Magnetic Anisotropy Of Nanoparticles Research Articles

Testing the validity of the core-shell-surface layer model on the size dependence of effective magnetic anisotropy in magnetic nanoparticles

The stability of the stored information in magnetic recording media depends on the anisotropy energy Ea (=KeffV) of nanoparticles (NPs) of volume V or diameter D. Therefore, it is important to know how the effective anisotropy constant Keff varies with size D of the NPs. In a recent paper [Appl. Phys. Lett. 110, 222409 (2017)], the observed Keff versus D variation in NPs of maghemite (γ-Fe2O3) was explained on the basis of the core-shell-surface layer (CSSL) model given by Eq.: Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]−3–1}, where Kb,KS, and Ksh are the anisotropy constants of spins in the core, surface layer, and a shell of thickness d, respectively. This CSSL model is an extension of an earlier core-surface layer (CSL) model described by Keff = Kb + (6KS/D) [Phys. Rev. Lett. 72, 282 (1994)] proposed to explain the Keff versus D variation in Fe NPs. For the NPs of γ-Fe2O3, the additional term of the CSSL model involving Ksh was found to be necessary to fit the data for sizes D < 5 nm. In this paper, we report the validity of the CSSL model for NPs of several other systems viz. Co, Ni, NiO, and Fe3O4 using the available data from literature. In selecting the data, care was taken to consider data only for non-interacting NPs since the interparticle interactions generally overshadow the actual value of Keff in NPs. It is shown that the new CSSL model describes very well the Keff vs. D variation for all particle sizes whereas the CSL model fails for smaller particles with the notable exception of Fe NPs. This validation of the CSSL model for the NPs of Co, Ni, NiO, Fe3O4, and γ-Fe2O3 suggests its general validity for magnetic NPs. Discussion is also presented on the comparative magnitudes of the parameters Kb, KS, and Ksh obtained from the fits to the CSSL model.

Optimization of magnetic fluid hyperthermia with respect to nanoparticle shape-related parameters: case of magnetite ellipsoidal nanoparticles

Issues related to the optimization of heat transfer mechanisms dominated by superparamagnetic relaxation are considered in the case of AC (alternating current) magnetic field hyperthermia procedures. The key role in the conversion of electromagnetic energy to the thermal one via the superparamagnetic relaxation mechanism is played by the magnetic anisotropy of nanoparticles, easily to be controlled via the shape anisotropy component. The optimization process has been discussed in the case of magnetite (Fe3O4) ellipsoidal nanoparticles with dominant shape anisotropy dispersed in different media. Nanoparticles of different sizes and aspect ratios have been considered in correlation with those specific parameters of the actuating AC magnetic field which respect an established biological safely criterion. It has been proven that the dissipated power can be maximized for a given set of biological compatible RF (radiofrequency) field parameters (frequency and field amplitude at the sample space) only for specific pairs of particle sizes and aspect ratios. For instance, it has been shown that ellipsoidal magnetite nanoparticles with 10 nm equatorial size and aspect ratio of 2 are optimal for a maximum transferred power under radiofrequency excitations of 250 kHz and field amplitude of 20 kA/m, if high viscosity dispersion media are used. The methodology for deriving the optimal shape (geometrical) parameters of a specific type of nanoparticles in conditions of using available radiofrequency excitations, or vice versa, for deriving the optimal radiofrequency working parameters in the case of ferrofluids with specific nanoparticles (type and geometry) is described and discussed in detail.

Enhanced magnetic performance of aligned wires assembled from nanoparticles: from nanoscale to macroscale.

Magnetic wires in highly dense arrays, possessing unique magnetic properties, are eagerly anticipated for inexpensive and scalable fabrication technologies. This study reports a facile method to fabricate arrays of magnetic wires directly assembled from well-dispersed α″-Fe16N2/Al2O3 and Fe3O4 nanoparticles with average diameters of 45 nm and 65 nm, respectively. The magnetic arrays with a height scale of the order of 10 mm were formed on substrate surfaces, which were perpendicular to an applied magnetic field of 15 T. The applied magnetic field aligned the easy axis of the magnetic nanoparticles (MNPs) and resulted in a significant enhancement of the magnetic performance. Hysteresis curves reveal that values of magnetic coercivity and remanent magnetization in the preferred magnetization direction are both higher than that of the nanoparticles, while these values in the perpendicular direction are both lower. Enhancement in the magnetic property for arrays made from spindle-shape α″-Fe16N2/Al2O3 nanoparticles is higher than that made from cube-like α″-Fe16N2/Al2O3 ones, owing to the shape anisotropy of MNPs. Furthermore, the assembled highly magnetic α″-Fe16N2/Al2O3 arrays produced a detectable magnetic field with an intensity of approximately 0.2 T. Although high-intensity external field benefits for the fabrication of magnetic arrays, the newly developed technique provides an environmentally friendly and feasible approach to fabricate magnetic wires in highly dense arrays in open environment condition.

The effect of magneto-crystalline anisotropy on the properties of hard and soft magnetic ferrite nanoparticles.

Recent advances in the field of magnetic materials emphasize that the development of new and useful magnetic nanoparticles (NPs) requires an accurate and fundamental understanding of their collective magnetic behavior. Studies show that the magnetic properties are strongly affected by the magnetic anisotropy of NPs and by interparticle interactions that are the result of the collective magnetic behavior of NPs. Here we study these effects in more detail. For this purpose, we prepared CoxFe3−xO4 NPs, with x = 0–1 in steps of 0.2, from soft magnetic (Fe3O4) to hard magnetic (CoFe2O4) ferrite, with a significant variation of the magnetic anisotropy. The phase purity and the formation of crystalline NPs with a spinel structure were confirmed through Rietveld refinement. The effect of Co doping on structure, morphology and magnetic properties of CoxFe3−xO4 samples was investigated. In particular, we examined the interparticle interactions in the samples by δm graphs and Henkel plots that have not been reported before in literature. Finally, we studied the hyperthermia properties and observed that the heat efficiency of soft Fe3O4 is about 4 times larger than that of hard CoFe2O4 ferrite, which was attributed to the high coercive field of samples compared with the external field amplitude.

Magnetization reversal in ferromagnetic Fibonacci nano-spirals

Abstract Nanostructured ferromagnetic materials are applicable in a broad range of technologies, especially in the areas of data storage and spintronics. In addition, special structures are of high interest in basic research, aiming at understanding magnetization reversal processes on the nano-scale and thus possibly creating new applications. The strong influence of the shape anisotropy in magnetic nanoparticles enables tailoring magnetization reversal processes by the shape of such a particle. Typically, symmetrical structures are investigated, such as nano-dots, doughnuts, squares, rectangles, etc. Here we report on spiral nano-structures with constantly varying bending radii, modelled according to the Fibonacci spiral which is well-known from diverse plants and biological processes and also related to many mathematical problems. The magnetic Fibonacci nano-spirals with a thicker and a thinner lateral structure size as well as a mirrored form, building a heart-like shape, show large numbers of steps in the hysteresis loops, corresponding to stable intermediate states, due to nucleation and annihilation of domains which do not propagate along the spirals. The numbers of these stable intermediate states which could be utilized for data storage as well as the coercive fields depend strongly on the spiral dimensions and the orientation of the external magnetic field. Simulating minor loops showed the high stability of the intermediate states and thus underlined the possibility to use such structures for data storage.

Magnetization of superparamagnets in the state of mechanical anisotropy

The internal energy of magnetic anisotropy in some particles dominates over the thermal energy, even at room temperature. The existence of strong magnetic anisotropy of nanoparticles can significantly affect the process of magnetization of superparamagnets. However, if the axes of magnetic anisotropy of nanoparticles are randomly oriented, then their presence does not affect the process of magnetization, which occurs according to the classical Langevin theory. However, if the axes of nanoparticles are polarized (mechanical anisotropy), then the magnetization curve of a superparamagnet under the conditions of mechanical anisotropy lies between the Langevin curve and the curve of hyperbolic tangent and with increasing anisotropy moves progressively farther from the Langevin curve and approaches the curve of hyperbolic tangent. It has also been shown that, in the case of powder superparamagnets, the presence of mechanical anisotropy leads to significant changes in the Curie constant.

Hexagonal CoO nanoparticles as studied by electron spin resonance

We report the electron spin resonance (ESR) results of hexagonal CoO nanoparticles those are different in average size. The temperature evolution of ESR spectra was found to be complicated in nature. The anomalous ESR behaviour is ascribed to (i) uncompensated magnetic sublattice, (ii) magnetic anisotropy of nanoparticles, and (iii) spatial distribution of the anisotropy axis with respect to the magnetic field. Anomalous changes of resonance field and linewidth were observed near the Néel temperature, TN. According to the temperature dependence of ESR intensity, the TN values are found to be 225, 255, and 285 K for hexagonal CoO nanoparticles with sizes of 38, 49, and 67 nm, respectively. We found that the size dependence of TN fits well with the Boltzmann curve.

Spectroscopic Investigation of Nano-Sized Strontium Ferrite Particles at Different Annealing Temperatures

Strontium ferrite enjoys a high degree of chemical stability and is completely nontoxic, which makes it ideal for a wide range of applications. Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized via the sol-gel technique employing ethylene glycol as the gel precursor. The phase morphology, particle diameter, and magnetic properties of the prepared samples were studied using X-ray diffractometry (XRD), scanning electron microscope (SEM), and a vibrating sample magnetometer (VSM), respectively. The effect of temperature on the crystal structure, morphology, and magnetic studies were carried out. Also, the thermal decomposition of as- synthesized powdered samples has been studied by thermogravimetric (TG) and differential thermal analysis (DTA) methods. The optical properties were analyzed using fl uorescence spectra. The XRD results showed that the samples synthesized at 600 °C, 800 °C, and 1000 °C were of single phase and smaller crystallite size. The intensity of the emission spectra of strontium ferrite was also examined. The yield percentage along with structure determination and VSM studies of the prepared samples are discussed in detail. Introduction. The enhanced interest of researchers in nanoobjects is due to the discovery of the unusual physical and chemical properties of these objects, which is related to the manifestation of the so-called quantum size effects. A key reason for the change in the physical and chemical properties of small particles as their size decreases is the increased fraction of the surface atoms, which occur under conditions (coordination number, symmetry of the local environment, etc. differing from those of the bulk atoms. From the energy standpoint, a decrease in the particle size results in an increase in the fraction of the surface energy in its chemical potential. Currently, the unique physical properties of nanoparticles are under intensive research. A special place belongs to the magnetic properties in which the difference between a massive (bulk) material and a nanomaterial is especially pronounced. In particular, it was shown that the magnetization and magnetic anisotropy of nanoparticles can be much greater than those of a bulk specimen, while differences in the Curie (TC) or Neel (TN) temperatures, i.e., the temperatures of spontaneous parallel or antiparallel orientation of spins, between nanoparticles and the corresponding microscopic phases reach hundreds of degrees. In addition, magnetic nanomaterials were found to possess a number of unusual properties, such as giant magneto-resistance, abnormally high magnetocaloric effect, and so on. The magnetic properties of nanoparticles are determined by many factors, the key of these including the chemical composition, the type and degree of defectiveness of the crystal lattice, the particle size and shape, the morphology, and the interaction of the particle with the surrounding matrix and neighboring particles. By changing the nanoparticle size, shape, composition, and structure, one can control to a certain extent the magnetic characteristics of the material based on them. However, these factors cannot always be controlled during the synthesis of nanoparticles nearly equal in size and chemical composition; therefore, the properties of nanomaterials of the same type can be markedly different. Magnetic nanomaterials are used in information recording and storage systems, in new permanent magnets, in magnetic cooling systems, as magnetic sensors, etc. All this accounts for the interest of various specialists in these systems. A nanoparticle is a quasi-zero-dimensional (0D) nano-object in which all characteristic linear dimensions are of the same

Temperature dependence of the effective anisotropies in magnetic nanoparticles with Néel surface anisotropy

We discuss the physical concept of the effective anisotropy in magnetic nanoparticles with surface anisotropy. A recently developed constrained Monte Carlo method allows evaluation of the temperature dependence of the energy surface in the whole temperature range, from which the effective anisotropy is determined. We consider nanoparticles of different shapes with cubic or uniaxial core anisotropy and Néel surface anisotropy. We demonstrate that at low temperatures surface effects can be dominant, leading to an overall cubic effective anisotropy even in spherical nanoparticles with uniaxial core anisotropy. This cubic anisotropy contribution decreases more rapidly with increasing temperature than the uniaxial core anisotropy, leading to a temperature-induced reorientation transition. We discuss the scaling behaviour of the effective anisotropy with magnetization in nanoparticles with surface anisotropy contribution. The scaling exponent deviates from that expected from Callen–Callen theory due to increased fluctuations of the surface spins.

Elaboration and characterization of magnetic nanocomposite fibers by electrospinning

We describe a simple method based on the electrospinning process to prepare heterogeneous hybrid submicronic fibers with magnetic behavior, consisting of Co nanoparticles embedded in a polyacrylonitrile (PAN) polymer. Quantity and anisotropy of magnetic nanoparticles are key parameters to improve the specific magnetic properties of fibers. We notably show that for higher Co nanoparticles concentration, their lower dispersity into the resulting fibers lead to dipolar interactions that become demagnetizing. The structural and morphological properties of Co nanodisks and of the resulting nanocomposite fibers are investigated by SEM, TEM, and EDX. The magnetic properties of the hybrid electrospun fibers have been evaluated with a SQUID magnetometer.

Determining magnetic nanoparticle size distributions from thermomagnetic measurements

Thermomagnetic measurements are used to obtain the size distribution and anisotropy of magnetic nanoparticles. An analytical transformation method is described which utilizes temperature-dependent zero-field cooling magnetization data to provide a quantitative measurement of the average diameter and relative abundance of superparamagnetic nanoparticles. Applying this method to self-assembled MnAs nanoparticles in MnAs–GaAs composite films reveals a log-normal size distribution and reduced anisotropy for nanoparticles compared to bulk materials. This analytical technique holds promise for rapid assessment of the size distribution of an ensemble of superparamagnetic nanoparticles.