Magnetization Reversal Research Articles

Magnetization reversal of the longitudinally magnetized film with cubic anisotropy by magnetic pulses

Magnetization reversal of the longitudinally magnetized film with cubic anisotropy by magnetic pulses

Magnetization Reversal of a Longitudinally Magnetized Film with Cubic Anisotropy by Magnetic Pulses

Magnetization Reversal of a Longitudinally Magnetized Film with Cubic Anisotropy by Magnetic Pulses

Evidence for surface spin structures from first order reversal curves in Co[formula omitted]Sn[formula omitted]S[formula omitted] and Fe[formula omitted]GeTe[formula omitted] magnetic topological semimetals

We study magnetization reversal and first order reversal curves for two different magnetic topological semimetals, Co3Sn2S2 and Fe3GeTe2, in a wide temperature range. For the magnetization reversal, we observe strong temperature dependence of the initial (low-temperature) step-like magnetization switchings, so the inverted hysteresis appears at high temperatures. Usually, the inverted hysteresis is a fingerprint of material with two independent magnetic phases, the inversion reflects the phase interaction. First order reversal curve analysis confirms the two-phase behavior even at the lowest temperatures of the experiment. While the bulk ferromagnetic magnetization shows strong temperature dependence, one of the observed phases demonstrates perfect stability below the Curie temperature. The obtained hysteresis loops are of the bow-tie type, which is usually ascribed to appearance of the skyrmionic phase. The described two-phase behavior is mostly identical for Co3Sn2S2 and Fe3GeTe2 magnetic topological semimetals, only the characteristic temperatures differ for these materials. The specifics of our experiment is the excellent temperature stability of the second phase, while the skyrmions are usually observed near the Curie temperature. On the other hand, temperature stability can be expected for surface-state induced spin textures due to the topological protection of surface states in topological semimetals. This also explains the universal behavior of the second phase for two different topological semimetals Co3Sn2S2 and FGT. Both these materials have strongly different bulk properties, the only similarity is the presence of the topological surface states. Thus, we can ascribe the second, temperature-stable magnetic phase to the surface states in topological semimetals.

Analysis of the magnetization reversal mechanism of heterogenous grain boundary diffused Nd-Fe-B sintered magnets

The grain boundary diffusion (GBD) method could effectively improve the coercivity of Nd-Fe-B sintered magnets, but the uneven distribution of heave rare earth (HRE) elements would lead to the decrease of squareness factor of the demagnetization curve of the magnets, which limited the application of this method. In this paper, three typical-structure regions were summarized based on microstructure characterization, and the magnetic properties and magnetization reversal mechanism were analyzed. The differences in nucleation positions and magnetization reversal processes of the three regions were discussed, and it found that the remarkable differences among them resulted in the low squareness factor of GBD Nd-Fe-B sintered magnets. In addition, the efficiency of Tb elements could be further improved to enhance the coercivity of Nd-Fe-B sintered magnets by forming homogeneous and complete core-shell structure grains, which could strengthen the nucleation field and even transfer the nucleation position from the grain surface defect layer to the grain interior region. Our results could provide guidance and reference for further improving the permanent magnetic properties of Nd-Fe-B sintered magnets by optimized GBD method.

Magnetic Behavior of the Arrays of Iron Cylindrical Nanostructures: Atomistic Spin Model Simulations

Cylindrical ferromagnetic nanowires are of particular interest in nanomaterials science due to various manufacturing methods and a wide range of applications in nanotechnology, with special attention given to those with diameters less than the single domain limit. In the current study, the simulations of magnetic properties of isolated iron nanowires with a diameter of 5 nm and various aspect ratios, as well as two types of arrays of such nanowires (with hexagonal and square arrangement), were performed using atomistic spin model. In the case of a single nanowire, change of coercive field for different applied field directions with aspect ratio was discussed. It was shown that the evolution of the magnetization reversal mechanism from coherent rotation to domain wall propagation appears with increasing length of single nanowire. For the arrays of cylindrical nanostructures, it was revealed that different number of nearest neighbors for each nanostructure in square and hexagonal arrays have an influence on their magnetostatic interactions, which are the most significant for shortest interwire distances. The corresponding spin configurations during the remagnetization process showed the appearance of intermediate magnetization states (when a part of wires is magnetized parallel and part antiparallel to the field direction), connected with Barkhausen effect, which influence the observed hysteresis curves.

Evolution of switching fields caused by reorientation of GdFeCo/Ir/GdFeCo synthetic ferrimagnet in magnetic field

Anomalous Hall effect (AHE) in GdFeCo/Ir/GdFeCo multilayered structures attracts great interest because all optical switching, spin-torque, and other effects promise effective application for ultrafast memory element creation. Since AHE is controlled by GdFeCo magnetization, domain dynamics has importance for practical applications. In our work, magnetization reversal in perpendicular GdFeCo/Ir/GdFeCo synthetic ferrimagnets is characterized by AHE measurements. The AHE hysteresis loop obtained with the field applied perpendicular to the sample plane is composed of three sub-loops, and two of them are symmetrically biased with respect to the third one. Switching magnetic fields for two of the three transitions are found to be dependent on magnetic history. In particular, exposure of the sample in the in-plane field leads to reduction of the out-of-plane switching fields in side sub-loops. A multiple series of perpendicular hysteresis loops recorded after exposure under high in-plane field reveals gradual (within 30 min) relaxation of the out-of-plane switching fields to their initial values observed in a non-magnetized sample. Domain wall mobility, limiting switching of the bilayer devices, is complicated due to the coupling between partial domains in each single layer. Unusual dynamics of double domain walls results in unexpected new phenomena affecting electrical processes in bilayer structures.

Controllable unidirectional magnetoresistance in ferromagnetic films with broken symmetry

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Paleomagnetism of the Ediacaran Avellaneda Formation (Argentina), Part II: Magnetic and chemical stratigraphy constraints on the onset of the Shuram carbon excursion

Magnetostratigraphy is a powerful technique for high-resolution stratigraphic correlation and accurate dating of the polarity history of the magnetic field. Up to now, magnetostratigraphic studies have been underutilized in Ediacaran successions. Here, we present a magnetostratigraphy coupled with a carbon isotope study in Ediacaran Avellaneda Formation (∼570 Ma), a sedimentary cover sequence in the Tandilia orogenic belt, of the Río de la Plata craton. Paleomagnetic results and δ13C profiles were obtained from three drill cores that cut across the sedimentary record of the Avellaneda Formation. Paleomagnetic results plotted stratigraphically allowed the identification of a prevalent reverse polarity separated by normal polarity interval. Our findings show a coincident correlative polarity transition at each drill core analyzed. δ13C values demonstrate that the magnetostratigraphic polarity pattern is laterally consistent. Furthermore, C-isotope curves show a transition from positive to negative values near the top of the Avellaneda Formation. This excursion is likely correlative with the onset of the Shuram excursion (ca. 570 Ma). Our findings suggest that the onset of Shuram excursion took place during a reverse polarity chron. Comparison with other coeval sequences validate this correlation. Geomagnetic polarity reversals pattern in the Avellaneda Formation and its correlative pattern, validates a magnetostratigraphy as a potential method to correlate Precambrian strata. Our new findings shed further light on the possible use of magnetic polarity reversals to correlate Ediacaran successions.

Determination of Magnetic Structures in Magnetic Microwires with Longitudinally Distributed Magnetic Anisotropy.

We studied the magnetic properties of a glass-covered amorphous microwire that was stress-annealed at temperatures distributed along the microwire length. The Sixtus-Tonks, Kerr effect microscopy and magnetic impedance techniques have been applied. There was a transformation of the magnetic structure across the zones subjected to annealing at different temperatures. The annealing temperature distribution induces the graded magnetic anisotropy in the studied sample. The variety of the surface domain structures depending on the longitudinal location has been discovered. Spiral, circular, curved, elliptic and longitudinal domain structures coexist and replace each other in the process of magnetization reversal. The analysis of the obtained results was carried out based on the calculations of the magnetic structure, assuming the distribution of internal stresses.

Efficient magnetization reversal by self-generated spin–orbit torque in magnetic bulk Rashba materials

In this paper, we demonstrate that V0.027Bi0.973TeI, a material with both giant bulk Rashba effect and ferromagnetism, can reverse its magnetization by self-generated spin–orbit torque. Through first-principles calculation, it is found that the giant bulk Rashba effect arises from both bulk space inversion asymmetry and strong spin–orbital coupling, while the ferromagnetism originates from the itinerant d-electrons of doped element vanadium. More importantly, its field-like spin–orbit torque efficiency is determined to be as high as 4.53 × 10−4 mT/(A cm−2), which is more than two orders of magnitude higher than that typically observed in magnetic heterostructures. It is further shown that by using such magnetic bulk Rashba material to form a homogenous spintronic device, the power consumption for magnetization switching can be significantly reduced.

Optically induced ultrafast magnetization switching in ferromagnetic spin valves.

The discovery of spin-transfer torque (STT) enabled the control of the magnetization direction in magnetic devices in nanoseconds using an electrical current. Ultrashort optical pulses have also been used to manipulate the magnetization of ferrimagnets at picosecond timescales by bringing the system out of equilibrium. So far, these methods of magnetization manipulation have mostly been developed independently within the fields of spintronics and ultrafast magnetism. Here we show optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free archetypal spin valves of [Pt/Co]/Cu/[Co/Pt] commonly used for current-induced STT switching. We find that the magnetization of the free layer can be switched from a parallel to an antiparallel alignment, as in STT, indicating the presence of an unexpected, intense and ultrafast source of opposite angular momentum in our structures. Our findings provide a route to ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism.

First-Order Reversal Curves of Sets of Bistable Magnetostrictive Microwires.

Amorphous microwires have attracted substantial attention in the past decade because of their useful technological applications. Their bistable magnetic response is determined by positive or negative magnetostriction, respectively. First-order reversal curves (FORC) are a powerful tool for analyzing the magnetization reversal processes of many-body ferromagnetic systems that are essential for a deeper understanding of those applications. After theoretical considerations about magnetostatic interactions among microwires, this work introduces a systematic experimental study and analysis of the FORC diagrams for magnetostrictive microwires exhibiting an individually bistable hysteresis loop, from a single microwire to sets of an increasing number of coupled microwires, the latter considered as an intermediate case to the standard many-body problem. We performed the study for sets of quasi-identical and different hysteretic microwires where we obtained the coercivity Hc and interaction Hu fields. In the cases with relevant magnetostatic interactions, FORC analysis supplies deeper information than standard hysteresis loops since the intrinsic fluctuations of the switching field generate a complex response. For sets of microwires with very different coercivity, the coercivity distributions of the individual microwires characterize the FORC diagram.

Structural, magnetic and Magnetic-Microstructural properties of sputtered FeCoNi thin films

This paper reports the effect of growth rate and substrate temperature on the composition, structure, microstructure, magnetic and magnetic microstructural properties of Fe-Co-Ni alloy thin films. 200 nm thick Fe-Co-Ni thin films were grown by high power impulse magnetron sputtering on Si 〈100〉 at different growth rates (viz., 1 – 2.4 Å/s). Films were also thermally processed by modifying the substrate temperatures. Room temperature deposited films showed the presence of FCC structure for all the films. However, films grown at higher substrate temperatures displayed presence of BCC phase co-existing with FCC phase. High saturation magnetization combined with low coercivity has been obtained for the Fe-Co-Ni films grown at higher growth rate and enhanced substrate temperature. Magnetization and magnetic microstructural studies using Kerr microscopy showed anisotropic magnetic properties along the plane of the film with the magnetization reversal dominated by domain wall motion and domain wall rotation.

Micromagnetic Study of the Dependence of Output Voltages and Magnetization Behaviors on Damping Constant, Frequency, and Wire Length for a Gigahertz Spin Rotation Sensor.

In this report, we studied the dependence of output voltage on the damping constant, the frequency of the pulse current, and the wire length of zero-magnetostriction CoFeBSi wires using multiphysics simulation considering eddy currents in micromagnetic simulations. The magnetization reversal mechanism in the wires was also investigated. As a result, we found that a high output voltage can be achieved with a damping constant of ≥0.03. We also found that the output voltage increases up to a pulse current of 3 GHz. The longer the wire length, the lower the external magnetic field at which the output voltage peaks. This is because the demagnetization field from the axial ends of the wire is weaker as the wire length is longer.

Space environment and magnetospheric Poynting fluxes of the exoplanet τ Boötis b

Context. The first tentative detection of a magnetic field on the hot-Jupiter-type exoplanet τ Boötis b was recently reported by Turner et al. (A&A, 645, A59). The magnetic field was inferred from observations of circularly polarized radio emission obtained with the LOFAR telescopes. The observed radio emission is possibly a consequence of the interaction of the surrounding stellar wind with the planet's magnetic field. Aims. We aim to better understand the near space environment of τ Boötis b and to shed light on the structure and energetics of its near-field interaction with the stellar wind. We are particularly interested in understanding the magnetospheric energy fluxes powered by the star-planet interaction and in localizing the source region of possible auroral radio emission. Methods. We performed magnetohydrodynamic simulations of the space environment around τ Boötis b and its interaction with the stellar wind using the PLUTO code. We investigated the magnetospheric energy fluxes and effects of different magnetic field orientations in order to understand the physical processes that cause the energy fluxes that may lead to the observed radio emission given the magnetic field strength proposed in Turner et al. (A&A, 645, A59). Furthermore, we study the effect of various stellar wind properties, such as density and pressure, on magnetospheric energy fluxes given the uncertainty of extrasolar stellar wind predictions. Results. We find in our simulations that the interaction is most likely super-Alfvénic and that energy fluxes generated by the stellar wind-planet interaction are consistent with the observed radio powers. Magnetospheric Poynting fluxes are on the order of 1–8 × 1018 W for hypothetical open, semi-open, and closed magnetospheres. These Poynting fluxes are energetically consistent with the radio powers in Turner et al. (A&A, 645, A59) for a magnetospheric Poynting flux-to-radio efficiency >10−3 when the magnetic fields of the planet and star are aligned. In the case of lower efficiency factors, the magnetospheric radio emission scenario is, according to the parameter space modeled in this study, not powerful enough. A sub-Alfvénic interaction with decreased stellar wind density could channel Poynting fluxes on the order of 1018W toward the star. In the case of a magnetic polarity reversal of the host star from an aligned to anti-aligned field configuration, the expected radio powers in the magnetospheric emission scenario fall below the observable threshold. Furthermore, we constrain the possible structure of the auroral oval to a narrow band near the open-closed field line boundary. The strongest emission is likely to originate from the night side of the planet. More generally, we find that stellar wind variability in terms of density and pressure does significantly influence magnetospheric energy fluxes for close-in magnetized exoplanets.

Pinning of Vortices during the Passage of Turbulent Magnetization Reversal Waves in Antidot Films with Through and Non-Through Holes

Pinning of Vortices during the Passage of Turbulent Magnetization Reversal Waves in Antidot Films with Through and Non-Through Holes

Simulation of Rolling in a Magnetostriction Fe–Ga Alloy

Based on the measured physical properties of the Fe–Ga alloy, it was found that a decrease in the material thickness to 0.3 mm leads to an acceptable level of eddy current losses at high-frequency magnetization reversal (20 kHz). Recommendations have been developed on the choice of rolling modes to achieve the required thickness. The modeling of rolling according to the proposed mode is implemented, the distributions of deformation, stresses, and temperature for each pass were obtained. The results obtained are analyzedin the context of the relation between the features of deformation and the processes of crystallographic textureformation.

Thickness dependent microstructural and magnetic studies of iron embedded PVA nanocomposite films

This paper presents the structural and magnetic properties of polyvinyl alcohol/iron (PVA/Fe) nanocomposite films. Iron (Fe) films of different thicknesses were deposited on a PVA substrate using ion beam sputtering. The Fe nanoparticle layer thickness ranges from 3 to 50 nm on the PVA substrate. Grazing incidence x-ray diffraction has been carried out to study the structural behavior of the prepared films. An atomic force microscope records the indicated remarkable change in the roughness of the nanocomposite film as a result of initiation of vertical growth of sputtered Fe nanoparticles.Magneto-optic Kerr effect (MOKE) magnetometry and MOKE spectroscopy have been employed to evaluate the magnetic property and simultaneously study the real time growth of the magnetic domain structure for both 0° and 90° azimuthal angles. MOKE microscopic images indicated the magnetization reversal commensurate with the hysteresis loop of the nanocomposites. All the films show soft ferromagnetic behavior. Gradual development of the domain structure is observed in the MOKE micrograph with the increase in the thickness of Fe nanoparticle deposition. Ex situ magnetic force micrographs of the magnetic domain structures supported the observation of MOKE microscopic studies, which indicated switching of in-plane magnetization to out-of-plane magnetization near an Fe nanoparticle deposition thickness of 40 nm. The PVA matrix appears to be an effective material to support the growth of magnetic properties in the PVA/Fe nanocomposite system. The evolution of Fe nanostructures on PVA and the resulting magnetic behavior have been discussed.

Disentangling the magneto-optic Kerr effect of manganite epitaxial heterostructures

The magneto-optic Kerr effect can probe the process of magnetization reversal in ferromagnetic thin films and can, thus, be used as an alternative to magnetometry. The Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign, vanishing at particular wavelengths. We investigate the epitaxial heterostructures of ferromagnetic manganite, La0.7Sr0.3Mn0.9Ru0.1O3, by using the polar Kerr effect and magnetometry. The manganite layers are separated by or interfaced with a layer of nickelate, NdNiO3. The Kerr rotation hysteresis loops of trilayers, with two manganite layers of different thicknesses separated by a nickelate layer, have intriguing humplike features when measured with light of 400 nm wavelength. By investigating additional reference samples, we disentangle the contributions of the individual layers to the loops: we show that the humps originate from the opposite sense of the Kerr rotation of the two different ferromagnetic layers, combined with the additive behavior of the Kerr signal. The change of sign of the Kerr rotation for the thinner manganite layer is most likely caused by optical interference.

The Effect of Milling with Combined Surfactants
 on the Magnetic Properties and Microstructure
 of Submicron Sm2Fe17N3 Powders

The results of the application of a combination of several surfactants in preparing submicron hard magnetic Sm2Fe17N3 powders by milling in a centrifugal mill are reported. Along with methyl caproate, to protect the powder against oxidation, the efficiency of siloxane has been studied. The application of the combined surfactants allowed us to increase both the (BH)max of the Sm2Fe17N3 powder to 23.6 MG Oe and its corrosion resistance. The effect of milling kinetics on the angular dependence of the coercive force Hc has been determined. Information on the primary mechanism of magnetization reversal of the powders has beenobtained.