Arrays Of Magnetic Nanoparticles Research Articles

High-frequency magnetic response of superparamagnetic composites of spherical Fe and Fe3O4 nanoparticles

The response of arrays of magnetic nanoparticles (MNPs) of cubic anisotropy materials (magnetite or iron) to high-frequency (sub-GHz) field is investigated utilizing micromagnetic simulations. Composites of MNPs embedded in dielectric media are of interest as core materials for high-frequency power microconverters with low eddy-current losses. Going beyond the macrospin approximation for MNPs (including internal spin structure), we simulate the magnetization dynamics of arrays of spherical and periodically arranged MNPs under periodic boundary conditions and in the presence of thermal fluctuations of the magnetization within the stochastic thermal field. Analyzing the dependence of the dynamical response on the size of the MNPs, we find the size to be crucial for exciting regular magnetization oscillations at room temperature and reducing hysteresis loss. The efficiency of the dynamical response of composites of high-magnetization-metal (iron) MNPs is compared to that of magnetite nanocomposites. We find the sub-GHz susceptibility of magnetite-containing nanocomposite to be comparable to that of the iron MNPs despite the lower saturation magnetization of Fe3O4, (at the volumetric ratio of the magnetic phase of 0.13, susceptibility is χ≈1.6 for 5 nm Fe and ≈1.4 for 12 nm Fe3O4 MNPs at T=300 K and frequency of 0.1 GHz).

Confined In-Situ Encapsulation of Co/C Composites with Increased Heterogeneous Interface Polarization for Enhanced Electromagnetic Performance.

It is an urgent problem to realize reliable microwave absorption materials (MAMs) with low density. To address this issue, a series of controlled experiments w ere carried out, which indicated that the tubular structure enables excellent microwave absorption properties with a lower powder filling rate. This performance is attributable to the combined dielectric and magnetic loss mechanisms provided by Co/C and the interface polarization facilitated by multiple heterogeneous interfaces. Particularly, Co@C nanotubes, benefiting from the enhanced heterointerface polarization due to their abundant specific surface area and the reduced electron migration barrier induced by their 1D stacked structure, effectively achieved a dual enhancement of dielectric loss and polarization loss at lower powder filling ratios. Furthermore, the magnetic coupling effect of magnetic nanoparticle arrays in tubular structures is demonstrated by micromagnetic simulation, which have been few reported elsewhere. These propertied enable Co@C nanotubes to achieve minimum reflection lossand maximum effective absorption broadband values of 61.0dB and 5.5GHz, respectively, with a powder filling ratio of 20wt% and a thickness of 1.94mm. This study reveals the significance of designing 1D structures in reducing powder filling ratio and matching thickness, providing valuable insights for developing MAMs with different microstructures.

Tunability and Ordering in 2D Arrays of Magnetic Nanoparticles Assembled via Extreme Field Gradients

AbstractColloidal magnetite nanoparticles self‐assemble onto a disk drive medium as directed by magnetic field gradients created where the medium magnetic moment switches direction over single nanometer distances. Here, it is shown that for two such reversals or transitions that are closely spaced, the nanoparticles self‐assemble into a single feature centered between the transitions, rather than forming separate features at the transitions, and the resulting 2D assembly achieves hexatic ordering. Langevin dynamics simulations are used to explain these results, and it is found that the detailed magnetic properties of the medium play a critical role in determining assembly location. Slight changes to solvent polarity disrupt the hexatic ordering and push the nanoparticles toward the transitions, suggesting an alternate mechanism to precisely tune the self‐assembly process.

Tailoring heat dissipation in linear arrays of dipolar interacting magnetic nanoparticles

The main aim of the present work is to analyse the effect of dipolar interaction strength $\lambda$, particle size $D$ and temperature $T$ on the hysteresis mechanism in ordered arrays of magnetic nanoparticles (MNPs) using computer simulations. The anisotropy axes of the MNPs are assumed to have random orientation to mimic the real system. In the absence of thermal fluctuations and dipolar interaction, the hysteresis follows the Stoner and Wohlfarth model irrespective of $D$, as expected. The hysteresis loop area is minimal for particle sizes $D \approx8-16$ nm at $T=300$ K and $\lambda=0.0$, indicating the dominance of superparamagnetic character. Switching magnetic interaction on is able to move the MNPs from superparamagnetic to a ferromagnetic state even at room temperature; therefore, magnetic interaction of enough strength enhances the hysteresis loop area. Interestingly, the hysteresis loop area is significant and is the same as that of Stoner and Wohlfarth particle even $T=300$ K and negligible dipolar interaction for ferromagnetic MNPs ($D>16$ nm). The coercive field $\mu^{}_oH^{}_c$ and blocking temperature $T^{}_B$ also get enhanced with an increase in $\lambda$ and $D$. The rigorous analysis of the coercive field $\mu^{}_oH^{}_c$ vs temperature data also reveals significant deviation from $T^{3/4}$ dependence of $\mu^{}_oH^{}_c$ because of dipolar interaction. The amount of heat dissipated $E^{}_H$ and $\mu_oH^{}_c$ decrease rapidly with $T$ for $D\approx 8-16$ nm and $\lambda\leq0.6$. On the other hand, $E^{}_H$ and $\mu^{}_oH^{}_c$ depend weakly on $T$ with $D>16$ nm, even in the weak dipolar limit. The present work should provide a better understanding of magnetic hyperthermia to researchers working on this subject. For physicists, it would be interesting to test experimentally the results described in this article.

Anisotropic effect of dipolar interaction in ordered ensembles of nanoparticles

We implement extensive computer simulations to investigate the hysteresis characteristics in the ordered arrays (lx × ly) of magnetic nanoparticles as a function of aspect ratio Ar = ly/lx, dipolar interaction strength hd, and external magnetic field directions. We have considered the aligned anisotropy case, α is the orientational angle. It provides an elegant en route to unearth the explicit role of anisotropy and dipolar interaction on the hysteresis response in such a versatile system. The superparamagnetic character is dominant with weak dipolar interaction (hd ≤ 0.2), resulting in the minimal hysteresis loop area. Remarkably, the double-loop hysteresis emerges even with moderate interaction strength (hd ≈ 0.4), reminiscent of antiferromagnetic coupling. These features are strongly dependent on α and Ar. Interestingly, the hysteresis loop area increases with hd, provided Ar is enormous, and the external magnetic field is along the y-direction. The coercive field μoHc, remanent magnetization Mr, and the heat dissipation EH also depend strongly on these parameters. Irrespective of the external field direction and weak dipolar interaction (hd ≤ 0.4), there is an increase in μoHc with hd for a fixed α and Ar ≤ 4.0. The dipolar interaction also elevates Mr as long as Ar is huge and the field is along the y-direction. EH is minimal for negligible and weak dipolar interaction, irrespective of Ar, α, and the field directions. Notably, the magnetic interaction enhances EH if Ar is enormous and the magnetic field is along the long axis of the system. These results are beneficial in various applications of interest such as digital data storage and spintronics. (a) Schematic of the two-dimensional array of nanoparticles. The variation of heat dissipation EH as a function of dipolar interaction strength and anisotropy axis orientation is shown in (b)-(d). The magnetic field is applied along the x-direction in (b). In contrast, the magnetic field is applied along y-direction in (c) and (d).

Glucose-Sensing Photonic Nanochain Probes with Color Change in Seconds.

Glucose‐sensing photonic crystals are promising for the significant advance of continuous glucose monitoring systems due to the naked‐eye colorimetric readouts and noninvasive detection of diabetes, but the long response time hampers their practical applications. Here, for the first time probes of photonic nanochains (PNCs) are demonstrated that are capable of continuously and reversibly sensing glucose concentration ([glucose]) variation within seconds by color change without power consumption, much faster by 2–3 orders of magnitude than previous ones. They are comprised of 1D equidistant arrays of magnetic nanoparticles enveloped by tens‐of‐nanometer‐thick phenylboronic acid‐functionalized hydrogels, and fabricated by developing selective concentration polymerization of monomers in binary microheterogeneous solvents of dimethyl sulfoxide (DMSO) and H2O. In this process, both 3‐acrylamido phenylboronic acid (AAPBA) and N‐2‐hydroxyethyl acrylamide (HEAAm) are preferentially dissolved in the small volume of free DMSO concentrated in the vicinity of poly vinylpyrrolidone coated Fe3O4 colloidal nanoparticles (Fe3O4@PVP), yielding Fe3O4@PVP@poly(AAPBA‐co‐HEAAm) PNCs after UV irradiation under magnetic field. The PNCs in phosphate buffered solution have a wavelength‐shift range up to 130 nm when [glucose] changes from 0 to 20 × 10−3 m. The results can facilitate real‐time glucose monitoring and provide an alternative to produce functional organic–inorganic nanostructures.

Magnetic nanoarrays on flexible substrates.

In this work, we used nanosphere lithography to fabricate large area 2-D magnetic nanoparticle (MNP) arrays on a flexible polyimide substrate (Kapton). Samples were fabricated by assembling polystyrene (PS) spheres on thin films of Co capped with Au. Etched PS spheres were used to mask Co–Au particle arrays. The MNP arrays were subjected to superconducting quantum interference device measurements; flat samples (10 nm Co coated with 10 nm Au) exhibited an Ms of 117.3 emu g−1, which was lower than the reported literature value for bulk Co (162.7 emu g−1). When compared to the flat film, coercivity, Hc, increased in a linear fashion with respect to particle size. These preliminary results reveal that future investigations of the magnetic properties on flexible substrates should account for residual Co remaining in the polymeric material, the unique MNP shape, the effect of order (or lack or order) of the 2D array, and positioning with respect to the direction of the magnetic field.Graphical Supplementary InformationThe online version contains supplementary material available at 10.1557/s43580-021-00193-z.

Dipolar interaction and sample shape effects on the hysteresis properties of 2d array of magnetic nanoparticles

We study the ground-state and magnetic hysteresis properties of 2d arrays ( $$L^{}_x\times L^{}_y$$ ) of dipolar interacting magnetic nanoparticles (MNPs) by performing micromagnetic simulations. Our primary interest is to understand the effect of sample shape, $$\Theta $$ , the ratio of the dipolar strength to the anisotropy strength and the direction of the applied field $$\vec {H} = H_{0}{\hat{e}}^{}_H$$ on the ground state and the magnetic hysteresis in an array of MNPs. To study the effect of the shape of the sample, we have varied the aspect ratio $$A^{}_r=L^{}_y/L^{}_x$$ , which in turn, is found to induce shape anisotropy in the system. Our main observations are: (a) When the dipolar interaction is strong $$(\Theta >1)$$ , the ground-state morphology has an in-plane ordering of magnetic moments, (b) the ground-state morphology has randomly oriented magnetic moments that are robust regarding system sizes and $$A^{}_r$$ for weakly interacting MNPs ( $$\Theta <1$$ ), (c) micromagnetic simulations suggest that the dipolar interaction decreases the coercive field $$H^{}_c$$ , (d) the remanence magnetisation $$M^{}_r$$ is found to be strongly dependent not only on the strength of dipolar interaction but also on the shape of the sample and (e) due to the anisotropic nature of dipolar interaction, a strong shape anisotropy effect is observed when the field is applied along the long axis of the sample. In such a case, the dipolar interaction induces an effective ferromagnetic coupling when the aspect ratio is enormous. These results are of vital importance in high-density recording systems, magneto-impedance sensors, etc.

Hysteresis in two dimensional arrays of magnetic nanoparticles

We perform computer simulations to probe the magnetic hysteresis in a two-dimensional (Lx×Ly) assembly of magnetic nanoparticles as a function of dipolar interaction strength hd, temperature T, aspect ratio Ar=Ly/Lx, and the applied alternating magnetic field’s direction. In the absence of magnetic interaction (hd≈0) and thermal fluctuations (T=0 K), the hysteresis follows the Stoner and Wohlfarth model, as expected. For weak dipolar interaction and substantial temperature, the hysteresis has the dominance of superparamagnetic behaviour, irrespective of the applied magnetic field’s direction and Ar. Interestingly, the hysteresis curve has all the characteristics of antiferromagnetic interaction dominance for Ar≤6 and considerable dipolar interaction strength (hd>0.2), independent of applied magnetic direction. When the magnetic field is applied along the system’s shorter axis (x-direction), a non-hysteresis straight line is observed with large hd. In the case of the magnetic field applied along the long axis of the sample (y-direction), ferromagnetic interaction dominates the hysteresis for large hd and Ar>6. Irrespective of hd and applied magnetic field’s direction, the coercive field μoHc and remanence Mr are minimal for Ar≤6 and significant temperature. They are found to increase with hd when Ar is enormous. There is a strong dependence of μoHc and Mr on temperature for weakly interacting MNPs. While for large hd, they depend weakly on temperature. Remarkably, the variation of hysteresis loop area EH as a function of these parameters is the same as that of the coercive field variation. We believe that the concepts presented in this work are relevant in various technological applications such as spintronics and magnetic hyperthermia, in which such self-assembled nanoparticle arrays are ubiquitous.

Magnetic array-templated method for fabrication of polymer nanoporous films

This paper describes the development of a novel method of producing nanoporous polymeric membranes in a cost-effective and reproducible manner. The novelty of the technique hinges on the exploitation of a new type of sacrificial material & structures - self-assembled arrays of magnetic nanoparticles. The arrays are obtained through application of an external magnetic field to a thin layer of colloidal solution of superparamagnetic nanoparticles in a polymerizable monomer; this is followed by photopolymerisation. The resulting columnar structures form the pore templates which when selectively etched away leave an array of nanopores spanning across the polymeric film. The morphological characterisation of the nanopores by scanning electron microscopy and ionic conductivity revealed a very unusual sponge-like pore morphology. The applications which would benefit from the specific pore morphology and arrayed manufacturing are discussed.

Nanoparticle-Induced Anomalous Hall Effect in Graphene.

Schemes for introducing magnetic properties into graphene are of fundamental interest and could enable the development of electrically controlled magnetic devices, thereby extending graphene's applications from conventional electronics to spintronics. Proximity-induced ferromagnetism (PIFM) has been reported for graphene coupled to adjacent ferromagnetic insulators (FMIs). PIFM from an FMI preserves graphene's high carrier mobility and does not introduce a parallel current path. However, few FMIs other than yttrium-iron-garnet are suitable for practical applications due to difficulties in their growth and deposition and to their typically low Curie temperatures. Furthermore, it is difficult to obtain a high-quality FMI/graphene interface by graphene transfer methods, which are essential for obtaining the required interfacial exchange coupling. Here, we report the observation of the anomalous Hall effect (AHE) in graphene proximity coupled to an array of magnetic nanoparticles. This observation of AHE in graphene in proximity to a discontinuous magnetic structure opens the door to realizing magnetic properties in graphene from a greatly expanded range of materials and offers new possibilities for realizing patterned spintronic devices and circuitry.

Maximal tunnel magnetoresistance in magnetic nanoparticle arrays with perpendicular anisotropy

We study tunnel magnetoresistance (TMR) ratio of self-assembled $2d$ magnetic nanoparticle (MNP) arrays by modeling them as interacting dipoles on a triangular lattice, which is representative of experimentally obtained assemblies. Low-temperature Monte Carlo simulations are performed to understand the effects of dipolar interactions and uniaxial anisotropy on TMR behavior. In magnetic tunnel junctions with a perfect antiparallel state, the TMR amplitude reaches the ideal value $2{P}^{2}/(1\ensuremath{-}{P}^{2})$, where $P$ is the spin polarization. We show that, for MNPs with their anisotropy axes perpendicular to the array, the TMR amplitude can reach a value as large as $2400%$, which represents an order of magnitude improvement over previous studies. We evaluate the conditions for which these large amplitudes could be obtained experimentally at room temperature and find that spherical MNPs with large magnetocrystalline anisotropy or nanorods are suitable candidates. Our theoretical results should stimulate experimental groups into elaborating the samples proposed in this work, which could display large TMR amplitudes.

Self-Assembly of CoPt Magnetic Nanoparticle Arrays and its Underlying Forces.

Nanoparticles covered with surfactants are often used to study particle motion patterns and self-assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self-assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.

Micromagnetic simulation study of a disordered model for one-dimensional granular perovskite manganite oxide nanostructures

Chemical techniques are an efficient method to synthesize one-dimensional perovskite manganite oxide nanostructures with a granular morphology, that is, formed by arrays of monodomain magnetic nanoparticles. Integrating the stochastic Landau-Lifshitz-Gilbert equation, we simulate the dynamics of a simple disordered model for such materials that only takes into account the morphological characteristics of their nanograins. We show that it is possible to describe reasonably well experimental hysteresis loops reported in the literature for single La_0.67Ca_0.33MnO_3 nanotubes and powders of these nanostructures, simulating small systems consisting of only 100 nanoparticles.

Role of dipolar interactions on morphologies and tunnel magnetoresistance in assemblies of magnetic nanoparticles

We undertake comprehensive simulations of 2d arrays (Lx×Ly) of magnetic nanoparticles (MNPs) with dipole-dipole interactions by solving LLG equations. Our primary interest is to understand the correspondence between equilibrium spin (ES) morphologies and tunnel magnetoresistance (TMR) as a function of Θ – the ratio of the dipolar to the anisotropy strength, sample size Lx, aspect ratio Ar=Ly/Lx and the direction of the applied field H→=HêH. The parameter Θ is varied by choosing three distinct particles: (i) α-Fe2O3(Θ≃0), (ii) Co (Θ≃0.37) and (iii) Fe3O4(Θ≃1.28). Our main observations are as follows: (a) For weakly interacting spins (Θ≃0), the morphology has randomly oriented magnetic moments for all sample sizes and aspect ratios. The TMR exhibits a peak value of 50% at the coercive field Hc. It is robust with respect to Lx and Ar, and isotropic with respect to êH. (b) For strong interactions (Θ>1), the moments order in the plane of the sample. The ES morphology comprises of magnetically aligned regions interspersed with flux closure loops. For fields along x or y, the maximum TMR amplitude decrease to ∼30%. For êH=ẑ, it drops to ∼3%. The TMR is robust with respect to Lx and Ar and isotropic in the x and y directions only. (c) In strongly interacting samples (Θ>1) with Lx comparable to the size of a flux closure loop, increasing Ar creates ferromagnetic chains in the sample oriented along y or -y. Consequently, for êH=ŷ, the TMR magnitude for Ar=1 is ∼33% while that for Ar=32 drops to ∼16%. For êH=x̂ on the other hand, it is ∼30% and independent of Ar. The TMR of long ribbons of MNPs has a strong dependence on Ar and is anisotropic in all three directions.

Effect of dipolar interactions and DC magnetic field on the specific absorption rate of an array of magnetic nanoparticles

We address the issue of inter-particle dipolar interactions in the context of magnetic hyperthermia. More precisely, the main question dealt with here is concerned with the conditions under which the specific absorption rate is enhanced or reduced by dipolar interactions. For this purpose, we propose a theory for the calculation of the AC susceptibility, and thereby the specific absorption rate, for a monodisperse two-dimensional assembly of nanoparticles with oriented anisotropy, in the presence of a DC magnetic field, in addition to the AC magnetic field. We also study the competition between the dipolar interactions and the DC field, both in the transverse and longitudinal configurations. In both cases, we find that the specific absorption rate has a maximum at some critical DC field that depends on the inter-particle separation. In the longitudinal setup, this critical field falls well within the range of experiments.

Fabrication of DNA nanotubes with an array of exterior magnetic nanoparticles

Described here a methodology for arraying of magnetic nanoparticles (MNPs) on the surface of DNA nanotubes (DNTs). Positioning of magnetic nanoparticles at exterior surface of DNTs were shaped after self-assembling of oligonucleotide staples within an M13mp18 DNA scaffold via an origami process. The staples were partially labeled with biotin to be arrayed at the surface of DNTs. Gel retardation assay of the DNTs carrying magnetic nanoparticles indicated a reversely behavioral electrophoretic movement in comparison to the nanotubes have been demonstrated previously. Also, high resolution transmission electron microscopy confirmed positioning magnetic nanoparticles at the exterior surface of DNTs, correctly. Ultrastructural characteristics of these DNA nanotubes using atomic force microscopy demonstrated topographic heights on their surfaces formed through positioning of magnetic nanoparticles outside the tubules. This nanoarchitecture would be potential for multiple arraying of nanoparticles that those be useful as functionalized chimeric nanocarriers for developing novel nanodrugs and nanobiosensors.

Top-down design of magnonic crystals from bottom-up magnetic nanoparticles through protein arrays

We show that chemical fixation enables top-down micro-machining of large periodic 3D arrays of protein-encapsulated magnetic nanoparticles (NPs) without loss of order. We machined 3D micro-cubes containing a superlattice of NPs by means of focused ion beam etching, integrated an individual micro-cube to a thin-film coplanar waveguide and measured the resonant microwave response. Our work represents a major step towards well-defined magnonic metamaterials created from the self-assembly of magnetic nanoparticles.

Magnetic circular dichroism of non-local surface lattice resonances in magnetic nanoparticle arrays.

Subwavelength metallic particles support plasmon resonances that allow extreme confinement of light down to the nanoscale. Irradiation with left- and right hand circularly polarized light results in the excitation of circular plasmon modes with opposite helicity. The Lorenz force lifts the degeneracy of the two modes in magnetic nanoparticles. Consequently, the confinement and frequency of localized surface plasmon resonances can be tuned by an external magnetic field. In this paper, we experimentally demonstrate this effect for nickel nanoparticles using magnetic circular dichroism (MCD). Besides, we show that non-local surface lattice resonances in periodic arrays of the same nanoparticles significantly enhance the MCD signal. A numerical model based on the modified long wavelength approximation is used to reproduce the main features in the experimental spectra and provide design rules for large MCD effects in sensing applications.

Enhanced Collective Magnetic Properties in 2D Monolayers of Iron Oxide Nanoparticles Favored by Local Order and Local 1D Shape Anisotropy.

Magnetic nanoparticle arrays represent a very attractive research field because their collective properties can be efficiently modulated as a function of the structure of the assembly. Nevertheless, understanding the way dipolar interactions influence the intrinsic magnetic properties of nanoparticles still remains a great challenge. In this study, we report on the preparation of 2D assemblies of iron oxide nanoparticles as monolayers deposited onto substrates. Assemblies have been prepared by using the Langmuir-Blodgett technique and the SAM assisted assembling technique combined to CuAAC "click" reaction. These techniques afford to control the formation of well-defined monolayers of nanoparticles on large areas. The LB technique controls local ordering of nanoparticles, while adjusting the kinetics of CuAAC "click" reaction strongly affects the spatial arrangement of nanoparticles in monolayers. Fast kinetics favor disordered assemblies while slow kinetics favor the formation of chain-like structures. Such anisotropic assemblies are induced by dipolar interactions between nanoparticles as no magnetic field is applied and no solvent evaporation is performed. The collective magnetic properties of monolayers are studied as a function of average interparticle distance, local order and local shape anisotropy. We demonstrate that local control on spatial arrangement of nanoparticles in monolayers significantly strengthens dipolar interactions which enhances collective properties and results in possible super ferromagnetic order.