Characteristic Relaxation Research Articles

The timescale identification decoupling complicated kinetic processes in lithium batteries

<h2>Summary</h2> A comprehensive understanding of multiple Li kinetics in batteries is essential to break the limitations of mechanism study and materials design. Various kinetic processes with specific relaxation features can be clearly identified in timescales. Extracting and analyzing the timescale information in batteries will provide insights into investigating kinetic issues such as ionic conductions, charge transfer, diffusions, interfacial evolutions, and other unknown kinetic processes. In this regard, the timescale identification is an important method to combine with the non-destructive impedance characterizations in length scale for online battery monitoring. This perspective introduces and advocates the timescale characterization in the views of the basic timescale property in batteries, employing the concept of distribution of relaxation time (DRT) and presenting successful applications for battery diagnosis. In the future, we suggest that timescale characterizations will become powerful tools for data extraction and dataset building for various battery systems, which can realize data-driven machine learning modeling for practical application situations such as retired battery rapid sorting and battery status estimations.

Dynamic characterization of multiphase methane during CO2-ECBM: An NMR relaxation method

Injecting CO2 into coal reservoirs is essential both for enhancing coalbed methane recovery (CO2-ECBM) and CO2 geological sequestration. However, due to the difficulty in simultaneously monitoring multiphase methane (adsorbed and free phases) by conventional testing methods, the real-time variations in different phases of methane throughout the whole process of CO2-ECBM still remains to be clarified. In this study, with the introduction of self-designed nuclear magnetic resonance (NMR) measurement, three subbituminous and anthracite coals collected from deep-well drilling were used to investigate the dynamic characteristics of multiphase methane in CO2-ECBM. The results show that the adsorbed methane desorption efficiency could be additionally improved by ∼ 14%–26% with the injection of CO2 after conventional reservoir depressurization. In the process of CO2-ECBM, three different CO2-CH4 displacement rates emerged: the rate rapidly decreased during the initial CO2 injection time period and then began to slowly decrease until reaching CO2-CH4 competitive-adsorption equilibrium. Comparison of the NMR transverse relaxation (T2) characteristics of CO2-ECBM after natural and in-situ methane adsorption suggests that a low concentration ratio of CH4/CO2 could significantly improve the methane recovery. The promising results from the CO2-CH4 displacement experiments demonstrate the great significance of the CO2-ECBM method in enhancing methane recovery and CO2 geological sequestration for coals.

Effect of oxygen vacancy on the dielectric relaxation characteristics of PMNT ceramics

The as-sintered 0.68 Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 ceramics were firstly prepared using the Columbite technique and then further annealed in specific atmosphere. The effect of annealing atmospheres, e.g. O2, N2 and air conditions, on the phase structure, microstructure and dielectric properties of ceramics were systematically discussed. The obtained results suggested that, compared to as-sintered sample, there is no obvious difference in phase structure and microstructure for ceramics annealed in O2 atmosphere, showing pure perovskite structure and higher density. While the appearance of second pyrochlore phase and porosity can be found as sample was annealed in nitrogen or air condition. It is worth noting that the dielectric properties of ceramics are very sensitive to the annealing atmosphere and there is a significant increase in dielectric constant and decrease in dielectric loss as the PMNT ceramic was treated in O2 atmosphere, while sample annealed in N2 or air condition would show the opposite trend.

Insights into ion transport in biodegradable solid polymer blend electrolyte based on FTIR analysis and circuit design

This study reports the use of the solution casting method to prepare a biodegradable polymer blend electrolyte composed of chitosan (CS) and potato starch (PS). Different concentrations of potassium thiocyanate (KSCN) salt as an ionic provider were added to the CS:PS host. The results of X-ray diffraction (XRD) revealed a substantial reduction in the crystalline phase of the host polymers of up to 40 wt% of the added KSCN salt. Fourier transform infrared (FTIR) spectroscopy was used to detect microstructural modifications to the films to explore the complexation between the blended polymer chains and ions of the salt. Deconvoluted FTIR spectra were used to compute the free ions, contact ion pairs, and ion aggregates, which were then used as a sensitive method to determine the parameters of ion transport, including mobility (μ), carrier density (n), and the diffusion coefficient (D). Electrochemical impedance spectroscopy (EIS) was used based on electrical equivalent circuit modeling to study the electrical properties of the electrolyte films. A circuit design for each electrolyte was presented based on the curves of fitting of the EIS data. The parameters associated with elements of the circuit were of significant interest, especially for determining the conductivity of the films. The sample with the highest KSCN concentration of 50 wt% exhibited a reduction in conductivity due to the formation of ion pairs, the recrystallization of salt, and a decline in the amorphous fraction of the system, which was also visualized from its surface morphology by using an optical microscope. An analysis of properties of the dielectric manifested an enhancement upon the addition of salt, with characteristics of dispersive relaxation that verified the non-Debye behavior of the solid polymer electrolyte films. This behavior was further confirmed through the appearance of a distorted arc in the Argand plot. A decrease in the relaxation time was noted with the addition of salt, where this was in agreement with the results of EIS and AC conductivity measurements. An examination of the electric modulus revealed the process of viscoelastic relaxation whereby ion hopping was supported by coupling with segmental chain dynamics.

NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers.

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of 1H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole–Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

Effects of type 2 diabetes on the viscoelastic behavior of human trabecular bone

SummaryType 2 diabetes (T2D) is a well-known disease that impaired bone mechanical properties and increases the risk of fragility fracture. The bone tissue is a viscoelastic material that means the loading rate determines its mechanical properties. This study investigates the impact of T2D on the viscoelastic properties of human bone and its association with microstructure and biochemical properties. IntroductionViscoelasticity is an important mechanical property of bone and for this the interaction of individual constituents of bone plays an important role. The viscoelastic nature of bone can be affected by aging and diseases, which can further influence its deformation and damage behavior. MethodsThe present study investigated the effects of T2D on the viscoelastic behavior of trabecular bone. The femoral heads of T2D (n = 26) and non-T2D (n = 40) individuals with hip fragility fractures were collected for this investigation. Following the micro-CT scanning of all bone samples, the stress relaxation and dynamic mechanical analysis (DMA) tests were performed to quantify the viscoelasticity of bone. Further, a correlation analysis was performed to investigate the effects of alteration in bone microstructural and biochemical parameters on viscoelasticity. ResultsThe stress relaxation and frequency sweep responses of T2D and non-T2D trabecular bone specimens were not found significantly different. However, the storage modulus, initial stiffness, and initial stress were found lower in T2D bone. The significant correlation of percentage stress relaxed is obtained between the mineral content (r= - 0.52, p-value = 0.003), organic content (r = 0.40, p-value = 0.02), and mineral-to-matrix ratio (r = - 0.43, p-value = 0.009). Further, storage and loss modulus were correlated with bone volume fraction (BV/TV) for both groups. The stress relaxation and frequency sweep characteristics were not found significantly connected with the other chemical, structural, or clinical parameters. ConclusionThis study suggests that T2D does not affect the time-dependent response of human femoral trabecular bone. The viscoelastic properties are positively correlated with organic content and negatively correlated with mineral content.

Energy dissipation during collision for anti-relaxation coatings in alkali-metal vapor cells

Abstract Anti-relaxation surface coatings can reduce collisions between alkali-metal atoms and cell walls, thereby prolonging the spin-polarization lifetime of alkali-metal atoms. This phenomenon can considerably improve the performance of quantum sensors. Researchers have focused on the surface topography and composition of anti-relaxation coatings in alkali-metal vapor cells and on methods that provide an increase in the relaxation time of alkali metals. We proposed a novel method to characterise the coating performance of alkali-metal cells by directly measuring the energy dissipation during collisions between the anti-relaxation coating surface and atoms in the cell. We found that the interactions between alkali-metal atoms and the anti-relaxation coating resemble elastic collisions, which can be indirectly characterised by the energy dissipation during interactions between an atomic force microscopy tip and atoms. In addition, we confirmed that the energy dissipation between collisions is positively correlated with the relaxation characteristics of alkali metals at various temperatures. This discovery is of great significance as it can be utilised to characterise the anti-relaxation coating performance of alkali-metal cells by directly analysing the energy dissipation of anti-relaxation coatings instead of measuring the relaxation time.

The kinetics of nanostructural relaxation in electrodeposited Ni upon low-temperature annealing: an in-situ X-ray diffraction investigation

The rather limited microstructural evolution in electrodeposited nanocrystalline Ni isothermally annealed at 453–493 K is investigated by using in situ X-ray diffraction technique. The nanostructural relaxation features roughly linear lattice contraction, based on which the kinetics analysis yields the activation energy of about 0.57 eV/atom, in good agreement with the fast diffusion of excess vacancies on the nonequilibrium surfaces and/or interfaces for Ni. The lattice contraction of Ni may result from a reduction in the grain boundary excess volume. The integral breadth of diffraction line profiles decreases with annealing time in an exponential manner, implying the gradual improvement of the lattice perfections during nanostructural relaxation. The microhardness evaluation demonstrates the initial invariance or slight softening and subsequent remarkable hardening of nanocrystalline Ni subjected to low-temperature annealing.

Impact of silver ions on dielectric properties and conductivity of lithium silicate glass system mixed with red lead

Glasses of the composition (40-x)Li2O-10Pb3O4-50SiO2 doped with different contents of Ag2O were fabricated and were subsequently subjected to heat-treatment. The quantitative analysis of the results of different characterization techniques indicated increasing magnitude of internal connectivity of the glass network with increasing Ag2O content. Several dielectric parameters viz., ε′, M, Z and σacin the frequency (ω) region 0.01 Hz – 1 MHz and in the region of temperature (T) 30 –250 °C of these glasses were measured as functions of Ag2O content. ε′ showed a decreasing tendency with Ag2O content which may be attributed to the decrease in the space charge polarization (scp) due to the increasing order of augmentation of glass network. Such a decrease of scp was attributed to the growing presence of PbO4 units and also Ag0 particles in these samples. The plots of electric modulus vs ω and T indicated relaxation character. Such relaxation phenomena were discussed in detail with the help of Cole-Cole diagrams. Probable dipoles responsible for such effects were recognized and analyzed. σac (conductivity) exhibited a decreasing trend with Ag2O content. The mechanism of conduction was analyzed using polaronic tunneling effect in the intermediate ω and high T ranges, whereas in the low T range it is predicted to follow QMT model. These conclusions were observed to follow the results of various spectroscopic studies. In conclusion, presence of red lead seemed to be an obstruction for the conductivity in the fast ion conducting glasses.

Estimating Water Content in Water–Oil Mixtures and Porous MEDIA They Saturate: Joint Interpretation of NMR Relaxometry and Dielectric Spectroscopy

This article deals with the topical problem of estimating water content in water–oil mixtures within porous media they saturate, according to low-field NMR relaxometry and dielectric spectroscopy. The aim of the research is experimental validation of the capability of complex data interpretation to acquire information on the filtration-volumetric properties of drill cuttings, relaxation characteristics of oil-containing fluids, the water/oil ratio in water–oil mixtures, and their saturation of drill cuttings to control the composition of liquids produced from boreholes. The studies are carried out on samples of cuttings and oils taken from fields in the Northern regions of the West Siberian oil-and-gas province, where NMR studies have not been performed before. Based on the experimental data obtained, the possibility of water content assessment in water-in-oil mixtures and porous media they saturate were proved through NMR relaxometry. With the use of the proposed methodology, the amount of water in oil–water mixtures was established, and their main NMR characteristics were determined. The relative error in evaluating the proportion of water in mixtures based on high-viscosity oils is less than 10%, and about 20% for those based on light oils. When determining the oil–water ratio in the pore space of the drill cuttings, the error is about 15%. It was proven that joint use of these two techniques makes it possible to increase the reliability of the oil–water ratio assessment of all the samples studied. Furthermore, it was revealed that the NMR spectrum shifts to the right, and the spectrum of the complex permittivity shifts downwards during the transition from high-viscosity oils to light ones.

Non-Fourier modeling and numerical simulations on heat and transfer in tangent hyperbolic nanofluid subjected to chemical reactions

Two-phase chemical reactions (homogeneous-heterogeneous) in tangent hyperbolic fluid with nanoparticles subjected to heat transfer are modeled using mathematical laws and models. The solutions of governing models are derived by the finite element method (FEM). The solutions are checked to be grid-independent and convergent solutions are obtained which are further used for simulations of effects of various parameters. Shear-thinning and shear-thickening behaviors are predicted from the FEM solutions for the various values of the power-law index and other parameters. The tangent hyperbolic rheological behavior is determined through variation of Wessienberg number. The diverse behavior of fluid motion for the values of the Wessienberg number is observed. Electric current experiences resistance by the particles of the fluid. Hence, more heat is dissipated by the Ohmic mechanism. It is also noted that hybrid nanofluid dissipates more heat than that by mono nanofluid. The Lorentz force induced by the motion of pure and nanofluid is less than that induced by the motion of fluid with hybrid nanoparticles. Thermal relaxation characteristics reinforces the fluid in maintaining its thermal equilibrium and therefore temperature field for both types of fluids has to show a decreasing behavior for an increase in thermal relaxation parameter. The increasing behavior of homogeneous chemical reaction strength on concentration profile is noted whilest the oppositein concentration profile trend is observed for heterogeneous chemical reaction strength.

On the Cattaneo–Christov heat flux theory for mixed convection flow due to the rotating disk with slip effects

The maximum heat conduction difficulty has been recently termed and scrutinized using Fourier’s law. It is acknowledged that this established diffusion theory may collapse when one is absorbed in the transient problems in a minimal period, extraordinary flux, or for precise low temperatures. In this article, a mechanism of heat flux model with a mixed convective flow field is considered. Here fluid motion under the applied magnetic field and velocity slip condition take place due to the rotating disk. After using the Cattaneo–Christov approach, the heat equation is derived for the heat flux model. The thermal relaxation features are examined in detail. The modeled partial differential framework is reduced to an ordinary differential framework through an appropriate transformation, which is solved numerically by implementing the shooting technique with Runge–Kutta–Fehlberg method. The velocity and fluid temperature are simulated for involved physical parameters. The heat generation/absorption factor has a reverse impact on the temperature field. Furthermore, the fluid temperature decays for the thermo-relaxation factor. The results for skin friction and Nusselt number are also computed.

Modeling of Λ-graded InxGa1−xN solar cells: comparison of strained and relaxed features

The optical properties of Λ-graded indium gallium nitride (InGaN) solar cells are studied. Graded InGaN well structures with the indium composition increasing to xmax and then decreasing in a Λ-shaped pattern have been designed. Through polarization doping, this naturally creates alternating p- and n-type regions. Separate structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations ranging from 20% to 90%, while maintaining a constant overall structure thickness of 100 nm. The solar cell parameters under fully strained and relaxed conditions are considered. The results show that a maximum efficiency of ≅5.5 % under fully strained condition occurs for xmax = 60 % . Solar cell efficiency under relaxed conditions increases to a maximum of 8.3% for xmax = 90 % . Vegard’s law predicts the bandgap under relaxed conditions, whereas a Vegard-like law is empirically determined from the output of nextnano™ for varying indium compositions to calculate the solar cell parameters under strain.

Dynamic Characteristic Master Curve and Parameters of Different Asphalt Mixtures

Using an AMPT tester and based on laboratory tests, this paper performed a comparative study on the dynamic characteristics of different asphalt mixtures, analyzed the influence of different asphalt binders on the characteristic parameters of the dynamic modulus master curve and the phase angle master curve of asphalt mixture, and expounds the evaluation function of the phase angle master curve for mixture relaxation characteristics. The results show that the modulus master curve parameters of the asphalt mixture are closely related to voids in the mineral aggregate, mixture density, and asphalt content of the asphalt mixture. For the same kind of asphalt mixture, because the gradation of mineral aggregate is fixed and the volume parameters are almost the same, the ultimate modulus of the mixture at different temperatures is unique; when the temperature changes or the asphalt changes, the shape parameter β of the modulus master curve changes regularly, which brings different dynamic responses, and the lower β will show the characteristics of a higher modulus. Asphalt is the source of the viscoelasticity of the asphalt mixture. Although the influence of particle gradation of the mixture will bring about the change of modulus, the phase angle of the mixture depends on the viscoelastic properties of asphalt, and the initial phase angle in the main curve is positively correlated with asphalt penetration and negatively correlated with the softening point and viscosity, while the peak phase angle A is negatively correlated with penetration, and the softening point viscosity is positively correlated. The viscoelastic interval, represented by ω, is negatively correlated with penetration but positively correlated with the softening point and viscosity. The peak position, parameter ωc, of the phase angle master curve can evaluate the relaxation characteristics of the mixture, and the crack resistance of different mixtures can be compared without complex model calculation. In the comparison of the relaxation time of asphalt mixture, the relaxation time of foam cold-recycled mixture is the largest, which is significantly higher than that of other forms of cement mixture; the emulsified asphalt cold-recycled mixture is equivalent to AC20 and LSPM30 mixtures; the SBS-modified asphalt mixture has the best relaxation characteristics.

Significance of Lorentz forces on Jeffrey nanofluid flows over a convectively heated flat surface featured by multiple velocity slips and dual stretching constraint: a homotopy analysis approach

Abstract Motivated by the temporal relaxation feature of the Jeffrey model and its practical uses in the rheological modeling of several vital liquids, this study aimed to present a theoretical analysis of three-dimensional MHD Jeffrey nanofluid flows over a dual stretching surface with velocity slip conditions. By adopting the nonhomogeneous nanofluid model along with the passive control approach of nanoparticles, the current flow problem is solved semi-analytically via the homotopy analysis method for convective heating and multiple slip conditions. Dynamically, the magnetic and viscoelastic parameters have a declining effect on the velocity distributions in both directions in the existence and absence of slip effects, while the Deborah number has generally an escalating influence on the flow distributions. On the other hand, the variations of the velocity profiles in both directions are always greater in the presence of slip effect as compared to the nonslip case. Besides, the velocity stretching factor rises the velocity profiles in both directions. Furthermore, this increasing impact is dominant for the velocity distribution along the $y{\rm{-}}$direction as compared to the velocity field along the $x{\rm{-}}$direction. Thermally, the greater Biot number increases the temperature distribution. However, the bigger Schmidt number reduces the concentration distribution.

Numerical study on non-Fourier heat and mass transfer in partially ionized MHD Williamson hybrid nanofluid

This article aims to analyze the combined effects of Hall and ion slip currents, Darcy-Forchheimer porous medium, and non-uniform magnetic field under the suspension of hybrid nanoparticles. The flow situation is transformed into mathematical problems using conservation laws and correlations among thermophysical properties of base fluid nano and hybrid nanoparticles. These mathematical models are approximated under boundary layer (BL) approximations. The similarity analysis is performed and the finite element method (FEM) is deployed for the simulations and further investigations regarding non-Fourier behavior of heat and mass transfer. The present results have good agreement with the published benchmark which is a special case of present work. The numerical simulations for the motion of fluid particles under the variation of Weissenberg number are observed and it is noticed that fluid motion decelerates for higher values of Weissenberg number. Significant retardation to the motion of the fluid is seen because of an increase in the Darcy porous medium parameter as an increase in it implies a decrease in the permeability of the porous. Thermal relaxation is the property of the fluid due to which it avoids thermal changes to maintain thermal equilibrium. Therefore, a remarkable decrease in the temperature of fluid exhibiting thermal relaxation characteristics is noticed. It is also noted from numerical experiments that Hall and ion slip currents are responsible to slow down the Ohmic dissipation phenomenon. Thus the property of ionized fluid helps in controlling the thickness of thermal BL. Numerical experiments have shown that the solutal relaxation time for hybrid nanofluid is greater than that for mono nanofluid. Mass diffusivity of the fluid with hybrid nanofluid is less than that of mono nanofluid.

Test for stress relief cracking susceptibility in creep resistant chromium-molybdenum steels

An externally restrained stress relief cracking test was developed and demonstrated in testing susceptible and resistant to cracking welds in Cr–Mo steels. Compared to other externally restrained tests, it simultaneously applies stress and compensates thermal expansion during heating to post-weld heat treatment temperature and utilises digital image correlation for quantification of key characteristics of the stress relaxation and stress relief cracking phenomena. In contrast with resistant to stress relief cracking materials, susceptible materials experienced lower levels of stress relaxation, strain absorption, and sustained mechanical energy, with accelerated kinetics of strain accumulation and strain localisation leading to failure. The processes of stress relief cracking and stress relaxation were quantified as low strain – slow strain rate – low energy phenomena.

Simulation methodology development of warpage estimation for epoxy molding compound under considerations of stress relaxation characteristics and curing conditions applied in semiconductor packaging

The thermal mismatch between different components has been a serious issue in the reliability of electronic products. The effect of epoxy molding compound (EMC) on the warpage of electronic package should be considered. The finite element method (FEM) is widely adopted to predict the warpage induced by the molding process. However, no previous studies focused on the warpage during the molding process and post process. In this study, several material characteristics of the EMC, such as cure reaction, cure shrinkage, and viscoelasticity, are measured. This study analyzes the corresponding mathematical model completely and substitutes the above properties into the FEM while the process conditions are considered through measurement. A process-oriented simulation methodology is proposed, and the estimated warpage results show that the simulation methodology with stress relaxation of EMC is in good agreement with the experimental value. The characteristics of stress relaxation behavior undergoing the molding process are discussed through the evolution of warpage. The warpage is different from the two process steps at room temperature due to the stress relaxation. This study demonstrates how the conditions of the post molding cure (PMC) process affect the warpage. The warpage can decrease with the extension of PMC time but the degree of reduction has a limit.

Enhanced dielectric, ferroelectric, and ferromagnetic properties of 0.7Bi1−xTmxFeO3–0.3BaTiO3 ceramics by Tm-induced structural modification

In this study, Tm-doped 0.7Bi1−xTmxFeO3–0.3BaTiO3 (x = 0–0.05) ceramics have been fabricated using traditional solid-phase reactions and their structural, morphological, and dielectric as well as multiferroic properties are investigated. X-ray diffraction and Rietveld refinement indicated the formation of a Tm-modulated morphotropic phase boundary (MPB) from the rhombohedral (R3c) and tetragonal (P4mm) phases to a single P4mm phase for Tm content of x = 0.02–0.03. Tm occupation of lattice sites is strongly suggested at both the A and B sites in the ABO3-type perovskite structure, which is supported by the obtained Raman spectra, the calculated tolerance factor, and dielectric relaxation characteristics. With the Tm-induced aforementioned–structural modification, the dielectric constant, remnant polarization (Pr), and remnant magnetization (Mr) values indicate that the dielectric and multiferroic properties are greatly improved. The obtained maximum remnant polarization (2Pr = 35.28 μC/cm2) and remnant magnetization (2Mr = 0.56 emu/g) are approximately two and seven times larger, respectively, than those of the sample without Tm substitution (x = 0). With increasing the Tm content, the remnant polarization, and the remnant magnetization exhibit almost the same trends in variation, indicating the possible existence of a relatively strong magnetoelectric coupling effect. Our study may provide a substitution strategy (structural modification by substituting the same element into two different lattice sites) to efficiently enhance the dielectric and multiferroic properties of BiFeO3–BaTiO3-based ceramics.

Magnetic relaxation in two dimensional assembly of dipolar interacting nanoparticles

We perform extensive kinetic Monte Carlo simulations to investigate the magnetic relaxation characteristics in a two-dimensional (Lx×Ly) assembly of magnetic nanoparticles (MNPs) as a function of dipolar interaction strength hd, aspect ratio Ar=Ly/Lx, and temperature T. The anisotropy axes are taken as randomly oriented in three-dimensional space. In the presence of small dipolar interaction (hd≤0.3) and substantial temperature, the magnetic relaxation follows the Néel Brown model as expected. Interestingly, the dipolar interaction of enough strength is found to induce antiferromagnetic coupling in the square arrangement of MNPs (Ar=1.0), resulting in the fastening of magnetic relaxation with hd. There is also a rapid increase in relaxation even with Ar<100 above a particular dipolar interaction strength hd⋆, which gets enhanced with Ar. Remarkably, magnetization relaxes slowly with hd for the highly anisotropic system, i.e. Ar>100. It is because the dipolar interaction induces ferromagnetic interaction in such a case. The temperature also affects relaxation drastically. For weak dipolar interaction, magnetization relaxes rapidly with T because of enhancement in thermal fluctuations. The effective Néel relaxation time τN also depends strongly on these parameters. In the presence of strong dipolar interaction (hd>0.3) and Ar=1.0, τN decreases with hd for a given temperature. On the other hand, there is an increase in τN with hd for huge Ar(>100). The comparison of relaxation behaviour and τN with aligned anisotropy cases also reveals a strong dependence of these characteristics on the anisotropy axes orientation. These results are beneficial in data storage and other spintronics based applications.