Magnetic properties of arrays of interacting Co nanocrystals

Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the film-plane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance.     

Fine structure of ferromagnetic-resonance spectra of disperse magnets

The technique of ferromagnetic resonance (FMR) was used for the investigation of nickel powders dispersed in a diamagnetic solid matrix. The fine structure of FMR was studied, which was observed against a background of broad featureless FMR lines of conventional unoriented polycrystals. A model is suggested according to which the narrow lines observed appear in the FMR spectra because of jumplike changes in the resonance conditions caused by a sharp change in the magnetization of the sample due to a change in the external magnetic field. In contrast to the Barkhausen effect, the fine structure detected in the FMR spectra in this case is observed in stronger fields characteristic of the processes of magnetization rotation. It is shown that the physical origin of magnetization jumps in this case may be magnetic interparticle interactions as well as complex anisotropy of particles.     

Magnetization Dynamics in Two-Dimensional Arrays of Square Microelements

The collective magnetization motion in an array of magnetostatically interacting ferromagnetic square elements is studied theoretically and experimentally. Dispersion laws are obtained for collective modes for some particular cases of the distribution of the topological charges π _T of magnetic vortices in particles. Resonance curves are plotted with allowance for dissipation for various values of π _T . An experimental investigation of the ferromagnetic resonance in a two-dimensional array of particles qualitatively supports the calculation results.     

Ferromagnetic resonance of cobalt nanoparticles used as a catalyst for the carbon nanotubes synthesis

Catalyst is considered to be the most crucial parameter for the growth of carbon nanotubes. In this work we study the ferromagnetic resonance (FMR) spectra of the catalyst nanoclusters. Moreover we report for the first time the angle FMR studies of catalyst particles with and without CNT layer. The dependencies of the FMR spectra, X-ray diffraction (XRD) patterns, Raman spectra and morphology of the CNT layers on the growth conditions are discussed.     

Magnonic crystal as a medium with tunable disorder on a periodical lattice

We show that periodic magnetic nanostructures represent a perfect system for studying excitations on disordered periodical lattices because of the possibility of controlled variation of the degree of disorder by varying the applied magnetic field. Magnetic force microscopy images and ferromagnetic resonance (FMR) data collected inside minor hysteresis loops for a periodic array of Permalloy nanowires were used to demonstrate correlation between the type of FMR response and the degree of disorder of the magnetic ground state.     

Co-SiO2颗粒膜中颗粒相互作用的温度和尺寸依赖关系

我们在真空离子束溅射设备中制备了Co-SiO2颗粒膜，其结构和磁性分别利用透射电子显微术（TEM）和铁磁共振（FMR）来研究。TEM分析表明Co纳米颗粒相互独立地分布在SiO2介质中，且尺寸分布非常窄。室温转角FMR实验显示薄膜具有单轴各向异性，变温FMR实验被用来研究在100－300K温度范围内的颗粒间相互作用强度随温度变化的关系（我们利用一理论模型计算了颗粒集合体中的铁磁部分所占比例和超顺磁部分所占比例）。我们发现颗粒间相互作用强度除了与温度有关之外，还与Co颗粒的尺寸直接相关。     

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.     

FMR study of superparamagnetic Ni particles with weak and strong magnetic anisotropy

The method of ferromagnetic resonance (FMR) was used to study the process of thermal decomposition of the layered double hydroxides of lithium-aluminum and nickel-aluminum with intercalated EDTA complexes of nickel. The magnetic resonance spectra of nickel superparamagnetic nanoparticles were recorded at two temperatures (300 and 77 K). A computer simulation of FMR spectra was based on a modified statistic model which assumes the resonance of single-domain particles randomly oriented in an amorphous matrix. It is suggested that the line of the magnetic resonance of superparamagnetic particles narrows due to effects similar to those of dynamic narrowing in electron spin resonance and nuclear magnetic resonance spectra. In the framework of the model used, a fairly good agreement was achieved between calculated and experimental data. The formation of the two types of particles with strong (about 2·10⁶ erg/cm³) and weak (about 2·10⁵ erg/cm³) effective magnetic anisotropy was established.     

Bifurcations and loss of orbital stability in nonlinear viscoelastic beam arrays subject to parametric actuation

The collective dynamic response of microbeam arrays is governed by nonlinear effects, which have not yet been fully investigated and understood. This work employs a nonlinear continuum-based model in order to investigate the nonlinear dynamic behavior of an array of N nonlinearly coupled micro-electromechanical beams that are parametrically actuated. Investigations focus on the behavior of small size arrays in the one-to-one internal resonance regime, which is generated for low or zero DC voltages. The dynamic equations of motion of a two-element system are solved analytically using the asymptotic multiple-scales method for the weakly nonlinear system. Analytically obtained results are verified numerically and complemented by a numerical analysis of a three-beam array. The dynamic responses of the two- and three-beam systems reveal coexisting periodic and aperiodic solutions. The stability analysis enables construction of a detailed bifurcation structure, which reveals coexisting stable periodic and aperiodic solutions. For zero DC voltage only quasi-periodic and no evidence for the existence of chaotic solutions are observed. This study of small size microbeam arrays yields design criteria, complements the understanding of nonlinear nearest-neighbor interactions, and sheds light on the fundamental understanding of the collective behavior of finite-size arrays.     

Width of the ferromagnetic resonance line in an anisotropic magnet in misoriented resonance and scanning magnetic fields

The dependence of the width ΔH_β of the ferromagnetic resonance (FMR) line in anisotropic magnets on the angle of misorientation β between the static (resonance) and scanning magnetic fields was investigated both experimentally and theoretically. The change in the linewidth is due to the dependence of the equilibrium orientation of the magnetization vector on the direction of the magnetic field upon the passage through the resonance. Using iron garnet films as an example, it was shown that under such a misorientation (a nonzero angle β) the linewidth is smallest. It was also shown that a two-dimensional representation of FMR spectra, in contrast to one-dimensional angular dependences of the resonance parameters, contains full information on the spectral characteristics of the film, including noncollinear field configurations.     

铁磁共振实验中值得注意的几个问题

与顺磁样品相比铁磁样品的磁共振信号不仅是强得多,铁磁样品的许多特点都会在铁磁共振(FMR)谱中反映出来.诸如,起始磁化过程的FMR谱线与有剩余磁化强度Mr的FMR谱是不同的,起始磁化过程中的巴克豪森跳跃也会在FMR谱中反映出来.样品的磁晶各向异性、形状各向异性,铁磁高温顺磁的磁性相变等等都能用FMR方法加以研究.     

Ferromagnetic resonance spectroscopic response of magnetite chains in a biological matrix

In this study ferromagnetic resonance (FMR) spectroscopy using different frequencies (4.02, 9.47, and 34.16 GHz) was applied to randomly distributed magnetotactic bacteria characterized by their alignment of equidimensional magnetite particles in a chain. Using this technique the coupling field between the aligned magnetite particles of 55 mT was determined. The non-linear frequency dependence of the spectral linewidth is interpreted by the strong influence of the anisotropy properties of the studied magnetic composite.     

Influence of dipole-dipole interactions on the angular dependence of ferromagnetic resonance spectra in arrays of Fe/Fe<Subscript>x</Subscript>O<Subscript>y</Subscript> core/shell nanocubes

We present a detailed theoretical study of the role of long-range dipole-dipole interactions on the angular dependence of ferromagnetic resonance spectra in a two-dimensional array of nanocubes. Variations of polar (φ) and azimuthal (θ) angles are studied numerically and analytically to illustrate the effect of the magnetocrystalline properties and the dipole-dipole interactions, forming complex resonance bands. In addition, we show that when the static magnetic field lies in the arrays’ plane under the angle of 129° with the edge of the array or when its tilted around 15° to the plane’s normal, the spectra of absorption transform into a plateau spanning from 0.1 T to 0.4 T, which is prominent enough for experimental observation.     

FMR Investigation of the Magnetic Anisotropy in Films Synthesized by Co<Superscript>+</Superscript> Implantation into Si

Thin ferromagnetic films with the uniaxial magnetic anisotropy were synthesized by Co⁺ implantation into single-crystal silicon in the magnetic field. It was concluded that the formation of the induced magnetic anisotropy is due to the directional atomic pair ordering (Neel–Taniguchi model). The synthesized films were studied by the ferromagnetic resonance (FMR) method in the temperature range from 100 to 300 K. The FMR linewidth is almost independent of temperature, which is in agreement with the Raikher model describing the magnetic resonance of uniaxial magnetic particles. It is found that the temperature dependence of the anisotropy constant is linear. This dependence can be associated with the difference in the coefficients of thermal expansion of the Si (111) substrate and the ion-beam-synthesized cobalt silicide films.     

Surface and frustration evidence in Co–Ni–B nanoparticles by FMR measurements

We present ferromagnetic resonance (FMR) measurements on ∼3 nm amorphous magnetic nanoparticles of Co–Ni–B as a function of temperature (T). The particles were studied in powder form and dispersed in a polymer matrix to study the interparticle interaction effect. In both samples the FMR responses are similar down to T∼10 K, where the powder sample shows an intensity increase not followed by the dispersed sample. We argue that the general characteristics are compatible with previous magnetization measurements and Monte Carlo simulations indicating large surface contributions to the effective anisotropy. In this case the frustration of the single-particle behavior observed in the powder sample at very low T (T ⩽ 10 K) is due to interparticle interactions.     

Surface plasmon resonance in near-field coupled gold cylinder arrays fabricated by EUV-interference lithography and hot embossing

Arrays of metal nanoparticles with nanometer-scale gaps between the particles is highly interesting for plasmonic field enhancement applications. We report a simple method to fabricate arrays of closely spaced Au particles with inter-particle separation down to 20 nm. We used extreme ultraviolet interference lithography (EUV-IL) and a mechanical press to fabricate two-dimensional arrays of Au nanoparticles. Lithographically produced particle arrays were modified by hot pressing in a nanoimprint machine and the gap was varied in a range from 50 nm to below 20 nm. Optical measurement shows two resonances at 520 nm and 620 nm, with the latter gaining strength as the gap is reduced. The experimental and theoretical investigations using a FDTD algorithm demonstrate that the low-energy resonance can be assigned to a collective surface plasmon resonance arising from the strong near-field coupling between the nanoparticles. Surface Enhanced Raman Spectroscopy (SERS) experiments performed on a model molecule (BPE) show a large gain in signal intensity as a result of the reduced gaps between the particles.     

Fe/[NiFe/Cu]30多层膜线列阵的铁磁共振研究

用铁磁共振(FMR)技术研究Fe/［NiFe/Cu］₃₀多层膜线列阵的特性,线列阵采用激光全息光刻和离子束刻蚀技术加工,线密度为250线/mm和500线/mm.线列阵样品的FMR谱在稍高于主峰（声学峰）的共振场下有光学峰出现,相当层间交换耦合为反铁磁型.光学峰与声学峰的峰位间距沿易轴小于沿难轴.又,来自非磁Cu夹层的顺磁共振峰的幅度远大于原始的连续膜,说明线列阵加工致使层结构退化. 关键词：     

Magnetic properties of ZnFe2O4 nanoparticles

Fine particles of ZnFe₂O₄ were synthesized by a wet chemical method in the (80 wt.% Fe₂O₃ + 20 wt.% ZnO) system. The morphological and structural properties of the mixed system were investigated by scanning electron microscopy, X-ray diffraction, inductively coupled plasma atomic emission, and X-ray photoelectron spectroscopy. The major phase was determined to be the ZnFe₂O₄ spinel with particle size of 11 nm. The magnetic properties of the material were investigated by ferromagnetic resonance (FMR) in the temperature range from liquid helium to room temperature. A very intense, asymmetric FMR signal from ZnFe₂O₄ nanoparticles was recorded, which has been analyzed in terms of two Callen-lineshape lines. Temperature dependence of the FMR parameters was obtained from fitting the experimental lines with two component lines. Analysis of the FMR spectra in terms of two separate components indicates the presence of strongly anisotropic magnetic interactions.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

Micromagnetic simulation of ferromagnetic resonance of perpendicular granular media: Influence of the intergranular exchange on the Landau-Lifshitz-Gilbert damping constant

A micromagnetic approach was used to simulate ferromagnetic resonance frequency (FMR) profiles of perpendicular granular CoCrPt-oxide thin films. From the obtained FMR line-width we computed the effective damping constant. The influence of the intergranular exchange on the effective damping was investigated, showing an increase in the damping constant with increase in intergranular exchange coupling. Moreover, the effective damping constant increases with decrease in mesh size of the model, and eventually saturates for mesh sizes of about 1 nm. These dependencies are explained in terms of different modes that can be excited in the granular medium due to interactions between the individual spins.