Magnetic Dipole Interaction Research Articles

Controlling the creation/annihilation and distribution of magnetic skyrmions by manipulating an externally applied voltage

Magnetic skyrmions are currently gaining attention owing to their potential to act as information carriers in spintronic devices. However, conventional techniques which rely on modulating the electric current to write or manipulate information using skyrmions are not energy efficient. Therefore, in this study, a Ta/Co–Fe–B/Ta/MgO junction that hosts a skyrmion was utilized to fabricate a device to investigate the effect of applying a voltage in the direction perpendicular to the film plane. Magneto-optical Kerr effect microscopy was performed in a polar configuration to observe the difference in the perpendicular magnetic anisotropy by observing the behavior of the magnetic domain structure and the skyrmions. Our findings suggest that voltage-induced magnetic domain structure modulation and the creation/annihilation of skyrmions are both possible. Furthermore, manipulation of skyrmions was realized by utilizing repulsive magnetic dipole interaction between the voltage-created skyrmion and skyrmion, exhibiting Brownian motion, outside the top electrode. Thus, our proposed method can enable controlling the creation and annihilation of skyrmions and their positions by manipulating the externally applied voltage. These findings can advance unconventional computing fields, such as stochastic and ultra-low-power computing.

Superparamagnetic hyperthermia and cytotoxicity properties of bimagnetic core–shell nanoparticles synthesized by solvothermal reflux method

Magnetic properties of spinel ferrites nanoparticles (NPs) were well studied for various applications, including biomedical applications such as magnetic hyperthermia agents, magnetic resonance imaging (MRI) contrast agents, targeted drug delivery agents, etc., Magnetic properties of these nanoparticles were altered by metal cations substitution as well as variation of size and shape of nanoparticles. Here hard–soft magnetic materials based magnetic exchange coupled bimagnetic CoFe2O4-CaFe2O4, and CaFe2O4-CoFe2O4 core–shell nanoparticles (CSNPs) with narrow size distribution were synthesized to alter their magnetic properties. Structural, morphology, magnetic, superparamagnetic hyperthermia (SPHT), and cytotoxicity properties of these NPs were reported. Unimagnetic phase CoFe2O4, CaFe2O4 nanoparticles were also studied for comparison. Though CaFe2O4 macrocrystals crystallizes in orthorhombic structure, CaFe2O4 nanocrystals crystallizes in cubic spinel structure whereas both macro- and nanoparticles of CoFe2O4 crystallizes in spinel structure. Scanning electron nanographs (electrongraphs) of all samples show narrow size distributed spherical particles of average particle size of around 10 nm and is confirmed by transmission electron nanographs. Room temperature magnetic hysteresis loop of all samples were shown superparamagnetic nature with very low coercive field. Bimagnetic CSNPs show higher specific heat generation rate (SHR) or specific magnetic field energy absorption rate (SAR) of superparamagnetic nanoparticles (474 W/g for CaFe2O4-CoFe2O4 CSNPs under H = 35.5 kAm−1 and f = 316 kHz field parameters ) compared to counterpart unimagnetic nanoparticles (407 W/g for CaFe2O4 NPs). However, the SAR decreases with increase of magnetic nanoparticles (MNPs) concentration in the dispersion medium due to reduced superparamagnetic relaxation time resulting from magnetic dipole interactions among dispersed nanoparticles. This is also confirmed in SPHT properties of dispersant removed CSNPs. Cytotoxicity studies show that cell viability increases with MNPs concentration due to increased colorimetric interference of iron oxide and inhomogeneous distribution of MNPs, due to magnetic interactions, among cell culture wells, at higher concentration. These CSNPs may be used for localized SPHT in cancer therapy.

Nuclear Magnetic Resonance Dipolar Cross-Relaxation Interaction between Nanoconfined Fluids and Matrix Solids.

Many different methods have been developed to investigate fluid-solid interactions in nanoporous systems. These methods either only work in the liquid phase or provide an indirect measurement by probing the fluid-solid interaction based on a measured property change of the fluid or solid under different sample conditions. Here, we report a direct measurement technique using NMR dipolar cross-relaxation between the nanoconfined fluids and the matrix solids. The method was tested using a methyl-functionalized mesostructured silica saturated with methanol as a model sample. A formal theory was established to describe the enhanced dipolar cross-relaxation interaction between the nanoconfined fluids and the matrix solids. Both the experiment and theory showed that nanoconfinement of the fluids enhances the dipolar cross-relaxation interaction between the fluid and the matrix solids, which can be applied to investigate the fluid-solid interaction for various materials of a similar nanostructure.

Emergence of zero-field non-synthetic single and interchained antiferromagnetic skyrmions in thin films

Antiferromagnetic (AFM) skyrmions are envisioned as ideal localized topological magnetic bits in future information technologies. In contrast to ferromagnetic (FM) skyrmions, they are immune to the skyrmion Hall effect, might offer potential terahertz dynamics while being insensitive to external magnetic fields and dipolar interactions. Although observed in synthetic AFM structures and as complex meronic textures in intrinsic AFM bulk materials, their realization in non-synthetic AFM films, of crucial importance in racetrack concepts, has been elusive. Here, we unveil their presence in a row-wise AFM Cr film deposited on PdFe bilayer grown on fcc Ir(111) surface. Using first principles, we demonstrate the emergence of single and strikingly interpenetrating chains of AFM skyrmions, which can co-exist with the rich inhomogeneous exchange field, including that of FM skyrmions, hosted by PdFe. Besides the identification of an ideal platform of materials for intrinsic AFM skyrmions, we anticipate the uncovered knotted solitons to be promising building blocks in AFM spintronics.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

PH-dependent phase transitions in ferrofluids: A Monte Carlo simulation study using an extended DLVO model

The present survey focuses on the pH-dependent phase behavior of acidic ferrofluids using Monte Carlo simulations. Samples based on CoFe2O4@γ-Fe2O3 core@shell nanoparticles with five different mean sizes were investigated. The interparticle interactions were modeled employing an extended DLVO theory, which includes the anisotropic magnetic dipolar interaction. The electrostatic repulsive component considers the dependence of the surface charge density with pH and nanoparticle mean size. The simulations highlight the effect of pH on the phase behavior of the samples, where the pH increase towards neutral induces an increase in the number of agglomerates, while also reducing first-neighbor distances, especially for samples based on larger nanoparticles. The results obtained in these simulations agree with experimental observations carried out on acidic ferrofluid samples and allow a reliable characterization of the stability domains of these systems. Ultimately, the general pH-dependent phase behavior is confirmed through a detailed analysis of the results of the simulated samples.

Controllable Topological Magnetic Transformations in the Thickness-Tunable van der Waals Ferromagnet Fe5GeTe2.

Recent observations of topological meron textures in two-dimensional (2D) van der Waals (vdW) magnetic materials have attracted considerable research interest for both fundamental physics and spintronic applications. However, manipulating the meron textures and realizing the topological transformations, which allow for exploring emergent electromagnetic behaviors, remain largely unexplored in 2D magnets. In this work, utilizing real-space imaging and micromagnetic simulations, we reveal temperature- and thickness-dependent topological magnetic transformations among domain walls, meron textures, and stripe domain in Fe5GeTe2 (FGT) lamellae. The key mechanism of the magnetic transformations can be attributed to the temperature-induced change of exchange stiffness constant within layers and uniaxial magnetic anisotropy, while the magnetic dipole interaction as governed by sample thickness is crucial to affect the critical transformation temperature and stripe period. Our findings provide reliable insights into the origin and manipulation of topological spin textures in 2D vdW ferromagnets.

Magnetorheological liposomes. An alternative approach for drug delivery driven by mutual magnetic dipole–dipole interaction

In this study consideration is given to an alternative approach for drug release by the use magnetorheological liposomes with the aim to eliminate the need for an external oscillatory magnetic field (hyperthermia) for rupture of the liposome. Contrariwise traditional magnetic liposomes – using nano magnetic particles or ferrofluids, here the magnetic liposome contains a magnetorheological particle (i.e., of micrometric size) which makes the dipole–dipole magnetic interaction between particles strong enough to break up the bilipid layer and then releasing the drug. Utilizing a simplified physical model an expression for the liposome rupture by dipole–dipole magnetic interaction as a function of the drug and magnetorheological particle fraction was derived. It is shown that rupture of the bilipid layer by dipole–dipole magnetic interaction could be feasible when the magnetorheological particle fraction inside the liposome is around a 20 per cent or higher.

Hyperfine structure studies of neutral praseodymium with Fourier transform spectroscopy

The hyperfine structure patterns of some neutral praseodymium (Pr I) lines in the range of 2969 − 34998 cm−1 were investigated based on the archival data from the Fourier transform spectrometer at the US National Solar Observatory. The hyperfine magnetic dipole interaction constants A are presented by fitting the spectra with our spectral analysis program. The HFS constants A for 56 levels of Pr I from 0.0 to 27602.42 cm−1 were determined.

Unveiling the anisotropic fractal magnetic domain structure in bulk crystals of antiskyrmion host (Fe,Ni,Pd)3P by small-angle neutron scattering

Intermetallic Pd-doped (Fe,Ni)3P, which crystallizes in a non-centrosymmetric tetragonal structure with S 4 symmetry, has recently been discovered to host magnetic antiskyrmions, antivortex-like topological spin textures. In this material, uniaxial magnetic anisotropy and dipolar interactions play a significant role, giving rise to finely branched magnetic domain patterns near the surface of bulk crystals, as revealed by a previous magnetic force microscopy (MFM) measurement. However, small-angle neutron scattering (SANS) is a more suitable method for characterizing bulk properties and fractal structures on the mesoscopic length scale. In this study, using SANS and MFM, the magnetic domain structure in bulk single crystals of (Fe0.63Ni0.30Pd0.07)3P is quantitatively investigated. The SANS results demonstrate that the magnetic domain structure exhibits anisotropic fractal character on length scales down to the width of the magnetic domain walls. The fractal features are gradually lost in magnetic fields, and different field dependencies are observed at 300 and 2 K due to a temperature-dependent anisotropy. This study quantifies the fractality of the highly anisotropic magnetic domain structures in an antiskyrmion material, and highlights the versatility of SANS for the study of fractal structures in magnetic systems.

Hybrid Bloch-Néel spiral states in Mn1.4PtSn probed by resonant soft x-ray scattering

Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype $D_{2d}$ compound Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic spin textures such as first-reported anti-skyrmions or elliptic Bloch-type skyrmions. While less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via resonant small-angle soft x-ray scattering experiments on high-quality single crystals of Mn$_{1.4}$PtSn at low temperatures, we evidence how the underlying $D_{2d}$ symmetry of the DMI in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the N\'{e}el-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period. These results broaden the reachness of Heusler $D_{2d}$ materials exotic magnetic phase diagram and extend its tunability, thus enhancing a relevant playground for further fundamental explorations and potential applications in energy saving technologies.

Magnetic Interactions of the Centrosymmetric Skyrmion Material Gd_{2}PdSi_{3}.

The experimental realization of magnetic skyrmion crystals in centrosymmetric materials has been driven by theoretical understanding of how a delicate balance of anisotropy and frustration can stabilize topological spin structures in applied magnetic fields. Recently, the centrosymmetric material Gd_{2}PdSi_{3} was shown to host a field-induced skyrmion crystal, but the skyrmion stabilization mechanism remains unclear. Here, we employ neutron-scattering measurements on an isotopically enriched polycrystalline Gd_{2}PdSi_{3} sample to quantify the interactions that drive skyrmion formation. Our analysis reveals spatially extended interactions in triangular planes, and large ferromagnetic interplanar magnetic interactions that are modulated by the Pd/Si superstructure. The skyrmion crystal emerges from a zero-field helical magnetic order with magnetic moments perpendicular to the magnetic propagation vector, indicating that the magnetic dipolar interaction plays a significant role. Our experimental results establish an interaction space that can promote skyrmion formation, facilitating identification and design of centrosymmetric skyrmion materials.

Magnetic field effects in an octupolar quantum spin liquid candidate

Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in ${\mathrm{Ce}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ pyrochlore, and its absence in ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$. Therefore, our results identify ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ as a strong candidate for an octupolar $U(1)$ QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.

Magnon squeezing enhanced entanglement in a cavity magnomechanical system

We investigate the generation of entanglement in a cavity magnomechanical system, which consists of magnons and cavity microwave photons and phonons; the magnon–photon and magnon–phonon couplings are achieved by the magnetic dipole interaction and the magnetostrictive interaction, respectively. By introducing magnon squeezing induced by magnon self-Kerr nonlinearity, the magnon–photon, magnon–phonon, and photon–phonon entanglements are enhanced compared with the case without inserting magnon squeezing. We find that an optimal parameter of the squeezing exists, which yields the maximum entanglement. This study provides a new idea for exploring the properties of quantum entanglement in cavity magnomechanical systems, and may have some potential applications in quantum state engineering.

Magnetoresponsive Functionalized Nanocomposite Aggregation Kinetics and Chain Formation at the Targeted Site during Magnetic Targeting

Drug therapy for vascular disease has been promoted to inhibit angiogenesis in atherosclerotic plaques and prevent restenosis following surgical intervention. This paper investigates the arterial depositions and distribution of PEG-functionalized magnetic nanocomposite clusters (PEG_MNCs) following local delivery in a stented artery model in a uniform magnetic field produced by a regionally positioned external permanent magnet; also, the PEG_MNCs aggregation or chain formation in and around the implanted stent. The central concept is to employ one external permanent magnet system, which produces enough magnetic field to magnetize and guide the magnetic nanoclusters in the stented artery region. At room temperature (25 °C), optical microscopy of the suspension model’s aggregation process was carried out in the external magnetic field. According to the optical microscopy pictures, the PEG_MNC particles form long linear aggregates due to dipolar magnetic interactions when there is an external magnetic field. During magnetic particle targeting, 20 mL of the model suspensions are injected (at a constant flow rate of 39.6 mL/min for the period of 30 s) by the syringe pump in the mean flow (flow velocity is Um = 0.25 m/s, corresponding to the Reynolds number of Re = 232) into the stented artery model. The PEG_MNC clusters are attracted by the magnetic forces (generated by the permanent external magnet) and captured around the stent struts and the bottom artery wall before and inside the implanted stent. The colloidal interaction among the MNC clusters was investigated by calculating the electrostatic repulsion, van der Waals and magnetic dipole-dipole energies. The current work offers essential details about PEG_MNCs aggregation and chain structure development in the presence of an external magnetic field and the process underlying this structure formation.

Magnetic Alignment for Plasmonic Control of Gold Nanorods Coated with Iron Oxide Nanoparticles.

Plasmonic nanoparticles that can be manipulated with magnetic fields are of interest for advanced optical applications, diagnostics, imaging, and therapy. Alignment of gold nanorods yields strong polarization-dependent extinction, and use of magnetic fields is appealing because they act through space and can be quickly switched. In this work, cationic polyethyleneimine-functionalized superparamagnetic Fe3 O4 nanoparticles (NPs) are deposited on the surface of anionic gold nanorods coated with bovine serum albumin. The magnetic gold nanorods (MagGNRs) obtained through mixing maintain the distinct optical properties of plasmonic gold nanorods that are minimally perturbed by the magnetic overcoating. Magnetic alignment of the MagGNRs arising from magnetic dipolar interactions on the anisotropic gold nanorod core is comprehensively characterized, including structural characterization and enhancement (suppression) of the longitudinal surface plasmon resonance and suppression (enhancement) of the transverse surface plasmon resonance for light polarized parallel (orthogonal) to the magnetic field. The MagGNRs can also be driven in rotating magnetic fields to rotate at frequencies of at least 17Hz. For suitably large gold nanorods (148nm long) and Fe3 O4 NPs (13.4nm diameter), significant alignment is possible even in modest (<500Oe) magnetic fields. An analytical model provides a unified understanding of the magnetic alignment of MagGNRs.

Enantiodetection of Chiral Molecules via Two-Dimensional Spectroscopy.

Enantiodetection of chiral molecules is important to chemical reaction control and biological function designs. Traditional optical methods of enantiodetection rely on the weak magnetic-dipole or electric-quadrupole interactions, and in turn suffer from the weak signal and low sensitivity. We propose a new optical enantiodetection method to determine the enantiomeric excess via two-dimensional (2D) spectroscopy of the chiral mixture driven by three electromagnetic fields. The quantities of left- and right-handed chiral molecules are reflected by the intensities of different peaks on the 2D spectrum, separated by the chirality-dependent frequency shifts resulting from the relative strong electric-dipole interactions between the chiral molecules and the driving fields. Thus, the enantiomeric excess can be determined via the intensity ratio of the peaks for the two enantiomers.

Magnetic Skyrmionic Bubbles at Room Temperature and Sign Reversal of the Topological Hall Effect in a Layered Ferromagnet Cr0.87Te.

The search for materials that exhibit topologically protected spin configurations, such as magnetic skyrmions, continues to be fueled by the promise of outstanding candidate components for spin-based applications. In this study, in situ Lorentz transmission electron microscopy directly images Bloch-type magnetic skyrmionic bubbles in a layered ferromagnet Cr0.87Te single crystal. Owing to the competition between a magnetic dipole interaction and uniaxial easy axis anisotropy, nanoscale magnetic bubbles with random chirality can be observed in a wide temperature range covering room temperature when the external magnetic field is applied along the out-of-plane direction. Moreover, high-density and stable skyrmionic bubbles are successfully realized at zero magnetic field by appropriate field-cooling manipulation. Additionally, a sign reversal of the Hall effect and the derived topological Hall effect is observed and discussed. As quasi-two-dimensional materials, the binary chromium tellurides hosting magnetic skyrmions could have many applications in low-dimensional skyrmion-based spintronic devices in an ambient atmosphere.

Magnon control of light transmission in a  -symmetric-like cavity magnomechanical system

We theoretically study magnon control of light transmission in a cavity magnomechanical system, which consists of an active magnon mode and a passive microwave cavity mode. Phonon-magnon coupling and cavity photon-magnon coupling can be achieved via magnetostrictive interaction and magnetic dipole interaction respectively in this system. By driving a small ferromagnetic sphere which connected by an optical fiber to the inner wall of a microwave cavity, the effective magnon-gain can be obtained. We find that the transparent, absorptive and amplificatory windows can appear by adjusting the gain of the magnon mode, and the introduction of gain can realize significant amplification of transmitted probe power, especially at the EP point. In comparison with traditional optomechanically induced amplification, the amplification can be controlled not only by adjusting the driving field and magnon-microwave coupling, but also by adjusting the external magnetic field. Our results lays a theoretical foundation for the study of the magnon control in magnomechanical systems.

Field-free spin-orbit torque-induced perpendicularmagnetization switching in YIG/Ta/CoTb/Pt

<p indent="0mm">Spin-orbit torques (SOTs) provide a physical basis for designing new spintronic memory and logic devices. The key to the application is the realization of field-free SOT-driven magnetization switching. In this study, Ta/CoTb/Pt ferromagnetic multilayers were grown on yttrium iron garnet (YIG) magnetic insulating film. We achieved current-induced CoTb magnetization switching under zero magnetic field by using the magnetic dipole interaction field generated by the YIG magnetic insulating layer applied to the CoTb film with perpendicular magnetic anisotropy and the spin torque generated from Ta and Pt spin source layers. Experimentally, we measured the magnetization reversal behavior induced by pulse current through anomalous Hall effect and anomalous Hall resistance and observed the magnetic domain wall motion characteristics driven by pulse current under the magneto-optical Kerr microscope. Moreover, due to the influence of the magneto-optical Kerr microscope, it was found that for YIG/Ta/CoTb/Pt devices, the magnetic domain of ferromagnetic CoTb moves and completes magnetization switching by forming stripe-like features, which is significantly different from the magnetization switching characteristics of the Si/Ta/CoTb/Pt film. Our results may provide a new degree of freedom for the potential application value of the research of spintronic low-power devices, particularly field-free SOT-driven switching-based devices.