Influence Of Dipolar Interactions Research Articles

Influence of dipolar interactions on the magnetic properties of superparamagnetic particle systems

Magnetic effects caused by dipolar interactions in single-domain magnetic ensembles at finite temperatures are described. A modified superparamagnetic approach based on the mean field theory and random anisotropy model has been developed to describe the magnetization curves of nanoparticle assemblies. The model was used to fit experimental zero-field-cooled and field-cooled magnetization curves in Fe3O4 nanoparticles embedded in paraffin. The fitting parameters were based on structural properties of the materials and the strength of the interactions between nanoparticles. The model provides a quantitative description of the effects of the nanoparticle interaction with good agreement with the experiment. In addition, the model was adapted to describe magnetic properties of a NiFe thin film patterned into a nanodot array, showing potential to be used as a framework to predict magnetic interaction effects in high-density 2D arrays such as bit patterned media.

Dipolar interactions in Fe: A study with the neutron Larmor precession technique MIEZE in a longitudinal field configuration

While the influence of dipolar interactions on the spin-wave dispersion in ferromagnets with localized magnetic moments has been studied in detail, similar studies in itinerant electron systems are rather scarce due to experimental difficulties. Using the newly developed neutron Larmor precession technique MIEZE in a longitudinal field configuration, we succeeded to map out the spin-wave dispersion in Fe at small momentum $q$ and energy transfers $E$. The results demonstrate an excellent agreement of the magnon dispersion with the Holstein-Primakoff theory, which takes the dipolar interactions into account. At larger $q$, the data is in agreement with previous investigations by Collins et al., Phys. Rev. 179, 417 (1969). The $q$ dependence of the linewidth of the magnons is proportional to ${q}^{2.5}$ in agreement with dynamical scaling theory. The critical exponent for the stiffness, $\ensuremath{\mu}=0.35\ifmmode\pm\else\textpm\fi{}0.01$, agrees with field theory. The spin dynamics in Fe is now explored by neutron scattering over an energy range $15\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{eV}\phantom{\rule{4pt}{0ex}}l{E}_{\mathrm{sw}}l120\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, i.e., over about four orders of magnitude in energy.

Influence of dipolar interactions on the superparamagnetic relaxation time of γ-Fe2O3

Influence of dipolar interactions on the Néel superparamagnetic relaxation time, τ, of an assembly of ultrafine ferromagnetic particles (γ-Fe2O3) with uniaxial anisotropy and of different sizes has been widely studied using Mössbauer technique. These studies, based on different analytical approaches, have shown that τ decreases with increasing interactions between particles. To interpret these results, we propose a model where interaction effects are considered as being due to a constant and external randomly oriented magnetic field B(Ψ, ϕ). The model is based on the resolution of the Fokker-Planck equation (FPE), generalizes previous calculations and gives satisfactory interpretation of the relaxation phenomenon in such systems.

Influence of dipolar interactions on the magnetic susceptibility spectra of ferrofluids.

The frequency-dependent magnetic susceptibility of a ferrofluid is calculated under the assumption that the constituent particles undergo Brownian relaxation only. Brownian-dynamics simulations are carried out in order to test the predictions of a recent theory [A. O. Ivanov, V. S. Zverev, and S. S. Kantorovich, Soft Matter 12, 3507 (2016)1744-683X10.1039/C5SM02679B] that includes the effects of interparticle dipole-dipole interactions. The theory is based on the so-called modified mean-field approach and possesses the following important characteristics: in the low-concentration, noninteracting regime, it gives the correct single-particle Debye-theory results; it yields the exact leading-order results in the zero-frequency limit; it includes particle polydispersity correctly from the outset; and it is based on firm theoretical foundations allowing, in principle, systematic extensions to treat stronger interactions and/or higher concentrations. The theory and simulations are compared in the case of a model monodisperse ferrofluid, where the effects of interactions are predicted to be more pronounced than in a polydisperse ferrofluid. The susceptibility spectra are analyzed in detail in terms of the low-frequency behavior, the position of the peak in the imaginary (out-of-phase) part, and the characteristic decay time of the magnetization autocorrelation function. It is demonstrated that the theory correctly predicts the trends in all of these properties with increasing concentration and dipolar coupling constant, the product of which is proportional to the Langevin susceptibility χ_{L}. The theory is in quantitative agreement with the simulation results as long as χ_{L}≲1.

Influence of Dipolar Interactions in Ferroelectric Vinylidene Fluoride Copolymers on their Structure and Low-Temperature Molecular Mobility

Dielectric, IR spectroscopy and DSC measurements were made over a wide range of temperatures and frequencies in copolymers of vinylidene fluoride (VDF) with tetra- (TFE) and trifluoroethylene (TrFE) of different composition. Experimental results show that decrease of VDF content in VDF/TFE copolymers is accompanied by decrease of activation energy of kinetic units in the glassy state, though relaxation times of cooperative mobility in amorphous phase differ insignificantly. Substitution of TFE with TrFE in copolymers with the same VDF content increases activation energy values for the local mobility. The results of IR spectroscopy suggest that VDF/TFE copolymers with low VDF content have lower crystallinity, which may be explained by the secondary crystallization processes and absence of long isomers in planar zigzag conformation.

Directed selective tunneling of dipolar bosons in a driven triple well

We study the coherent control of quantum tunneling for dipolar bosons held in a driven triple-well potential. In the high-frequency region within the resonance case, based on the non-Floquet solutions of two dipolar bosons, the influence of dipolar interaction on tunneling is investigated analytically and numerically, in which the directed selective-tunneling of a single atom is demonstrated when the two bosons are located in the middle well initially. Further, the corresponding effect is exhibited numerically for more dipolar atoms $N>2$ and the directed tunneling of 1 or $(N\ensuremath{-}1)$ atoms occurs by adjusting the driving parameters. These results may be useful in the design of atomic devices.

Nanostructured Block-Random Copolymers with Tunable Magnetic Properties

It was recently shown that block copolymers (BCPs) produced room-temperature ferromagnetic materials (RTFMs) due to their nanoscopic ordering and the cylindrical phase yielded the highest coercivity. Here, a series of metal-containing block-random copolymers composed of an alkyl-functionalized homo block (C(16)) and a random block of cobalt complex- (Co) and ferrocene-functionalized (Fe) units was synthesized via ring-opening metathesis polymerization. Taking advantage of the block-random architecture, the influence of dipolar interactions on the magnetic properties of these nanostructured BCP materials was studied by varying the molar ratio of the Co units to the Fe units, while maintaining the cylindrical phase-separated morphology. DC magnetic measurements, including magnetization versus field, zero-field-cooled, and field-cooled, as well as AC susceptibility measurements showed that the magnetic properties of the nanostructured BCP materials could be easily tuned by diluting the cobalt density with Fe units in the cylindrical domains. Decreasing the cobalt density weakened the dipolar interactions of the cobalt nanoparticles, leading to the transition from a room temperature ferromagnetic (RTF) to a superparamagnetic material. These results confirmed that dipolar interactions of the cobalt nanoparticles within the phase-separated domains were responsible for the RTF properties of the nanostructured BCP materials.

The influence of dipolar interaction on magnetic properties in nanomagnets with different shapes

AbstractWith the Monte Carlo method, we investigate the magnetic properties of four nanomagnets of different shapes, i.e., the circular‐shaped, the square‐shaped, the elliptical, and the ring‐shaped nanomagnets. A systematic study of the effects of the dipolar interaction on the magnetic configurations is performed in these nanomagnets, and further the coercive field and the remanence as a function of dipolar interaction are analyzed. The results show that the magnetic configuration and thus the magnetization reversal process of nanomagnets are dependent strongly on the strength of dipolar interactions. For the case of small dipolar interaction, the magnetization reversal process is mainly dominated by spin rotation, while the reversal transforms to the vortex nucleation and propagation formations with increasing dipolar interaction. Moreover, if the dipolar interaction is neglected in the calculation of the total energy, no clear difference is found among hysteresis loops of four nanomagnets with same areas, but the inclusion of dipolar interaction can lead to different hysteresis loops for nanomagnets with same areas but different shapes. This indicates that the dipolar interaction is important for accounting for the shape effect of the magnetic properties in nanomagnets.

Influence of dipolar interaction on small magnetic dot arrays

The effects of dipolar interaction in an array of small magnetic dots with perpendicular anisotropy are studied numerically within the framework of the Landau–Lifshitz–Gilbert equation. In the absence of a magnetic field, three typical configurations of the magnetic moments are found, depending on the dipolar coupling strength. Magnetization processes both parallel and perpendicular to the array are studied. The hysteresis loops are found to be highly sensitive to the dipolar coupling strength. The in-plane hysteresis loops are dominated by the strong dipolar interaction between particles, while the out-of-plane hysteresis loops are dominated by the perpendicular anisotropy. The coercive field, saturation field, and remanence are also sensitive to the strength of dipolar interaction. Dipole interactions also affect the characteristic switching time in a coupled array. Depending on the packing density of the dots in an array, the switching time may be shortened or lengthened.

Influence of dipolar interactions on the conduction mechanism of Li +-β-alumina: Molecular Dynamics study

We present Molecular Dynamics simulation results of stoichiometric (s; Li 2Al 22O 34) and nonstoichiometric (ns; Li 2+0.43Al 22O 34+0.21) Li +-β-alumina in the temperature range 300–700 K. Structural properties and ion migration are studied. Particular attention is paid to the role of polarization interactions. All types of ions are considered to be polarizable. Induced dipole moments are calculated in two different ways. For oxygen ion polarizabilities less than 0.3 Å 3 an iterative method is used. For higher polarizability, a switching function is introduced to prevent a polarization catastrophe. The induced dipole moments are determined through an equation of motion. The simulations confirm the picture in which Li + ions move out of the conduction plane attaching themselves to the spinel blocks. A competition between the dipolar interactions of the Li + with each other and with the highly polarized oxygen ions situated in the spinel blocks near the conduction planes was observed. At oxygen ion polarizabilities less than 3.0 Å 3, the Li + dipole moments show a head to tail arrangement in the x– y plane, which lowers the Li +–Li + dipolar energy, stimulates the collective motion and therefore the self-diffusion. However, at a polarizability of 3.5 Å 3, the dipolar interactions of the oxygen ions become dominant, and the head to tail pattern vanishes. The Li + are now attached to the spinel blocks, leading to a decrease of the Li + ion diffusivity.

Fabrication and characterization of tunable magnetic nanocomposite materials

ABSTRACTFerromagnetic nanocomposites in which magnetic nanoparticles are embedded into a polymeric matrix can replace conventional ferrites in the near future in applications such as: filters, high frequency inductors, chokes, sensors, core-shape and planar transformers, hybrid circuits and transponders. These dense magneto-dielectrics will provide a new approach in the fabrication of soft magnetic materials. In a magnetic/polymeric nanocomposite solid, the resistivity can be drastically increased, leading to significantly reduced eddy-current losses. In addition, the coupling between neighbouring magnetic nanoparticles results in much better soft magnetic properties at high frequencies than those of conventional bulk materials or ferrites. In order to study the influence of dipolar interactions between ferritic nanoparticles, samples of varying ferritic density were prepared. A polymeric binder (pre-swollen in toluene) was added to the nanoparticles, which were then cold-pressed using a standard compaction method. Characterization of the materials was carried out by means of x-ray diffractometry, electron microscopy, magnetometry, and high-frequency complex permeability measurements. Initial results show that tunable static and dynamic magnetic properties of the nanocomposite materials may be achievable.

Spin dynamics in Ni with SANS: dipolar effects and parallel fluctuations

The critical scattering of a Ni single crystal has been investigated by means of small-angle neutron scattering (SANS) below the ferromagnetic transition. The data have been interpreted with a recent mode-coupling theory that includes the dipolar interactions. The explicit susceptibility of the parallel fluctuations as well as the influence of dipolar interactions have been taken into account.

The Influence of Dipolar Interactions between Mesogenic Units on the Aggregation Behavior of a Series of Isomeric Double-Chained Ammonium Amphotropes

A series of novel double-chained ammonium amphotropes has been synthesized which differ in the dipole orientation of their mesogenic units along the long molecular axis. The mesogenic units consist of an azobenzene core which has an electron donating oxy unit at the 4-position and an electron withdrawing carboxy unit at the 4‘-position. This mesogenic unit has an intrinsically high dipole moment. The effects of the orientation of the dipoles of the mesogens on the thermotropic and lyotropic aggregation behavior of these compounds have been investigated. The thermotropic properties can be rationalized by assuming that the antiparallel orientation of the mesogenic units is the most favorable one. Compound 1, which has both dipoles pointing away from the headgroup, gives almost fully extended bilayers with parallel mesogens upon sonication in water. Compounds 2, with alternating dipoles, and 3, which has both dipoles pointing toward the headgroup, give interdigitated bilayers. The compounds also form monolay...

Long-range ferromagnetism in hybrid compounds: The role of dipolar interactions

The influence of dipolar interactions, as compared with quantum through-bond exchange coupling, is discussed in hybrid organic–inorganic layered ferromagnets with large interlayer spacing. A model is proposed to explain the occurrence of 3D ferromagnetic order, with a sizeable transition temperature, despite the fact that exchange interactions between layers are negligible. In such low-dimensional systems, it is shown that the exponential divergence of the in-plane correlation length at low temperature promotes superspins within the magnetic layers which couple ferromagnetically through sizeable dipolar interactions. The model evidences the occurrence of a spontaneous magnetisation for a critical temperature that depends weakly on the distance between magnetic layers but mostly on the rate of divergence of the 2D correlation length, i.e. on the strength of the in-plane exchange coupling. The behaviour of copper(II) layered ferromagnets with an interlayer spacing ranging from 29 to 40 Å and Curie temperatures ranging from 21 to 19 K, respectively, is shown to be well described by such a model.

Aging in a magnetic particle system.

The influence of dipolar interaction in a frozen ferrofluid has been experimentally studied. The ferrofluid consisted of particles of $\ensuremath{\gamma}$- ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ with mean diameter 70 \AA{}. Four samples with volume concentration of magnetic particles ranging from 0.03% to 17% have been investigated. The magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field---the system ages. The most diluted sample shows isolated particle dynamics and no aging.

Dipolar interactions in deuteron spin systems. I. Spin diffusion

The influence of dipolar interactions on the longitudianl relaxation of deuteron spin systems is investigated. Spin diffusion rates are evaluated, including approximate rates due to double quantum spin diffusion and three-spin flip–flop transitions. It is shown that slow molecular rotations in supercooled liquids do not affect the spin diffusion rates significantly provided that the motional correlation times are below the average spin lattice relaxation time T1 which becomes on the order of one second close to the glass transition temperature Tg. However, the broad distribution of deuteron T1 values at T<Tg results in a large effect of spin diffusion upon the long time decay of the longitudinal magnetization in T1 experiments. These effects are estimated in terms of a simple model in agreement with recent experiments. It is also shown that the initial decay determining the average rate 〈T−11〉 remains unaffected by spin diffusion. Finally, we show that small amplitude motions on a time scale of 10−6–10−3 s may cause temperature dependent spin diffusion effects.

Susceptibility phenomena in a fine particle system

Abstract The concentration dependence of the peak in the dc susceptibility of a weakly interacting fine particle system consisting of Fe 3 O 4 particle has been studied. Measurements of the initial susceptibility of the dispersion in the solid state show that the susceptibility values were depressed at low temperatures. The temperature of the peak ( T g ) in the susceptibility versus temperature curve was found to occur at higher temperatures as the concentration was increased. The variation of T g with concentration shows a simple power law relationship. The influence of dipolar interactions between the particles on the value of T g is discussed by considering the effects of particle interactions on the blocking temperature. In this study we develop an effective blocking temperature model, and apply it to our system.