3D calculation of hysteresis loops,magnetic orientations and reversal processes for exchange-spring bilayers with perpendicular anisotropy

Hysteresis loops and magnetic reversal processes have been determined by a three dimensional (3D) micromagnetic model for exchange-coupled Nd₂Fe₁₄B/α-Fe bilayers and carefully compared with a popular one-dimensional (1D) micromagnetic model. It is found that the calculated hysteresis loops, the critical fields and the magnetic phase diagrams agree well with the results given by the 1D model. However, the calculated nucleation mode is a quasi-curling one where the magnetic moment exhibits a curling in the film plane and varies in the thickness direction, in contrast with the reported quasi-coherent mode. The calculated spatial distribution of the magnetization orientation in the thickness direction at various applied fields signifies a three-step magnetic reversal process, which includes nucleation, growth and displacement of the domain wall as well as the rotation and the reversal of magnetization in the hard phase. The magnetic reversal of the hard phase is much slower than that given by the 1D model, leading to a more slant hysteresis loop near the coercivity point.     

Effects of thin MnAl buffer layer on structural and magnetic properties of MnAl films

Thin Mn(2 nm)/Al(2 nm) bilayers serving as buffer layers have been prepared prior to the deposition of MnAl films. The ferromagnetic τ-phase forms in the buffer layers at an optimum substrate temperature. As a template it induces the growth of following MnAl film. Compared with the case of film without buffer layer, the growth of non-ferromagnetic phase is suppressed and the structural and magnetic properties of MnAl film are improved. Weak dipolar inter-grain coupling is revealed in the MnAl film, and the magnetic reversal process is dominated by magnetic moment rotation.     

Direct imaging of asymmetric magnetization reversal in exchange-biased Fe/MnPd bilayers by x-ray photoemission electron microscopy

X-ray photoemission electron microscopy is used to probe the remnant magnetic domain structure in high quality, single-crystalline, exchange-biased Fe/MnPd bilayers. It is found that the induced unidirectional anisotropy strongly affects the overall magnetic domain structure. Real space images of the ferromagnetic domains provide direct evidence for an asymmetric magnetization reversal process after saturation along the ferromagnetic hard direction. The magnetization reversal occurs by moment rotation for decreasing fields while it proceeds by domain nucleation and growth for increasing fields. The observed domains are consistent with the crystallography of the bilayers and favor a configuration that minimizes the overall magnetostatic energy of the ferromagnetic layer.     

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.     

末端形状对NiFe纳米薄膜磁反转动力学特性的影响

基于微磁学模拟方法研究末端形状对NiFe纳米薄膜的磁反转和自旋波本征动力学特性的调制及磁反转与自旋波模式软化间的内在联系.纳米薄膜微磁结构的相变总是伴随着某种自旋波模式的软化,软化自旋波模式空间分布预示微磁结构相变的路径.存在一临界裁剪度（h₀）.当裁剪度h<h₀时,磁振荡局域于末端边缘的EM自旋波软化诱导磁反转从磁体末端边缘磁矩失稳开始,边缘失稳区域向中央扩展形成反转畴,最后反转畴逐渐移出膜面外而实现反转.当h≥h₀时,形状各向异性导致边缘局域化模式自旋波被抑制,反转场附近一致模式自旋波的软化诱导磁体一致反转.     

Magnetic layer thickness dependence of magnetization reversal in electrodeposited CoNi/Cu multilayer nanowires

The magnetization reversal of electrodeposited CoNi/Cu multilayer nanowires patterned in an array using a hole template has been investigated. The reversal mode is found to depend on the CoNi layer thickness t(CoNi); with increasing t(CoNi) a transition occurs from coherent rotation to a combination of coherent and incoherent rotation at around t(CoNi)=51 nm. The reversal mode has been identified using the magnetic hysteresis loops measured at room temperature for CoNi/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The nanowire samples have a diameter of ∼250 nm and constant Cu layer thickness of 4.2 nm with various t(CoNi) ranging from 6.8 nm to 7.5 μm. With increasing t(CoNi), the magnetic easy axis moves from the direction perpendicular to nanowires to that parallel to the nanowires at around t(CoNi)=51 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped CoNi layers (t(CoNi)=6.8, 12 and 17 nm) is of a coherent rotation type, while that for long rod-shaped CoNi layers (t(CoNi)=150 nm, 1.0, 2.5 and 7.5 μm) can be consistently explained by a combination of coherent rotation and a curling mode. The effects of dipole–dipole interactions between nanowires and between adjacent magnetic layers in each nanowire on the reversal process have been discussed.     

The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co₃₅Fe₆₅/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.     

Coercivity mechanisms in nanostructured permanent magnets

Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes.By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes.However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well.Nucleation and pinning are the two main coercivity mechanisms in permanent magnets.The former signifies the beginning of the magnetic reversal, whilst the latter completes it.Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field(self-pinning), which has the attributes of both traditional nucleation and pinning.Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.     

Conditions for infinite reversible susceptibility as magnetic systems reach irreversible switching

The reversible susceptibility approaching a magnetization phase transition corresponding to irreversible switching is examined both analytically and numerically. It is shown that the reversible susceptibility becomes infinite at switching if the reversible rotation just prior to irreversible motion has a component in the switching direction. For the numerical examples of non-uniform processes, as well as analytic analysis of two interacting grains, infinite susceptibility is seen only if the prior reversible rotation is virtually entirely along the subsequent reversal direction.     

Improved selective 180° radiofrequency pulses for magnetization inversion and phase reversal

Many efforts have been made to find a good selective 180° rf pulse, which include using amplitude and phase modulations, applying optimization techniques, and employing double-pulse techniques. While adequate selective magnetization inversion pulses have been designed, problems remain in the design of selective phase reversal pulses. In this paper we describe a novel approach which achieves large nutation angle selective rf pulses by using a series of small nutation angle pulse twins with alternated gradients (SNAPTAG). The design scheme takes into account the special properties of the rotation matrix appropriate for describing an rf pulse in the presence of a static field gradient. These pulses are suitable not only for selective magnetization inversion but also for selective phase reversal.     

SAR reduced pulse sequences

Three techniques were considered for reducing the RF (radiofrequency) power deposition in the body while maintaining scan time efficiency: reducing the RF peak amplitude while increasing the pulse width, substituting gradient echoes for spin echoes, and reducing the flip angle of the phase reversal pulse. The use of gradient echoes was found to be the most efficient means to reduce the power delivered to the patient and to obtain rapid data acquisition. The effect upon SAR (specific absorption rate) and SNR (signal-to-noise ratio) was demonstrated on a phantom when the phase reversal pulse was reduced from the standard 180 degrees to 90 degrees. Data in the body indicated a fairly constant SNR down to a refocusing flip angle between 110 degrees and 135 degrees. An initial clinical evaluation was performed at three institutions using the method of reducing the flip angle of the phase reversal pulse. The scan with theta = 120 degrees was rated by readers in a blinded study as having acceptable diagnostic image quality while the 135 degrees scan had comparable image quality to a conventional 90 degrees - 180 degrees pulse sequence. The use of reduced phase reversal pulses was seen as an efficient protocol to obtain T1-weighted images at rapid data rates while reducing the power delivered to the body by about 40%.     

Switching kinetics of ferroelastic ferroelectrics

Switching kinetics of uniaxial ferroelastic ferroelectrics (FFs) in external electric and stress fields is studied using classical theory of nucleation and growth. The stage in which the polarization and deformation reversal involves the main body of the FF and the final stage (Ostwald ripening) of the FF switching are studied with allowance for the change in the repolarization and redeformation during the phase transition. The time dependences of the repolarization and redeformation are found, and equations are derived from which the polarization current and the deformation flux, as well as their time dependence, can be calculated. The calculated main characteristics of the FF switching are compared with the experimental data for switching of Rochelle salt single crystals.     

Effect of an external magnetic field on the orientational phase diagrams of (011) crystalline garnet ferrite plates with combined anisotropy

The effect of an external magnetic field on uniform magnetic states in (011) crystalline garnet ferrite plates with combined anisotropy is theoretically studied. Under the action of the field, the magnetic symmetry of the crystals is reduced and the pattern of spin-reorientation phase transitions, which depends significantly on the magnetic field strength and direction, changes. Curves of critical fields are found, and the general law of magnetization reversal in the (011) plates due to rotation is derived.     

Topological spin Hall states, charged skyrmions, and superconductivity in two dimensions

We study the properties of two dimensional topological spin Hall insulators which arise through spontaneous breakdown of spin symmetry in systems that are spin rotation invariant. Such a phase breaks spin rotation but not time reversal symmetry and has a vector order parameter. Skyrmion configurations in this vector order parameter are shown to have an electric charge that is twice the electron charge. When the spin Hall order is destroyed by condensation of Skyrmions superconductivity results. This may happen either through doping or at fixed filling by tuning interactions to close the Skyrmion gap. In the latter case the superconductor-spin Hall insulator quantum phase transition can be second order even though the two phases break distinct symmetries.     

Optical activity and time reversal

The rôle of time reversal symmetry in natural and magnetic optical activity is discussed. Natural optical rotation is shown to be generated by an anti-hermitian odd parity time-even operator and magnetic optical rotation by an anti-hermitian even parity time-odd operator. This shows that lack of time reversal invariance is not the source of natural optical rotation and that free atoms can show natural optical rotation without violating reversality, which leads to a fundamental distinction between the conditions necessary for natural optical rotation and a permanent space-fixed electric dipole moment. General transition optical activity and polarizability tensors between components of degenerate states are discussed with reference to possible new Raman experiments and new contributions to discriminating intermolecular forces between chiral molecules. Time reversal symmetry also leads to a new criterion for chiral objects and to the concept that natural optical activity provides an example of spontaneous symmetry breaking with respect to CP.     

Simulation of magnetization reversal processes in finite ferromagnets with defects

The magnetization and magnetization reversal processes that occur through the mechanism of incoherent rotation of magnetic moments in cubic ferromagnets with limited sizes are investigated theoretically. It is established that the appropriate model representation of magnetic inhomogeneities arising in the region of defects is provided by 0° domain walls. The influence of the external magnetic field on the structure and the stability region of the 0° domain walls is determined. This makes it possible to reveal the characteristic features of the magnetization reversal of real crystals as a function of the material and defect parameters, in particular, in the vicinity of the spin-reorientation phase transition.     

Lattice Boltzmann study of three-dimensional immiscible Rayleigh–Taylor instability in turbulent mixing stage

In this paper, we numerically studied the late-time evolutional mechanism of three-dimensional (3D) single-mode immiscible Rayleigh–Taylor instability (RTI) by using an improved lattice Boltzmann multiphase method implemented on graphics processing units. The influences of extensive dimensionless Reynolds numbers and Atwood numbers on phase interfacial dynamics, spike and bubble growth were investigated in details. The longtime numerical experiments indicate that the development of 3D singlemode RTI with a high Reynolds number can be summarized into four different stages: linear growth stage, saturated velocity growth stage, reacceleration stage and turbulent mixing stage. A series of complex interfacial structures with large topological changes can be observed at the turbulent mixing stage, which always preserve the symmetries with respect to the middle axis for a low Atwood number, and the lines of symmetry within spike and bubble are broken as the Atwood number is increased. Five statistical methods for computing the spike and bubble growth rates were then analyzed to reveal the growth law of 3D single-mode RTI in turbulent mixing stage. It is found that the spike late-time growth rate shows an overall increase with the Atwood number, while the bubble growth rate experiences a slight decrease with the Atwood number at first and then basically maintains a steady value of around 0.1. When the Reynolds number decreases, the later stages cannot be reached gradually and the evolution of phase interface presents a laminar flow state.     

Magnetization reversal studies of continuous and patterned exchange biased NiFe/FeMn thin films

In this article we present a detailed investigation of the structural and magnetic properties of exchange biased NiFe (ferromagnet)/FeMn (antiferromagnet) thin films. The influence of the shape anisotropy on exchange bias and the magnetization reversal mechanism in a sample with patterned lines is compared with a continuous two-dimensional reference sample. Polarized neutron reflectivity (PNR) is employed to study the magnetization reversal by analyzing the spin-flip and non-spin-flip reflectivities. PNR measurements show that the magnetization reversal in the reference two-dimensional film and patterned lines is by domain wall motion rather than coherent rotation of magnetization.     

Magnetoelectric effect induced by electron–electron interaction in three dimensional topological insulators

We compute the magnetoelectric response of an interacting topological insulator in three space dimensions with a short range interaction between electrons in different orbitals. We show that in the presence of interactions and inverted bands the chiral phase is gauged away and replaced by a topological angle (θ-term) which is determined by saddle point of the interacting action and the Fujikawa integration measure. The magnetoelectric response breaks time reversal symmetry which is restored at strong interactions. The effect is equivalent to the one in four dimensions without interaction; it can be observed by measuring the Faraday rotation under external stress.     

Asymmetric magnetization reversal in exchange-biased hysteresis loops

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M--> of polycrystalline Fe films exchange coupled to twinned (110) MnF (2) or FeF (2) antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e. , the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.