Anti-function solution of uniaxial anisotropic Stoner-Wohlfarth model

The anti-trigonometric function is used to strictly solve the uniaxial anisotropic Stoner-Wohlfarth (SW) model, which can obtain the relation of the angle α (θ) between the magnetization (the anisotropy field) and the applied magnetic field. Using this analytic solution, the hysteresis loops of uniaxial anisotropic SW particles magnetized in typical directions could be numerically calculated. Then, the hysteresis loops are obtained in randomly distributed SW particle ensembles while ignoring the dipole interaction among them with the analytic solution. Finally, the correctness of the analytic solution is verified by the exact solutions of remanence, switching field, and coercivity from the SW model. The analytic solution provides an important reference for understanding the magnetizing and magnetization reversal processes of magnetic materials.     

Topology-like dynamical behavior of magnetization reversal in exchange-bias systems

In an exchange-bias system, the barriers with intrinsic potential energy may be asymmetric due to unidirectional anisotropy. Based on the Stoner-Wohlfarth model, we show that the asymmetric barriers may lead to four kinds of dynamical process underlying the hysteresis-loop measurement. These kinds of dynamical processes are different in a topology-like property, which can be controlled by the orientation of the external field. In our study, a new analysis approach has been proposed to reveal the dynamical behaviors of magnetization reversal. With this approach, coercivity, exchange-bias field, and asymmetry of hysteresis loops can be quantitatively obtained.     

Implications of nanoparticle concentration and size distribution in the superparamagnetic behaviour of aging-improved maghemite xerogels

Evolution of static magnetic properties of a set of enhanced γ-Fe₂O₃/SiO₂ nanocomposites with different iron concentration has been studied on the basis of their corresponding hysteresis loops, zero-field/field-cooled (ZFC/FC) magnetization curves and transmission electron microscopy images. The lack of coercivity in all compositions, as well as the fulfillment of the H/T scaling law by the magnetization above the blocking temperature of each system under study, evidence a superparamagnetic behaviour in the iron oxide nanoparticles. In order to study the influence of iron content in the unblocking processes of nanoparticles, ZFC curves under different applied magnetic fields have been fitted to a model considering the systems under study as a distribution of energy barriers. Depart from the superparamagnetic model is discussed considering interparticle interactions.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Micromagnetic modeling studies on the effects of stress on magnetization reversal and dynamic hysteresis

The complex behaviors of magnetic materials subjected to magneto-electro-mechanical coupled fields call for a better understanding of the mechanism of multi-fields coupling. In this paper, micromagnetic modeling is carried out to study the effect of stress on hysteresis loops and dynamic magnetization reversal. The time-dependent Landau–Lifshitz–Gilbert equation which governs the evolution of magnetization is solved using the fast Fourier transform technique in reciprocal space. The simulation results show that the stress changes the distribution of easy directions and, therefore, leads to the change of magnetic properties. Moreover, the positive product of the stress and saturation magnetostriction coefficients increases the coercivity, hence, increases the area enclosed by the hysteresis loop. A stress-induced magnetization reversal is investigated.     

Hysteresis and rotational hysteresis of textured polycrystalline magnets

A system of magnetic particles with uniaxial anisotropy is considered. The orientation of the particles is described by a distribution function, representing the texture by a single integer n. In each particle, two elementary processes of the magnetization reversal of the particles are taken into account, the coherent rotation of the magnetization and the pinning of domain walls. In the framework of this model the hysteresis loops including minor loops and virgin curves and the rotational hysteresis are computed, where arbitrarv angles between the texture axis and the external field are taken into consideration.     

小尺寸正方形铁磁薄膜边界效应对磁化翻转过程的影响

用自旋动力学方法系统地研究磁偶极相互作用表现的边界效应对小尺寸正方形铁磁薄膜的磁化翻转过程的影响.在确定的磁偶极相互作用强度下,针对不同的单轴各向异性强度和不同的磁化角(外磁场与易轴间的夹角),具体给出矫顽场与磁化角及单轴各向异性强度之间的依赖关系和-些有代表性的磁滞回线,并给出磁化翻转过程中-些有代表性的微观磁结构.模拟结果表明:磁偶极相互作用表现的边界钉扎作用与单轴各向异性场之间的竞争决定磁滞回线的形状和矫顽场的大小,从而在不同磁化角情况下会导致不同的矫顽场机理.本文提出可有效地描述正方形铁磁性薄膜复杂微观磁畴结构的形成与演变的五磁畴模型.这种五磁畴模型既能直接揭示单轴各向异性正方形铁磁薄膜的几何特性和物理特性,也方便于磁化翻转过程的分析.     

纳米晶永磁Pr8Fe87B5反磁化机理研究

用熔体快淬法制备了Pr2Fe14B/α-Fe纳米复合永磁样品.测量了样品的起始磁化、反磁化过程、矫顽力和剩磁与外场的关系，以及样品的磁粘滞性.经分析认为材料的矫顽力主要由非均匀的钉扎机理决定，但由于交换硬化的软磁相的可逆转动使得这种反磁化机理不同于单相永磁材料的钉扎行为.磁粘滞性表明热激活主要源于硬磁相的不可逆磁化行为. 关键词： 纳米复合永磁 矫顽力 剩磁 磁粘滞     

Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Element-specific magnetization reversal in Fe/Ce multilayers: : a study by X-ray magnetic circular dichroism and the magneto-optic Kerr effect

Fe/Ce multilayers are magnetically soft with coercive fields of a few Oersteds. In this artificial system, the itinerant 5d electrons of Ce are magnetically polarized by hybridization with the spin–split 3d states of Fe. To obtain an insight into the magnetization reversal process, the element selectivity of X-ray magnetic circular dichroism was used to measure the magnetization of the Ce-5d electrons as a function of an applied magnetic field. Comparison with the magnetization curves studied by the magneto-optic Kerr effect, which averages over the whole system, revealed that the coercivity in the hysteresis of the ordered Ce-5d moments is reduced by 50%. We propose that this is an effect of the magnetically disturbed interface or of the complex non-collinear magnetic structure of the Ce layers detected by recent experiments of X-ray resonant magnetic scattering. The results are compared to the X-ray dichroic and Kerr hysteresis loops of the multilayers Fe/La/Ce/La and Fe/CeH_2−δ. These systems are magnetically harder and their coercivities are identical.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Minor hysteresis loops model based on exponential parameters scaling of the modified Jiles–Atherton model

In this present work, the minor hysteresis loops model based on parameters scaling of the modified Jiles–Atherton model is evaluated by using judicious expressions. These expressions give the minor hysteresis loops parameters as a function of the major hysteresis loop ones. They have exponential form and are obtained by parameters identification using the stochastic optimization method “simulated annealing”. The main parameters influencing the data fitting are three parameters, the pinning parameter k, the mean filed parameter α and the parameter which characterizes the shape of anhysteretic magnetization curve a. To validate this model, calculated minor hysteresis loops are compared with measured ones and good agreements are obtained.     

Amorphous-crystalline dual-layer structures resulting from metastable liquid phase separation in(Fe₅₀Co₂₅B₁₅Si₁₀)₈₀Cu₂₀ melt-spun ribbons

(Fe50Co25B15Si10)80Cu20 ribbons are prepared by using the single-roller melt-spinning method.A dual-layer structure consisting of a(Fe,Co)-rich amorphous phase and a Cu-rich crystalline phase forms due to metastable liquid phase separation before solidification.The magnetic hysteresis loops of the as-quenched and annealed samples are measured at room temperature.It is indicated that the coercivity of the ribbon is almost zero in the as-quenched state.The crystallization leads to the increase of coercivity and decrease of saturation magnetization.     

Effect of porosity on the magnetic behaviour of nickel ferrites

The magnetic behaviour of polycrystalline materials is closely related to their specific manufacturing process. Beside composition, the various factors that affect this behaviour and cause the particular microstructure of these materials include non-magnetic inclusions, grain size, pressing, temperature, sintering time, and cooling rate. A study has been done of magnetization curves and static hysteresis loops in a set of polycrystalline nickel ferrites. The experimental results were fitted by a second-order rational function, which easily allowed us to calculate their characteristic parameters, and analyze them according to the non-magnetic inclusions model.     

Enhancement of the remanence and coercivity in Co-Cr layers

An enhancement of the remanent magnetisation and of the coercivity of Co-Cr layers is observed after applying a magnetic field which does not saturate the sample compared to measurements after saturating the sample. An enhancement of the remanent magnetisation up to 35% is observed by measuring remanence curves. A similar effect can be observed by measuring minor hysteresis loops. This effect can be explained by allowing the domain period to be an independent parameter in the Kooy and Enz model. This effect decreases with increasing coercivity of Co-Cr layers prepared at different substrate temperatures. This is a new indication that Co-Cr layers gradually change from a continuous medium to a more particulate-like medium with increasing coercivity.     

Stray field interaction of stacked amorphous tapes

In this study, magnetic cores made of amorphous rectangular tape layers are investigated. The quality factor Q of the tape material decreases rapidly, however, when stacking at least two tape layers. The hysteresis loop becomes non-linear, and the coercivity increases. These effects are principally independent of the frequency and occur whether tape layers are insulated or not. The Kerr-microscopy was used to monitor local hysteresis loops by varying the distance of two tape layers. The magnetization direction of each magnetic domain is influenced by the anisotropy axis, the external magnetic field and the stray field of magnetic domains of the neighboring tape layers. We found that crossed easy axes (as the extreme case for inclined axes) of congruent domains retain the remagnetization and induce a plateau of the local loop. Summarizing local loops leads to the observed increase of coercivity and non-linearity of the inductively measured loop. A high Q-factor can be preserved if the easy axes of stacked tape layers are identical within the interaction range in the order of mm.     

SIMULATION ON LOCAL FLUX CREEP IN TYPE-Ⅱ SUPERCONDUCTORS

Using C programming language, we have simulated the flux creep process in nonideal type-Ⅱ superconductors. Global and local magnetization curves are calculated and the logarithmic time dependence of local magnetic induction B under a constant external field is examined. The effects of nonuniform pinning potential and self-organized criticality (SOC) model on the simulations are discussed. The results show that the main feature of flux creep is the relaxation effect. The form of hysteresis loops is dependent on the magnetic field sweep rate. SOC can account for the occurrence of fluctuation to a certain extent and nonuniform pinning potential can enhance the fluctuation.     

Mössbauer and magnetic study of mechanical alloying of Fe3Si

Using Mössbauer spectroscopy and measurement of hysteresis loops and thermomagnetic curves, phase composition and magnetic parameters of Fe₃Si mechanically alloyed powders were studied in dependence on milling time and subsequent heat treatment at a thermomagnetic experiment. Samples of as-prepared powders show high value of coercivity, the saturation magnetization and the content of amorphous Fe₃Si phase raise with increasing time of milling, the content of α-Fe diminishes. Heat treatment of samples with long enough milling time can produce almost perfect Fe₃Si alloy.