Distribution Of Easy Axes Research Articles

Improved magnetic properties in amorphous FeSiBCr soft magnetic composites with easy-plane anisotropy for high-frequency applications

The miniaturization and high frequency of future electromagnetic devices require soft magnetic composites with high permeability, high working frequency, and low core loss. In this work, easy-plane amorphous FeSiBCr powders with a thickness of approximately 1–2 μm are obtained by planetary ball milling, and are successfully fabricated into an arranged soft magnetic composite by using a uniform magnetic field. The complex permeability spectrums of the composites are measured and analyzed. Due to the planar distribution of easy magnetization axes, the amorphous easy-plane FeSiBCr/Si-resin composite shows a permeability of 31 at 1 MHz. The core loss of the samples is measured from 100 KHz to 5 MHz and is discussed in detail. The loss-separated results illustrate that the eddy current loss and excess loss can be effectively reduced in the amorphous easy-plane FeSiBCr/Si-resin composite. These results indicate that, compared with an amorphous non-easy-plane FeSiBCr/Si-resin composite, the permeability of the amorphous easy-plane FeSiBCr/Si-resin composite is effectively improved and the core loss is reduced, making it promising for high-frequency applications.

Comparative study on the high frequency performances of the easy-plane FeNi@SiO2 powder soft magnetic composite

In this work, an easy-plane FeNi@SiO2 powder soft magnetic composite (SMC) was imitated and fabricated and its high frequency magnetic properties were comparatively investigated with a non-easy-plane composite. Due to the planar distribution of easy magnetization axes, the easy-plane composite exhibits a constant permeability of 38 up to 100 MHz. Moreover, the easy-plane SMC exhibits a lower core loss at higher frequencies. Loss separated results show that the hysteresis loss plays a dominant loss component in the composite, rather than dominant excess loss in the non-easy-plane powder composite. These results indicate that, compared with non-easy-plane powder composite, the easy-plane powder composites exhibit comprehensive advantages of higher permeability, wider effective operating frequencies and lower loss, which suggest that the use of easy-plane ferromagnetic powder is a promising and efficient measure to develop a new generation of soft magnetic composites for higher frequency application.

Competing magnetic states and M–H loop splitting in core–shell NiO nanoparticles

Magnetic relaxation in a nanoparticles system depends on the intra-particle interactions, reversal mechanism, the anisotropy field, easy axis distribution, particle volume, lattice defects, surface defects, materials composite, etc. Here we report the competing magnetic states between superparamagnetic blocking and Néel transition states in 14 nm core–shell NiO nanoparticles. A crossover temperature of 50 K was observed for both these states from the zero field cooled/field cooled magnetization curves taken at different fields. At crossover temperature, an interesting M–H loop splitting is observed which is attributed to the slow spin relaxation. This anomalous M–H loop splitting behaviour was found to be particle size dependent and suppressed for diameters above and below 14 nm which indicates a critical size for these competing magnetic states. Additional neutron diffraction experiments confirmed this observation. This experimental study provides a new insight for the understanding of intra-particle interactions in fine antiferromagnetic nanoparticles and obtained results are an important step towards deeper understanding of the competing/non-competing modes between superparamagnetic blocked and Néel transition states.

Phase diagram of a three-dimensional dipolar model on an fcc lattice

The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice is investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $\lambda_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic, quasi long range ordered ferromagnetic and spin-glass regions is thus considered in the temperature, $\lambda_u$ plane. This system is aimed at modeling the magnetic phase diagram of supracrystals of magnetic nanoparticles.

The diameter effect on the magnetization switching time of sphere-shaped ferromagnets using micromagnetic approach

In this work, the magnetization switching time has been studied as the diameter variation of sphere-shaped ferromagnets by means of micromagnetic simulation. Some ferromagnetic elements such as cobalt, iron, nickel, and permalloy were numerically simulated with diameter size variation from 50 nm to 100 nm. The micromagnetic simulation was performed by public software Object Oriented Micromagnetic Framework (OOMMF) based on the Landau-Lifshitz-Gilbert (LLG) equation. The ferromagnetic nano-sphere was induced by the quasi-static magnetic field to observe its magnetization response. Generally, it is observed that the switching time increases as the diameter size increases on the ferromagnetic elements. However, the switching time are relatively insensitive as the diameter increases in the Cobalt element for the diameter range from 60 nm to 90 nm. This behavior is contributed to the distribution of magnetization easy axis on the ferromagnetic elements. The understanding of domain structures during magnetization switching process is important in the development of nano-patterned magnetic memory storage.

Phase diagram of a three-dimensional dipolar Ising model with textured Ising axes

We study from tempered Monte Carlo simulations the magnetic phase diagram of a textured dipolar Ising model on a face centered cubic lattice. The Ising coupling of the model follow the dipole–dipole interaction. The Ising axes are distributed with a uniaxial symmetry along the direction with a Gaussian probability density of the polar angles. This distribution provides a quenched disorder realization of the dipolar Ising model making a continuous link between the parallel axes dipoles and the random axes dipole models. As expected the phase diagram presents three distinctive phases: paramagnetic, ferromagnetic and spin-glass. A quasi long range ferromagnetic and a reentrant spin-glass phases are obtained in the vicinity of the ferromagnetic spin-glass line. This model provides a way to predict the magnetic phases of magnetic nanoparticles supracrystals in terms of the texturation of the easy axes distribution in the strong anisotropy limit.

Micromagnetic model of exchange bias: effects of structure and AF easy axis dispersion for IrMn/CoFe bilayers

A micromagnetic model of an exchange bias bilayer is used to examine the impact of the physical structure and the easy axis dispersion of the antiferromagnetic (AF) layer on the exchange bias field (HEB) in an IrMn/CoFe system. Because of the different timescales, the magnetization dynamics of the IrMn and CoFe layers are modelled using respectively a kinetic Monte Carlo (kMC) approach and Landau–Lifshitz–Gilbert (LLG) equation. The easy axis dispersion is modelled using a Gaussian distribution. The calculations show that HEB increases with increasing IrMn thickness and grain size, in agreement with experimental work. Moreover, the model allows the visualization of the switching process at the micromagnetic level to reveal the reversal mechanism. We find that the effect of AF easy axis distribution not only strongly affects the reduction of HEB but also drives non-coherent behaviour in the reversal mechanism. This confirms that the easy axis distribution is an important factor with strong impact on the magnetic properties and exchange bias field of an exchange bias system.

CoCrPt@(TiO2,CoO) granular thin films grown on Ru/NixPd100-x/NiTa (x = 20, 50, 80)

Structural, morphological and magnetic properties of CoCrPt@(TiO2,CoO) granular thin films deposited on Ru/NixPd100-x/NiTa intermediate layers were investigated as a function of the NixPd100-x composition (x = 20, 50, 80). Using a Ni20Pd80 buffer-layer leads to a significant improvement of the crystallographic texture, grain separation and easy-axis angular distribution with respect to conventional systems using Ru/NiTa as intermediate layers. As a result, a meaningful enhancement of magnetic properties (anisotropy constant, K = 4.1 × 105 J/m3, out-of-plane coercivity, Hc⊥ = 470 kA/m, nucleation field, HN = −115 kA/m) with respect to conventional systems (K = 3.7 × 105 J/m3, Hc⊥ = 374 kA/m, HN = −71.6 kA/m) was achieved, thus indicating that Ni20Pd80 can potentially be used as buffer-layer to enhance the recording density of conventional Hard Disk Drives.

Thickness effect on the easy axis distribution in exchange biased Co/IrMn bilayers

It is commonly assumed that in uncompensated FM/AF interfaces, the easy axes of antiferromagnetic grains are aligned toward the magnetic field direction applied during the film growth. However, due to several intrinsic and extrinsic factors, these easy axes can be aligned away from the unidirectional axis. In this context, the aim of this work is to study the relationship between the easy axes distribution (EAD) and thickness of both the ferromagnetic (FM) and antiferromagnetic (AF) layers in Co/IrMn bilayer systems. A phenomenological model for the free energy of the system that takes into account a domain wall formation in the AF layer and a Gaussian-type distribution of their easy axes is used. Thus, we simulate and fit the magnetization curves and get several magnetic parameters like rotatable anisotropy field, exchange bias field, coercivity and magnetic anisotropy. Our results show that when the AF thickness is increased, the width of the easy-axis distribution also increases following a power-law relationship. However, no correlation is observed when the FM thickness is varied. Furthermore, considering an easy-axis distribution we observe a significant reduction in coercivity as well as a slight decrease of the exchange bias field.

Equilibrium magnetization of a quasispherical cluster of single-domain particles

Equilibrium magnetization curve of a rigid finite-size spherical cluster of single-domain particles is investigated both numerically and analytically. The spatial distribution of particles within the cluster is random. Dipole-dipole interactions between particles are taken into account. The particles are monodisperse. It is shown, using the stochastic Landau-Lifshitz-Gilbert equation that the magnetization of such clusters is generally lower than predicted by the classical Langevin model. In a broad range of dipolar coupling parameters and particle volume fractions, the cluster magnetization in the weak field limit can be successfully described by the modified mean-field theory, which was originally proposed for the description of concentrated ferrofluids. In moderate and strong fields, the theory overestimates the cluster magnetization. However, predictions of the theory can be improved by adjusting the corresponding mean-field parameter. If magnetic anisotropy of particles is additionally taken into account and if the distribution of the particles' easy axes is random and uniform, then the cluster equilibrium response is even weaker. The decrease of the magnetization with increasing anisotropy constant is more pronounced at large applied fields. The phenomenological generalization of the modified mean-field theory, that correctly describes this effect for small coupling parameters, is proposed.

A phenomenological approach to study the effect of uniaxial anisotropy on the magnetization of ferromagnetic nanoparticles

We study the effect of the uniaxial anisotropy in a system of ideal, noninteracting ferromagnetic nanoparticles by means of a thermodynamical model. We show that the effect of the anisotropy can be easily assimilated in a temperature shift Ta∗, in analogy to what was proposed by Allia et al. (2001) in the case of interacting nanomagnets. The phenomenological anisotropic Ta∗ parameter can be negative, indicating an antiferromagnetic-like behavior, or positive, indicating a ferromagnetic-like character as seen in the inverse susceptibility behavior as a function of temperature. The study is done considering an easy axis distribution to take into account the anisotropy axis dispersion in real samples (texture). In the case of a volumetric uniform distribution of anisotropy axes, the net effect makes Ta∗ to vanish, and the magnetic susceptibility behaves like a conventional superparamagnetic system, whereas in the others a finite value is obtained for Ta∗. When magnetic moment distribution is considered, the effect is to enhance the Ta∗ parameter, when the dispersion of the magnetic moments becomes wider.

2D Magnetic Texture Analysis of Co–Cu Films

Abstract The magnetic textures for the produced magnetic materials are important concepts in accordance with technical applications. Therefore, the aim of this article is to determine 2D magnetic textures of electrodeposited Co–Cu films by the measurement of hysteresis loops at the incremented angles. For that, Co–Cu films were deposited with different Co2+ in the electrolyte. In addition, the easy-axis orientation in the films from the squareness values of the angles, Mp(β) obtained by the hysteresis loops have been numerically studied using the Fourier series analysis. The differences observed in the magnetic easy-axis distributions were attributed to changes of the incorporation of Co in the films with the change of Co2+ in the electrolyte. The coefficients of Fourier series (A0 and A2n ) were also computed for 2D films. It is seen that a systematic and small decrease in A0 and an obvious decrease in A2n (n=1) were observed with increasing incorporated Co in the films. Results imply that interactions cause slightly demagnetization effect accordance with higher incorporation of Co in the films. Furthermore, the crystal structure of the Co–Cu films analysed by X-ray diffraction revealed that the films have dominantly face-centred cubic structure. Film contents analysed by energy-dispersive X-ray spectroscopy and film morphologies observed by scanning electron microscope also support the magnetic texture analysis results found by numerical computation.

Irreversible magnetization process and switching mechanism in L1 FePt thin films

The irreversible characteristics of the magnetization and the switching mechanism have been investigated in granular FePt films with L10 phase prepared by sputtering on a polymer substrate. The films display an extremely large magnetic anisotropy with a random distribution of the magnetization easy axis. The magnetic instabilities and the irreversible magnetization are found to be controlled by domain wall, which is responsible for the magnetization reversal. Through remanence curves and ΔM plot, the nature of magnetic interactions was revealed to be positive exchange coupling.

교환 결합력을 갖는 CoFe/MnIr 박막에서 강자성 공명 선폭의 각도 의존성 연구

Department of Physics, Andong National University, Andong 36729, Korea(Received 4 April 2016, Received in final form 11 April 2016, Accepted 18 April 2016)We analyzed the angular dependence of ferromagnetic resonance linewidth in exchange coupled CoFe/MnIr bilayers. The maximumand minimum linewidth was observed in the easy and hard direction of unidirectional anisotropy by exchange coupling, respectively, andit was well agreed with the angular dependence of exchange bias field. The maximum linewidth was due to the twist of CoFemagnetization near CoFe/MnIr interface from direction of pinned MnIr spin to direction of applied magnetic field. While, minimumlinewidth more higher than that of CoFe was related to rotatable anisotropy field, and explained by easy axis distribution of MnIr grains.Keywords: ferromagnetic resonance, linewidth, exchange bias, rotatable anisotropy, AF grain

Model of advanced recording media: The angular dependence of the coercivity including the effect of exchange interaction

We use a micromagnetic model based on the kinetic Monte-Carlo approach to investigate theoretically the magnetic properties of advanced recording media. The model is employed to examine the impact of the magnetostatic and exchange interaction between grains of realistic perpendicular recording media on the angular-dependent coercivity since the exchange field between grains is an important factor in recording performance. The micromagnetic model allows to take the easy axis distribution and the exchange interaction between grains into account. The results confirm the importance of exchange interaction since the variation of coercivity with angle between the applied field and the orientation of easy axis which is perpendicular to the film plane, (θ) is seen to broaden with decreasing exchange field. We show that a two-stage fitting procedure involving the separate determination of the exchange field and easy axis dispersion provides a useful tool for the characterization of media for perpendicular recording and heat assisted recording. We find excellent agreement between previous experimental results and the simulations including exchange interactions leading to estimate of the exchange coupling and easy axis dispersion.

Structure Optimization of FePt–C Nanogranular Films for Heat-Assisted Magnetic Recording Media

Systematic investigations for the optimization of FePt–C nanogranular films for heat-assisted magnetic recording media are overviewed. FePt–C films prepared by using compositionally graded process exhibit binomial distribution of grains with a maximum size distribution of 16%. In addition, the average grain size of FePt could be controlled well below 7 nm. Easy-axis distribution can be substantially reduced by growing FePt–C layers on a single crystalline substrate instead of a (001)-textured MgO seed layer, indicating the improvement of (001) texture in the seed layer would reduce the in-plane component. Finite-element micromagnetic simulations incorporating experimentally determined misorientation of FePt grains do not reproduce the experimentally observed demagnetization curve, suggesting the presence of large distribution in the anisotropy fields of FePt grains. A typical growth model of the FePt–C nanogranular films has been proposed on the basis of the thickness-dependent microstructure of the FePt–C nanogranular films, and the suitability of various seed layers has been discussed. The observed results suggest that the suppression of the coarsening of FePt grains is essential to keep the grain size smaller than 6 nm.

Magnetic-field dependence of Brownian and Néel relaxation times

The investigation of the rotational dynamics of magnetic nanoparticles in magnetic fields is of academic interest but also important for applications such as magnetic particle imaging where the particles are exposed to magnetic fields with amplitudes of up to 25 mT. We have experimentally studied the dependence of Brownian and Néel relaxation times on ac and dc magnetic field amplitude using ac susceptibility measurements in the frequency range between 2 Hz and 9 kHz for field amplitudes up to 9 mT. As samples, single-core iron oxide nanoparticles with core diameters between 20 nm and 30 nm were used either suspended in water-glycerol mixtures or immobilized by freeze-drying. The experimentally determined relaxation times are compared with theoretical models. It was found that the Néel relaxation time decays much faster with increasing field amplitude than the Brownian one. Whereas the dependence of the Brownian relaxation time on the ac and dc field amplitude can be well explained with existing theoretical models, a proper model for the dependence of the Néel relaxation time on ac field amplitude for particles with random distribution of easy axes is still lacking. The extrapolation of the measured relaxation times of the 25 nm core diameter particles to a 25 mT ac field with an empirical model predicts that the Brownian mechanism clearly co-determines the dynamics of magnetic nanoparticles in magnetic particle imaging applications, in agreement with magnetic particle spectroscopy data.

Magnetization reversal via a Stoner–Wohlfarth model with bi-dimensional angular distribution of easy axis

A numerical extension of the simple Stoner–Wohlfarth model to the case of bi-dimensional angular distributions of easy axis is provided. The results are particularized in case of step-like, Gaussian-like and user defined distributions. In spite of its simplicity, the model can be applied to magnetically textured thin films and multilayers with in-plane magnetic anisotropy, independently on the texture source. Exemplifications are provided for a simple ferromagnetic textured FeCo film as well as for a FeMn/FeCo/Cu/FeCo spin valve structure.

Effect of MgO underlayer misorientation on the texture and magnetic property of FePt–C granular film

A transmission electron microscope (TEM) based orientation mapping technique and micromagnetic simulations were applied to study the influence of easy axis distribution (EAD) on magnetic properties of FePt–C granular films which were deposited on a single crystalline MgO (001) substrate and a (001)-textured poly-crystalline MgO underlayer. The FePt–C film on the polycrystalline MgO underlayer shows smaller perpendicular coercivity, broader switching field distribution and visible in-plane minor loop compared with that deposited on the single crystalline MgO (001) substrate. Although the grain sizes and their distributions in both films look similar in TEM, orientation mapping and texture analysis revealed that the polycrystalline MgO underlayer introduces significant misorientation in the (001)-textured FePt grains. Micromagnetic simulations successfully reproduced the large hysteresis in the in-plane magnetization by introducing the specific misorientation distribution of the FePt grains obtained from the texture analysis. The misoriented FePt grains were found to be grown from misoriented MgO grains, indicating that the improvement of the (001) texture of the MgO underlayer is essential to reduce the in-plane component of FePt based recording media.

Intergrain exchange interaction estimation from the remanence magnetization analysis

Abstract Analysis of δ m ( H ) = [ M d ( H ) − M r ( ∞ ) + 2 M r ( H ) ] / M r ( ∞ ) curves constructed from dc demagnetization and isothermal remanent magnetization ( M d ( H ) and M r ( H ) respectively) is important for characterization of the interactions in ferromagnets. Up to now, it has been mainly used for qualitative deductions about them. In this work, the novel functional relation between the maximum of the δ m ( H ) plot and the microscopic parameters of the weakly coupled Stoner–Wohlfarth ensemble with the isotropic distribution of easy magnetization axes was established using computer modeling. It allows quantitative analysis in the frame of the model to be performed. Finally, a new method of estimating the intergrain exchange interaction constant for nanostructured high anisotropy magnets could be formulated taking into account the results of the modeling.