: Micromagnetic calculations elucidate potential origin of multiple Gaussian remanent coercivity componentsThe elongation distribution and grain morphologies strongly affect the shape of IRM curves for noninteracting fine‐grained particlesMagnetostatic interactions can overshadow the effect of grain morphologies and can also cause multiple Gaussian coercivity components [ABSTRACT FROM AUTHOR]

The energy and structure of two-dimensional domain walls under the action of an external magnetic field directed perpendicular to the easy axis of the film are investigated based on micromagnetic modeling. The transitions in the domain wall structure are considered in detail: with an increase in the external field, the transition of a single-vortex 180° Bloch DW to an intermediate two-dimensional Néel DW and, after removal of the external field, a transition to a 180° two-vortex structure. [ABSTRACT FROM AUTHOR]

Magnetic nanodisks have been recently proposed as biomedical tools for therapeutics at the nanoscale level, with a special focus on hyperthermia for cancer cure. Here we present a detailed study of permalloy nanodisks to be used in alternative to superparamagnetic iron oxide nanoparticles, as efficient heating agents that release heat via magnetic hysteresis. A micromagnetic modeling analysis is carried out to identify sizes and ac field parameters that maximize the specific loss power (SLP), guaranteeing the fulfillment of biophysical constraints (Hergt–Dutz limit) and vortex state at remanence (reduced agglomeration effects). The highest SLP (790 W g−1) is found for 100 nm diameter and 20 nm thickness nanodisks, excited at a frequency of 75 kHz. Further analysis elucidates the influence of magnetostatic interactions and local nanodisk‐field orientation on the SLP of nanodisk clusters, which originate from the deposition in target tissues. At high concentrations, magnetostatic interactions can lead to a reduction of 40–50% in the hysteresis losses. From thermal simulations, we finally demonstrate that in a murine model temperature increments comparable to that obtained in calorimetric measurements under quasi‐adiabatic conditions can be achieved only by using an order of magnitude larger dosage of nanodisks, due to blood perfusion effects. [ABSTRACT FROM AUTHOR]

Abstract Recent micromagnetic simulations predict that particles have an abnormal region of low magnetic stability for grain sizes near the domain state transitions from single domain to single vortex. The implications of this zone are not yet fully understood, but if the size range of this window changes as a function of applied field and temperature then it is likely to contribute to observational uncertainties in paleomagnetic recordings. The narrow size range of this zone means that its effects are difficult to identify when integrated across the much wider distribution of grain sizes present in a sample. Here we report a method of examining the unstable region using low temperature oxidation of magnetite. The rock magnetic experiments on partially oxidized particles with a median diameter close to the unstable zone are reinterpreted using micromagnetic simulations of a multilayer core‐shell structure. The predicted magnetic properties exhibit significantly improved agreement with experimental data than that of the previously reported single core‐shell coupled geometry. The observed changes in remanence and coercivity are associated with predicted domain structure changes near a proposed magnetic unstable zone (∼80–∼120 nm), and so provide the first experimental indication of the existence of such a region. We also demonstrate that magnetic remanence in particles with grain sizes outside this size range were stable to maghemitization. The research provides ideas for the experimental study of the “magnetic unstable zone” and is significant to the correct interpretation of magnetic recordings. Finally, the partially oxidized magnetite particles are able to record paleomagnetic signals.

System-level requirements for heat-assisted magnetic recording at high areal density are presented in this article. The primary factors that affect bit error rate (BER) are spatial, or media, signal-to-noise ratio (SNR), reader SNR, and channel bit density (CBD). Spatial SNR is largely determined by the interaction between the writer properties and the media properties. For example, how well the thermal gradient matches the grain size. Spatial SNR also depends on how well the reader cross-track resolution matches the track pitch, and the quality of the written pattern. For any given level of spatial SNR, there exists a combination of CBD and reader SNR that results in the desired level of performance. This article elucidates some of the tradeoffs that may be needed to maintain BER as areal density increases, for different bit aspect ratio scenarios. For very high areal density, we show that the geometrical requirements are extremely aggressive, which will most likely require significant invention in head and drive architecture design. [ABSTRACT FROM AUTHOR]

This work provides a review of the existing micromagnetic models of the interaction of magnetization structures with inhomogeneities with different magnetic properties and geometries. Most attention is paid to models of the interaction of domain walls with defects in thin magnetic films. This work also gives a brief overview of studies of magnetic structures associated with inhomogeneities of systems that are currently being intensively studied, such as antidot arrays in permalloy, cobalt, and composite films, skyrmions and skyrmion lattices, and materials with a granular and porous structure. [ABSTRACT FROM AUTHOR]

Recent micromagnetic simulations predict that particles have an abnormal region of low magnetic stability for grain sizes near the domain state transitions from single domain to single vortex. The implications of this zone are not yet fully understood, but if the size range of this window changes as a function of applied field and temperature then it is likely to contribute to observational uncertainties in paleomagnetic recordings. The narrow size range of this zone means that its effects are difficult to identify when integrated across the much wider distribution of grain sizes present in a sample. Here we report a method of examining the unstable region using low temperature oxidation of magnetite. The rock magnetic experiments on partially oxidized particles with a median diameter close to the unstable zone are reinterpreted using micromagnetic simulations of a multilayer core‐shell structure. The predicted magnetic properties exhibit significantly improved agreement with experimental data than that of the previously reported single core‐shell coupled geometry. The observed changes in remanence and coercivity are associated with predicted domain structure changes near a proposed magnetic unstable zone (∼80–∼120 nm), and so provide the first experimental indication of the existence of such a region. We also demonstrate that magnetic remanence in particles with grain sizes outside this size range were stable to maghemitization. The research provides ideas for the experimental study of the "magnetic unstable zone" and is significant to the correct interpretation of magnetic recordings. Finally, the partially oxidized magnetite particles are able to record paleomagnetic signals. Plain Language Summary: Magnetic minerals in geological samples can record variations of the geomagnetic field over billions of years and provide a way to investigate the evolution of the Earth. The recording ability of minerals varies with grain size and decreases for both the largest and smallest grains. Recent numerical models suggest that even within the intermediate grain sizes previously regarded as good recorders, there exists an unstable region whose characteristics are not yet fully understood. This prediction is of paramount importance for the fundamental accuracy of paleomagnetic studies, because by identifying and excluding such unstable recorders it is possible to increase the accuracy of paleointensity determinations. This study provides the first experimental evidence in support of the existence of the unstable zone. Key Points: Results of micromagnetic multilayer core‐shell models agree well with experimental observations of low temperature oxidized magnetiteThe variation of hysteresis parameters with the oxidation state is consistent with the existence of a magnetic unstable zoneThe effective identification of the magnetic unstable zone should improve the consistent observation of paleomagnetic signals from geological samples [ABSTRACT FROM AUTHOR]

Near-shore marine sediments deposited during the Paleocene-Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needleshaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the singledomain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change. [ABSTRACT FROM AUTHOR]

Coherent magnetization dynamics at terahertz (THz) frequencies is achieved by directly incorporating ferromagnetic Ni80Fe20 thin films into Fe/Au bilayer spintronic THz emitters. Electrooptical (EO) sampling demonstrates a generation of THz pulses centered near ~2.8 THz with a bandwidth in the 1–4 THz range. Ni80Fe20 magnetization is directly coupled to the magnetic component of the THz wave, in which the large-amplitude magnetization precession is found phase locked with the THz pulse. The THz-induced magnetization dynamics can be controlled by external bias magnetic fields. The micromagnetic and analytical models based on the Landau–Lifshitz–Gilbert (LLG) equation are in good agreement with experimental findings and verify that the underlying mechanism of the THz-induced magnetization dynamics is the Zeeman coupling between the magnetic component of the THz field and Ni80Fe20 magnetization. Our results open the opportunities for the use of low-cost metallic spintronic THz emitters in the studies of THz-induced magnetization dynamics in magnetic thin films. [ABSTRACT FROM AUTHOR]

Magnetotactic bacteria produce chains of nanoscopic iron minerals used for navigation, which can be preserved over geological timescales in the form of magnetofossils. Micrometer-sized magnetite crystals with unusual shapes suggesting a biologically controlled mineralization have been found in the geological record and termed giant magnetofossils. The biological origin and function of giant magnetofossils remains unclear, due to the lack of modern analogues to giant magnetofossils. Using distinctive Ptychographic nanotomography data of Precambrian (1.88 Ga) rocks, we recovered the morphology of micrometric cuboid grains of iron oxides embedded in an organic filamentous fossil to construct synthetic magnetosomes. Their morphology is different from that of previously found giant magnetofossils, but their occurrence in filamentous microfossils and micromagnetic simulations support the hypothesis that they could have functioned as a navigation aid, akin to modern magnetosomes., Competing Interests: Competing interests statement:The authors declare no competing interest.

Abstract Three‐dimensional geometries of silicate‐hosted magnetic inclusions from the Harcus intrusion, South Australia have been determined using focused‐ion‐beam nanotomography (FIB‐nt). By developing an effective workflow, the geometries were reconstructed for magnetic particles in a plagioclase (162) and a pyroxene (282), respectively. For each inclusion, micromagnetic modeling using Micromagnetic Earth Related Rapid Interpreted Language Laboratory provided averaged hysteresis loops and backfield remanence curves of 20 equidistributed field directions together with average Ms, Mrs, Hc, and Hcr. The micromagnetic structures within each silicate are single‐domain (SD), single‐vortex (SV), multivortex (MV) and multidomain states. They have been analyzed using domain‐state diagnostic plots, such as the Day plot and the Néel plot. SD particles can be subdivided into groups with dominant uniaxial anisotropy (Mrs/Ms ∼0.5 and 10 < Hc < 100 mT) and mixed uniaxial/multiaxial anisotropy (Mrs/Ms ∼0.7 and 10 < Hc < 30 mT). Most SV particles lie on a trend with 0 < Mrs/Ms < 0.1 and 0 < Hc < 10 mT, while others display a broad range of intermediate Mrs/Ms and Hc values. SV and MV states do not plot on systematic grain‐size trends. Instead, the multicomponent mixture of domain states within each silicate spans the entire range of natural variability seen in bulk samples. This questions the interpretation of bulk average hysteresis parameters in terms of grain size alone. FIB‐nt combined with large‐scale micromagnetic simulations provides a more complete characterization of silicate‐hosted carriers of stable magnetic remanence. This approach will improve the understanding of single‐crystal paleomagnetism and enable primary paleomagnetic data to be extracted from ancient rocks.

Abstract Realistic geometries of magnetite grains from the Stardalur volcano, Iceland, were obtained by Focused Ion Beam Scanning Electron Microscopy nanotomography. These magnetite grains are subdivided by oxidation‐exsolution lamellae of ilmenite. Magnetic properties of these grains were modeled without internal stress using the three‐dimensional micromagnetic code MERRILL. The influence of grain shape and size was isolated by modeling hysteresis loops of the same grains with and without exsolution microstructures. The resulting coercivities Hc are up to 1.5 times higher, and the Mrs/Ms ratios are twice as high for the grains with exsolution than for those without. Both modeled values are a factor of 10 smaller than the measured bulk data from the same sample. This difference between stress‐free models and measured hysteresis loops suggests that the internal stress due to the formation of the oxidation‐exsolution lamellae is the dominant mechanism of coercivity and remanence enhancement. By comparing the approach‐to‐saturation behavior of modeled and measured hysteresis loops, the internal stress is quantified to about 100 MPa. The formation of lamellae has two effects on magnetic properties. (1) The apparent grain size is geometrically reduced. This effect increases Mrs and Hc by up to a factor of 2. (2) The formation of lamellae produces internal stress fields, which provide additional anisotropy energy that deflect the magnetic spins and apparently increase Mrs and Hc by up to a factor of 10. Accordingly, stress dominates the remanent magnetic properties in the Stardalur basalts and may be the decisive effect explaining its unusual remanent‐dominated ground magnetic anomaly of up to 27,000 nT.

Three‐dimensional geometries of silicate‐hosted magnetic inclusions from the Harcus intrusion, South Australia have been determined using focused‐ion‐beam nanotomography (FIB‐nt). By developing an effective workflow, the geometries were reconstructed for magnetic particles in a plagioclase (162) and a pyroxene (282), respectively. For each inclusion, micromagnetic modeling using Micromagnetic Earth Related Rapid Interpreted Language Laboratory provided averaged hysteresis loops and backfield remanence curves of 20 equidistributed field directions together with average Ms, Mrs, Hc, and Hcr. The micromagnetic structures within each silicate are single‐domain (SD), single‐vortex (SV), multivortex (MV) and multidomain states. They have been analyzed using domain‐state diagnostic plots, such as the Day plot and the Néel plot. SD particles can be subdivided into groups with dominant uniaxial anisotropy (Mrs/Ms ∼0.5 and 10 < Hc < 100 mT) and mixed uniaxial/multiaxial anisotropy (Mrs/Ms ∼0.7 and 10 < Hc < 30 mT). Most SV particles lie on a trend with 0 < Mrs/Ms < 0.1 and 0 < Hc < 10 mT, while others display a broad range of intermediate Mrs/Ms and Hc values. SV and MV states do not plot on systematic grain‐size trends. Instead, the multicomponent mixture of domain states within each silicate spans the entire range of natural variability seen in bulk samples. This questions the interpretation of bulk average hysteresis parameters in terms of grain size alone. FIB‐nt combined with large‐scale micromagnetic simulations provides a more complete characterization of silicate‐hosted carriers of stable magnetic remanence. This approach will improve the understanding of single‐crystal paleomagnetism and enable primary paleomagnetic data to be extracted from ancient rocks. Key Points: The 3D geometries of more than 400 silicate‐hosted magnetite inclusions were measured using focused‐ion‐beam nanotomographyMicromagnetic modeling show that inclusions adopt multiple domain states, spanning the entire range of common domain‐state diagnostic plotsGeometric variability of naturally formed magnetite in silicate, explains scatter of magnetic parameters in the pseudo‐single‐domain region of the Day plot [ABSTRACT FROM AUTHOR]

Realistic geometries of magnetite grains from the Stardalur volcano, Iceland, were obtained by Focused Ion Beam Scanning Electron Microscopy nanotomography. These magnetite grains are subdivided by oxidation‐exsolution lamellae of ilmenite. Magnetic properties of these grains were modeled without internal stress using the three‐dimensional micromagnetic code MERRILL. The influence of grain shape and size was isolated by modeling hysteresis loops of the same grains with and without exsolution microstructures. The resulting coercivities Hc are up to 1.5 times higher, and the Mrs/Ms ratios are twice as high for the grains with exsolution than for those without. Both modeled values are a factor of 10 smaller than the measured bulk data from the same sample. This difference between stress‐free models and measured hysteresis loops suggests that the internal stress due to the formation of the oxidation‐exsolution lamellae is the dominant mechanism of coercivity and remanence enhancement. By comparing the approach‐to‐saturation behavior of modeled and measured hysteresis loops, the internal stress is quantified to about 100 MPa. The formation of lamellae has two effects on magnetic properties. (1) The apparent grain size is geometrically reduced. This effect increases Mrs and Hc by up to a factor of 2. (2) The formation of lamellae produces internal stress fields, which provide additional anisotropy energy that deflect the magnetic spins and apparently increase Mrs and Hc by up to a factor of 10. Accordingly, stress dominates the remanent magnetic properties in the Stardalur basalts and may be the decisive effect explaining its unusual remanent‐dominated ground magnetic anomaly of up to 27,000 nT. Key Points: Micromagnetic hysteresis models of magnetite grain shapes obtained by FIB‐SEM nanotomography are compared to measured hysteresis loopsStress‐free models of ~500‐nm‐long grains yield ~2 times higher Mrs/Ms and ~1.5 times higher Hc with microstructures than withoutComparison to 10 times higher measured bulk Mrs/Ms and Hc implies that internal stress from exsolution structures is the dominant effect [ABSTRACT FROM AUTHOR]

Here we study the spin-wave propagation in a system of Mach–Zehnder interferometers (MZI) based on ferrimagnetic yttrium-iron garnet (YIG) thin film. The angular momentum transfer during the excitation of a surface magnetostatic spin wave and the coupling of the spin-waves inside the adjacent arms of MZIs is studied using the micromagnetic modeling method. The local variation of the YIG magnetization inside the arms of MZI manifests itself both in the phase shift of the propagating spin-wave signal and coupling efficiency in the region where the adjacent arms of MZI are placed in the close proximity to each other. The switching/demultiplexing performance which is reflected in the spatial frequency selection of the spin-wave signal upon dynamical local change of the magnetization in each of four arms of the interferometers is demonstrated. The proposed MZI demonstrates the basis for the realization of the set of logical operations and could serve the facility of add and drop frequencies (channels) to and from a magnonic data bus in frequency division multiplexed magnonic networks. The use of laterally coupled interferometers opens new possibilities for the formation of the logical magnon devices tuned by the local variation of magnetization which could be realized with the locally focused laser heating. • Spin wave transmission in a tangentially magnetized system of Mach–Zehnder interferometers. • Logical magnonic devices tuned by the local variation of magnetization which could be realized with the locally focused laser heating. • Modeling a magnonic device based on interference principles to perform a set of logic operations. [ABSTRACT FROM AUTHOR]

• The magnetization curves of iron nanowires with diameter of 100 nm oriented within host polymer matrices and densities (1 × 108 cm-2, 5 × 108 cm-2 and 12 × 108 cm-2) were studied. • The saturation field in the magnetic field applied along the wire axis exhibit a correlation with nanowires density, according to effective medium model. • The magnetization rotation model, occurs to be useful for the estimation the inhomogeneity in nanowires orientation. Arrays of iron nanowires (NWs) obtained by template-assisted electrodeposition constitute a promising composite material characterized by a combination of high magnetization in the filler and perpendicular magnetic anisotropy. The properties of these composites arise from the interplay between the behavior of individual NWs and their magnetostatic interactions. In this study, we investigated NW arrays with identical wire diameters but varying spatial arrangements. Major hysteresis loops were studied under various field directions relative to the NW axis. Key parameters such as the slope of the magnetization curve, saturation magnetization, and coercive force were quantified. Additionally, FORC (First Order Reversal Curve) measurements were conducted with the field oriented longitudinally with respect to the NW, offering insights into the inhomogeneity of the demagnetizing field influenced by the NW array's configuration. In the sample with the highest NW density, we observed isotropic behavior of the effective demagnetizing field, and we proposed an explanation for this phenomenon using the effective media approach. Micromagnetic simulations revealed that the magnetic behavior of individual NWs with a 100 nm diameter can be described as an interchange between volumes characterized by vortex and uniform magnetization patterns. Calculations of the demagnetizing field using the effective medium model demonstrated excellent agreement with experimental data across arrays featuring different NW densities. Remarkably, the quantitative consistency of coercive field values obtained from micromagnetic simulations and experimental measurements in the range of angles from 0° to 45° for the studied samples underscores the structural homogeneity of the obtained NWs. [ABSTRACT FROM AUTHOR]

We introduce a formulation of a two-sublattice micromagnetic model, which can describe the statics and dynamics of ferromagnets, antiferromagnets, and ferrimagnets. Such a formulation is based on the normalization of exchange, Dzyaloshinskii–Moriya interaction, and anisotropy parameters with respect to the square modulus of the saturation magnetization for each sublattice. We show a study of the statics and dynamics of domain walls (DWs) focusing on antiferromagnets and ferrimagnets because of their potential ultrafast performance in terms of dynamics and robustness against external magnetic perturbations. Our results show a direction to design DW-based nanodevices in ferrimagnets with a superlinear velocity increasing as a function of the spin–orbit torque amplitude by working near, but still below, the threshold current for the excitation of self-oscillations. [ABSTRACT FROM AUTHOR]

Based on the micromagnetic model which takes into account the random distribution of the uniaxial magnetic anisotropy directions in crystallites of a nanocrystalline film, an effective method has been implemented for calculation of the magnetization dynamics in microwave fields. For a certain range of crystallite sizes, when the energy of the random magnetic anisotropy is comparable to the exchange energy, a significant change of the ferromagnetic resonance field, broadening of the resonance line, and the appearance of an asymmetry in the shape of the resonance curve were found. With an increase of the crystallite sizes, the resonance field first grows, then, it quickly decreases to its minimum, and then, it grows again to reach saturation. In this case, the steepness of the left slope of the broadening resonance curve first decreases faster than that of the right slope, leading to the symmetry breaking of the resonance curve shape, then, the curve becomes symmetrical again, and then, the steepness of the left slope becomes greater than that of the right slope. [ABSTRACT FROM AUTHOR]

Cylindrical magnetic nanowires show great potential for 3D applications such as magnetic recording, shift registers, and logic gates, as well as in sensing architectures or biomedicine. Their cylindrical geometry leads to interesting properties of the local domain structure, leading to multifunctional responses to magnetic fields and electric currents, mechanical stresses, or thermal gradients. This review article is summarizing the work carried out in our group on the fabrication and magnetic characterization of cylindrical magnetic nanowires with modulated geometry and anisotropy. The nanowires are prepared by electrochemical methods allowing the fabrication of magnetic nanowires with precise control over geometry, morphology, and composition. Different routes to control the magnetization configuration and its dynamics through the geometry and magnetocrystalline anisotropy are presented. The diameter modulations change the typical single domain state present in cubic nanowires, providing the possibility to confine or pin circular domains or domain walls in each segment. The control and stabilization of domains and domain walls in cylindrical wires have been achieved in multisegmented structures by alternating magnetic segments of different magnetic properties (producing alternative anisotropy) or with non-magnetic layers. The results point out the relevance of the geometry and magnetocrystalline anisotropy to promote the occurrence of stable magnetochiral structures and provide further information for the design of cylindrical nanowires for multiple applications.

Abstract: 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. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of thin M-type barium hexaferrite (BaFe 12 O 19 or BaM) films on silicon are reported. Multilayer in situ technique was employed to anneal the films at 850–900 °C for 10 min. The thickness dependence of the magnetic properties of the BaM films has been investigated using VSM. For the BaM 150 nm thickness film, acicular BaM grains were present having their c-axis randomly oriented. For the BaM films thicker than 150 nm, lattice relaxation favors the c-axis to be aligned in the film plane. The micromagnetic simulation was used to model the out-of-plane and the in-plane hysteresis loops. We have achieved good matching between the experimental data and the model. Using the micromagnetic model, we have estimated the deflection angle of c-axis from the normal plane θ = 25° for the 150 nm thick film. [ABSTRACT FROM AUTHOR]