Arrays Of Magnetic Nanoparticles Research Articles

Promotion of Self-Assembly Patterning of FePt Nanoparticles by Tuning the Concentration of Oleylamine/Oleic Acid Surfactants in a Coating Solution

To provide a bit-patterned perpendicular magnetic recording medium consisting of an array of FePt magnetic nanoparticles, the effect of the total dispersant concentrations of oleylamine and oleic acid in a FePt-nanoparticle/toluene coating solution on a self-assembled nanoparticle array with a long-range periodicity on a flat SiO2/Si substrate and two patterned substrates, respectively, was examined. At a surfactant concentration greater than 0.8 vol%, FePt nanoparticles with an average size of 4.4 nm were arrayed in a two-dimensional and hexagonal-close-packed ordered array with 8.3-nm pitches on the flat surface. Self-assembled nanoparticles were arrayed on the bottom surface of the patterned trench, the array exhibited a hexagonal-close-packed structure. Furthermore the line defects in the arrays are caused by the waviness of the trench walls, and the surface roughness.

Surface lattice resonances and magneto-optical response in magnetic nanoparticle arrays.

Structuring metallic and magnetic materials on subwavelength scales allows for extreme confinement and a versatile design of electromagnetic field modes. This may be used, for example, to enhance magneto-optical responses, to control plasmonic systems using a magnetic field, or to tailor magneto-optical properties of individual nanostructures. Here we show that periodic rectangular arrays of magnetic nanoparticles display surface plasmon modes in which the two directions of the lattice are coupled by the magnetic field-controllable spin–orbit coupling in the nanoparticles. When breaking the symmetry of the lattice, we find that the optical response shows Fano-type surface lattice resonances whose frequency is determined by the periodicity orthogonal to the polarization of the incident field. In striking contrast, the magneto-optical Kerr response is controlled by the period in the parallel direction. The spectral separation of the response for longitudinal and orthogonal excitations provides versatile tuning of narrow and intense magneto-optical resonances.

Taking a hard line with biotemplating: cobalt-doped magnetite magnetic nanoparticle arrays.

Rapid advancements made in technology, and the drive towards miniaturisation, means that we require reliable, sustainable and cost effective methods of manufacturing a wide range of nanomaterials. In this bioinspired study, we take advantage of millions of years of evolution, and adapt a biomineralisation protein for surface patterning of biotemplated magnetic nanoparticles (MNPs). We employ soft-lithographic micro-contact printing to pattern a recombinant version of the biomineralisation protein Mms6 (derived from the magnetotactic bacterium Magnetospirillum magneticum AMB-1). The Mms6 attaches to gold surfaces via a cysteine residue introduced into the N-terminal region. The surface bound protein biotemplates highly uniform MNPs of magnetite onto patterned surfaces during an aqueous mineralisation reaction (with a mean diameter of 90 ± 15 nm). The simple addition of 6% cobalt to the mineralisation reaction maintains the uniformity in grain size (with a mean diameter of 84 ± 14 nm), and results in the production of MNPs with a much higher coercivity (increased from ≈ 156 Oe to ≈ 377 Oe). Biotemplating magnetic nanoparticles on patterned surfaces could form a novel, environmentally friendly route for the production of bit-patterned media, potentially the next generation of ultra-high density magnetic data storage devices. This is a simple method to fine-tune the magnetic hardness of the surface biotemplated MNPs, and could easily be adapted to biotemplate a wide range of different nanomaterials on surfaces to create a range of biologically templated devices.

Magnetic memory effects in nickel ferrite/polymer nanocomposites

Memory effects are reported in the field cooled (FC) magnetization of pure nickel ferrite powders and nickel ferrite nanocomposites prepared by the solution casting method. Studies carried out at different concentrations of the nanocomposite indicate that memory effects are suppressed with increasing concentration of the magnetic component in the nanocomposite. This is linked to the increase in the dipolar interaction strength in the nanocomposites, which increase with increasing concentration, as confirmed by the Henkel plots. Model simulations of the FC magnetization carried out on an interacting array of monodispersed magnetic nanoparticles indicate that growing cluster sizes inhibit memory effects.

Magneto-transport and magnetization dynamics in magnetic nanoparticle assemblies

Abstract

Second harmonic generation in magnetic nanoparticles with vortex magnetic state

Nonlinear optical properties of a regular array of triangular-shaped vortex magnetic nanoparticles is studied using the optical second harmonic generation (SHG) technique. We demonstrate that the SHG azimuthal anisotropy is consistent with the 3$m$ symmetry of individual Co nanodots placed in a square surface lattice. Qualitatively different SHG magnetic hysteresis loops are obtained for circular and linear polarizations of the fundamental radiation. In the first case, a wide SHG hysteresis at zero DC magnetic field $H$ is observed, which is attributed to a macroscopic magnetic toroid moment in Co nanodots induced by a noncentrosymmetric distribution of the magnetization. On the contrary, for the linear pump polarization the SHG loop is similar to observed commonly in linear magnetooptics for vortex magnetic structures and reveals a rather narrow width at $H=0$. A phenomenological SHG description based on the introduction of the SHG polarization induced by a magnetic toroid moment in vortex magnetic nanostructures is presented.

Spatial Resolution in Micrometric Periodic Assemblies of Magnetotactic Bacteria and Magnetic Nanoparticles

We developed a simple method for obtaining micro arrays of magnetic nanoparticles using audio tapes. We present spatial micro-arrangements of magnetotactic bacteria (Magnetospirillum gryphiswaldense), magnetite (Fe3O4) and functionalized cobalt ferrite (CoFe2O4) nanoparticles. Computer generated square audio waves of different frequencies (100 Hz-10 kHz) were recorded leading to magnetic patterns of different micrometer spatial wavelengths. Drops of aqueous suspensions were deposited on the tapes to control particle density, and bacteria and particles were trapped at locations where magnetic energy is minimized, as observed using conventional optical microscopy after the dispersium medium was evaporated. We discuss the spatial limits of the magnetic nanoparticles and the bacteria assemblies, concluding that cell walls of bacteria inhibit agglomeration and optimize spatial organization.

Magnetic Nanoparticles Arrays for Quantum Calculations

In frames of a quantum computer implementation, the ordered array of magnetic dipoles nanoparticles is considered. The phase space calculated for system of dipoles, which interact through long-range magnetostatic field. The behavior of nanoarchitectures in an external magnetic field is studied. The degeneracy of the equilibrium magnetic states depending on the value of an external magnetic field and the spin excess of configurations are determined. The presence of degeneration is a classical analog of quantum superposition, and distribution of probability of magnetic state is a classical representation of such quantum phenomena as entanglement.

Magnetoresistive properties of Fe3O4 nanoparticles embedded in a Cu matrix

We studied ordered arrays of magnetic nanoparticles (NPs) in a nonmagnetic matrix. The influence of annealing temperature and measurement geometry (varying angle between sample surface and external magnetic field direction) on magnetoresistance and coercive field values was established. Measurements were done on the Au(2 nm)/Cu(20 nm)/Fe3O4(NPs)/SiO2/Si system.

A multiscale structural study of nanoparticle films prepared by the Langmuir–Blodgett technique

Arrays of magnetic nanoparticles (NPs) represent a very interesting challenge toward the development of new devices for magnetic applications such as data storage and spintronic. The final properties of such assemblies depending essentially on the spatial arrangement of NPs, it is of first importance to investigate precisely their structure. Here, the structure of monolayer and multilayer films of magnetic iron oxide NPs assembled by the Langmuir–Blodgett (LB) technique has been studied by usual techniques such as SEM, AFM and ellipsometry and by a new and an easy to process enhanced optical technique: the Surface Enhancement Ellipsometry Contrast (SEEC) microscopy. This technique is based on the use of a new generation of microscope slides used as substrates which allow the strong enhancement of the sample contrast to a point where it becomes possible to visualize the structure of monolayer and multilayer films at the nanoscale with a conventional optical microscope. The SEEC microscopy is demonstrated to be complementary to usual characterization techniques to study the structure of NPs films, especially for films containing very small nanosized NPs which are more difficult to analyze by usual techniques. While the film structure is investigated with lateral resolution of microns, the layer thickness is analyzed at the nanoscale (with a precision of 0.3 nm) with a close fit to the experimental measurements on local (AFM) and on larger (ellipsometry) areas. This technique presents the advantage to visualize directly the topography of NPs assemblies on very large areas by extracting information such as the height profile, the film roughness and generating 3D images.

Selective Localization of Preformed Nanoparticles in Morphologically Controllable Block Copolymer Aggregates in Solution

The development of nanodevices currently requires the formation of morphologically controlled or highly ordered arrays of metal, semiconducting, or magnetic nanoparticles. In this context, polymer self-assembly provides a powerful bottom-up approach for constructing these materials. The self-assembly of block copolymers (BCPs) in solution is a facile and popular method for the preparation of aggregates of controllable morphologies, including spherical micelles, cylindrical micelles, vesicles (or polymersomes), thin films, and other complex structures that range from zero to three dimensions. Researchers can generally control the morphology of the aggregates by varying copolymer composition or environmental parameters, including the copolymer concentration, the common solvent, the content of the precipitant, or the presence of additives such as ions, among others. For example, as the content of the hydrophilic block in amphiphilic copolymers decreases, the aggregates formed from the copolymers can change from spherical micelles to cylindrical micelles and to vesicles. The aggregates of various morphologies provide excellent templates for the organization of the nanoparticles. The presence of various domains, such as cores, interfaces, and coronas, in BCP aggregates allows for selective localization of nanoparticles in different regions, which may critically affect the resulting properties and applications of the nanoparticles. For example, the incorporation of quantum dots (QDs) into micelle cores solves many problems encountered in the utilization of QDs in biological environments, including enhancement of water solubility, aggregation prevention, increases in circulation or retention time, and toxicity clearance. Simultaneously it preserves the unique optical performance of QDs compared with those of organic fluorophores, such as size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors. Therefore, many studies have focused on the selective localization of nanoparticles in BCP aggregates. This Account describes the selective localization of preformed spherical nanoparticles in different domains of BCP aggregates of controllable morphologies in solution, including spherical micelles, cylindrical micelles, and vesicles. These structures offer many potential applications in biotechnology, biomedicine, catalysis, etc. We also introduce other types of control, including interparticle spacing, particle number density, or aggregate size control. We highlight examples in which the surface coating, volume fraction, or size of the particles was tailored to precisely control incorporation. These examples build on the thermodynamic considerations of particle-polymer interactions, such as hydrophobic interactions, hydrogen bonding, electrostatic interactions, and ligand replacement, among others.

Effect of the size distribution of magnetic nanoparticles on metastability in magnetization relaxation

We theoretically examine metastability occurring in magnetization relaxation for magnetic nanoparticles with size distributions. An array of magnetic nanoparticles is simulated using a spin $S=1$ ferromagnetic Blume-Capel model on a square lattice. The particle size distributions give rise to distributions of magnetic anisotropy. Including the distributions, we perform kinetic Monte Carlo simulations of magnetization relaxation at low temperatures for the Blume-Capel model. We compute the average lifetime of the metastable state from the simulations and the absorbing Markov chains method in the low-temperature limit. We also carry out similar simulations and calculations for a constant value of magnetic anisotropy for comparison. Our results suggest that the lifetime of the metastable state is determined by the smallest particle for a given system, and that the lifetime with size distributions obeys a modified Arrhenius-like law, where the energy barrier depends on even temperature and standard deviation of the distributions as well as magnetic field and magnetic anisotropy.

Functional Magnetic Nanoparticle Assemblies: Formation, Collective Behavior, and Future Directions

This Perspective describes recent progress in the development of functional magnetic nanoparticle assemblies. After describing the formation of two- and three-dimensional particle arrays in terms of the size-dependent driving forces, we focus on magnetic nanoparticle arrays. We discuss how the self-organized structure can modify the magnetic behavior, relative to that of isolated particles. We highlight an important development, described in this issue of ACS Nano by Kostiainen and co-workers, who have demonstrated not only the novel aqueous self-assembly of magnetic particles but also controlled and reversible disassembly. Finally, we explore two inter-related future directions for self-assembly of magnetic nanoparticles: the formation of more complex, hierarchical structures and the integration of self-assembly with fabrication techniques for electronic devices.

Light scattering by an array of electric and magnetic nanoparticles

Light scattering by an array of alternating electric and magnetic nanoparticles is analyzed in detailed. Specific geometrical conditions are derived, where such an array behaves like double-negative particles, leading to a suppression of the backscattered intensity. This effect is very robust and could be used to produce double-negative metamaterials using single-negative components.

Numerical simulations of collective magnetic properties and magnetoresistance in 2D ferromagnetic nanoparticle arrays

Magnetic properties and magnetoresistance (MR) in 2D magnetic nanoparticle (NP) arrays are investigated by solving the Landau–Lifshitz–Gilbert equation at T = 0 K. The interparticle interactions induce a decrease in the coercive field and in the MR amplitude compared with the non-interacting case, while in some cases, the variation of the remanent magnetization is found to be non-monotonic when increasing the dipolar strength. For different values of the anisotropy, these variations of the coercive field, the remanent magnetization and the MR ratio are reproduced and exhibit a scaling on the dipolar/anisotropy ratio. These results suggest that the magnetic properties of the assemblies can be described by an individual or collective behaviour depending on the balance between the magnetic anisotropy and the dipolar interactions. In the case of strongly interacting NPs, the corresponding configurations of the magnetic moments at the remanent state reveal the formation of a ferromagnetic order at moderate dipolar strength (increase in the remanent magnetization) while small ferromagnetic domains/chains coupled antiferromagnetically are obtained in the case of strongly interacting NPs (decrease in the remanent magnetization). Such domains lead to a reduction in the MR amplitude and to a deviation from the m2-law in the resistance-magnetic field [R(H)] characteristic of the non-interacting case.

Model for Hyperthermia with Arrays of Magnetic Nanoparticles: Spatial and Time Temperature Distributions in Tumor

A square array of micron-sized needles containing magnetic nanoparticles has been studied theoretically as potential systems to deliver heat to tumor tissue for the hyperthermia treatment. The spatiotemporal distributions of temperature in a tumor were calculated for different arrays of microneedles. The resulting temperature distributions are found to be more uniform within the tumor target than those caused by other heating systems. Moreover, the temperature profiles could be tuned by a proper choice of the combination of the system parameters: the needle radius, the spatial needles period, as well as the filling fraction and parameters of magnetic nanoparticles.

Highly ordered arrays of magnetic nanoparticles prepared via block copolymer assembly

In the present study we report a simple and reproducible method to prepare highly ordered arrays of Fe3O4 superparamagnetic nanoparticles (MNPs) via block copolymer (BCP) self assembly. Pre-synthesized MNPs with a mean diameter of 6.1 nm are selectively segregated within the lamellae or hexagonally packed cylinders composed of PVP blocks in poly(styrene-b-vinylpyridine) (PS-b-PVP) block copolymers without any additional surface modification. The density of the stabilizing shell in the MNPs as well as the position of pyridine nitrogen in the PVP block of the BCPs are found to be crucial for selective incorporation of MNPs into the PVP domains. The obtained results suggest a key importance of a mutual affinity between active blocks and the nanoparticles which should be maximized in order to attain high nanoparticles loadings and long-range structural orders.

Array of Magnetic Nanoparticles via Particle Co-operated Self-Assembly in Block Copolymer Thin Film

The influence of nanoparticles on the domain orientation in a particle co-operated self-assembly process in thin diblock copolymer films is investigated toward the preparation of ordered magnetic nanoparticle arrays. Thin films are prepared from a mixture of chemically masked iron oxide nanoparticles and a polystyrene-block-poly (methyl methacrylate) diblock copolymer. The resulting nanostructures are investigated with grazing incidence small-angle X-ray scattering, atomic force microscopy and scanning electron microscopy. Nanoparticles arrange themselves spontaneously inside the upright cylindrical domains due to the selective affinity to the poly (methyl methacrylate) minority phase during the microphase separation process and due to the balance of the surface free energies between the polymers and the nanoparticle coating after annealing. The incorporation of the nanoparticles inside the cylindrical domains increases the diameter of the cylindrical domains and the distance between two neighboring domai...

Dipolar mode localization and spectral gaps in quasi-periodic arrays of ferromagnetic nanoparticles

In this paper we study the spectral, localization, and dispersion properties of the ferromagnetic dipolar modes around a stable, saturated, and spatially uniform equilibrium in quasi-periodically modulated arrays of ferromagnetic nanoparticles based on the Fibonacci sequence. The Fibonacci sequence is the chief example of deterministic quasi-periodic order. The problem is reduced to the study of a linear-generalized eigenvalue equation for a suitable Hermitian operator connected to the micromagnetic effective field, which accounts for the magnetostatic, anisotropy, and Zeeman interactions. The coupling with a weak applied magnetic field, varying sinusoidally in time, is dealt with and the role of the losses is highlighted. By calculating the resonance frequencies and eigenmodes of the Fibonacci arrays we demonstrate the presence of large spectral gaps and strongly localized modes and we evaluate the pseudodispersion diagrams. The magnetization oscillation modes in quasi-periodic arrays of magnetic nanoparticles show, at microwave frequencies, behaviors that are very similar to those shown, at optical frequencies, by plasmon modes in quasi-periodic arrays of metal nanoparticles. The presence of band gaps and strongly localized states in magnetic nanoparticle arrays based on quasi-periodic order may have an impact in the design and fabrication of new microwave nanodevices and magnetic nanosensors.

Investigation of the Nonlinearity Thresholds of Magnetic Nanostructures by Computing the Bifurcation Points at Microwave Frequencies

The bifurcation analysis of the threshold for collective behavior, due to the instabil- ities (the parametric excitation) of magnetostatic waves (MSW) and dipole-exchange spin-waves (SW) in 3D magnetic nanostructures, is developed. The numerical method is applied to deter- mine the bifurcation points of the nonlinear Maxwell operator (the nonlinear Maxwell equations with electrodynamic boundary conditions complemented by the Landau-Lifshitz equation includ- ing the exchange term). The original computational algorithm is improved by combining it with a qualitative method of analysis, based on Lyapunov stability theory. The threshold magnitudes of the pumping electromagnetic waves (EMWs) in the magnetic particle arrays are determined by computing the bifurcation point for difierent size nanoparticles and for various separations. This technique, based on the bifurcation theory, is a pioneering approach in nano-electrodynamics, taking into account the constrained geometries. Magnetic nanocomposites of nanometer size magnetic particles embedded in a non-magnetic, insu- lating matrix provide a novel solution for low loss microwave materials up to mm wave frequencies. The electromagnetic properties of magnetic materials change drastically upon reducing the dimen- sions into the nano-range, including the early onset of nonlinear efiects, important for high power applications and non-linear signal processing. To investigate this efiect, the instability of para- metric excitation process of magnetostatic waves MSW and dipole-exchange spin-waves SW in 3D magnetic nanostructures should be simulated, taking into account the constrained geometries. In contrast to the bifurcation analysis of the instability of parametric excitation of electromag- netic oscillations in resonator structures with nonlinear planar ferrite inserts (1) and the parametric instability of MSW in thin fllm ferrite structures (2), in this work the parametric excitation of dipole-exchange SW in the arrays of magnetic nanoparticles is analyzed. For the analysis of non- linear phenomena (i.e., the parametric instability of SW) in magnetic nanoparticle systems the numerical method, developed by us earlier (3), is modifled here to determine the bifurcation points of the nonlinear Maxwell's operator including the Landau-Lifshitz equation with the exchange term.