Influence of coercive force and easy-axis bias on domain wall motion in NiFeCo films excited by hard-axis pulses

Domain wall motion in vacuum-deposited 1200 Å- 2000 Å-thick NiFe and NiFeCo films, excited by a 0.5 ns rise time/200 ns fall time hard-axis pulse field with an easy-axis dc field, is examined with regard to coercive force, easy-axis bias, and low-frequency creep results. Both NiFe and NiFeCo films have the same threshold field characteristics despite large differences in properties. The magnitude of the hard-axis pulse field necessary to cause creep increases with increasing difference between wall coercive force and easy-axis bias. Furthermore, the average creep displacement, for a given hard-axis pulse field magnitude, versus the easy-axis bias field normalized by wall coercive force results in almost identical curves for both NiFe and NiFeCo films except for a shift related to the threshold field. This result is consistent with previous low-frequency creep data and implies that the basic mechanism of low- and high-frequency creep may be closely related. The direction of the basic wall motion depends on the spin polarity in the wall and the durection of the hard-axis pulsed field. Motion in a direction opposite to the basic motion may be produced by an esay-axis bias field of sufficient magnitude but less than then wall coercive force, which is a new and significant result. A theory based on the dynamic torque equation and nonconservative spring coercive force model has been developed to explain the high-frequency creep phenomenon. This theory qualitatively predicts the observed experimental results very well.     

High-speed creep effects in magnetic films

Creep measurements were made on single domain walls in thin magnetic films using high-speed pulses with variable rise and fall times (0.4 ns to>1 mus) and durations (<1 ns to 3 μs). Combinations of these pulses and dc fields were applied along the hard axis while simultaneously easy-axis dc fields were applied. The two basic measurements that were made were onset of creep and the distance the wall crept per pulse as a function of applied fields. Definite rise-time effects were found, the exact behavior depending on the domain wall structure. For Bloch walls, gyromagnetic effects of the total wall (similar to wall streaming) are present for rise timeslsim 20ns, whereas for longer rise times the Bloch to Néel wall transition appears to be responsible. For films thinner than 900 Å the existence of a cross-tie structure was found to be necessary for creep. For this wall structure the exact mechanism which causes creep enhancement for rise times <100 ns is unknown.     

Magnetization creep in magnetic films

Magnetization reversal in planar magnetic films can occur for field amplitudes well below the static threshold. Repeated field transitions parallel to a film's hard axis in the presence of an easy-axis field can cause slow motion or creeping of the domain boundaries. Experimental observations of magnetization creep and proposed creep mechanisms are reviewed. Creep was studied in slow-rising sinewave and pulsed hard-axis fields. The effective creep thresholds obtained for these cases are compared. In an operating mode in which a bipolar easy-axis pulse and a unipolar word pulse are used, creep is reduced and the effective creep threshold is improved. Examples of domain growth in unipolar and bipolar hard-axis fields are shown in a series of Kerr optical photographs demonstrating the creep process.     

Magnetic properties and creep observations of Ni-Fe films 30 to 300 Å thick

Coercivity, induced uniaxial anisotropy field, angular dispersion, residual stress, magnetic domain wall, and magnetization creep of vapor-deposited 81.7 Ni-18.3 Fe films are reported. The thickness and temperature dependence of the coercivity and the induced uniaxial anisotropy field in the thickness range from 30 to 16 000 Å are described. A substrate temperature range is given for minimum residual stress. Lorentz micrographs of magnetic domain walls and magnetization creep of films in the thickness range from 30 to 300 Å are presented. It has been found that Néel walls creep under the influence of a unipolar hard-axis field pulse in the presence of a constant dc easy-axis field. The paper discusses the experimental results in the light of existing creep theories.     

Domain Wall Dynamics in Thin Magnetic Wires Under the Influence of Transversal Magnetic Field

We have studied the domain wall dynamics in thin amorphous glass-coating microwires under the influence of both axial and transversal magnetic fields. Two different regions of domain wall dynamics have been found according to the amplitude of the applied magnetic field rotated out of the wire's axis. At low amplitude, the effect of applied field rotation is reduced to the reduction of its axial component and the domain wall velocity decreases. For high amplitudes of magnetic field, the maximum velocity was found when applied field forms angle ${-}20^{circ}$ with the wire's axis. The results are explained in terms of different domain wall structure, being transversal for low amplitudes of magnetic field and vortex for high amplitude of applied magnetic fields.      

Cross-tie—Néel domain wall transition in thin magnetic films

A theoretical consideration of the hysteresis change of the domain wall structure in flat uniaxial thin magnetic films under the influence of a magnetic field parallel to the hard axis has been carried out. The theory is based on a cross-tie wall model defined in a more precise manner than that of Middelhoek. The circular Bloch line shift as a function of a magnetic field and estimates for the critical fields of both the direct “cross-tie Néel” wall transition and the inverse transition have been obtained.     

Domain wall velocity and interrupted pulse experiments

The velocity of magnetic domain walls in Bloch wall Permalloy thin films excited by interrupted easy axis drive fields is investigated and correlated with the available theory of transient wall contraction. The experimental technique is useful in determining the average wall velocity, and by comparing the velocity-field curves with field duration as a parameter it is possible to infer several features of the transient behavior. It is found that, when the displacement of the wall is of the order of a characteristic distance associated with the wall coercive force interaction, the threshold field for net motion as well as the mobility of the average velocity curves depend on the field duration. The theory together with a breakable-spring model of the coercive force interaction reasonably explain the observed phenomena.     

用于制备YBCO薄膜的光辅助MOCVD反应器内流动现象的数值研究

对一种制备YBCO薄膜的光辅助MOCVD反应器内的流动现象进行了三维数值模拟研究.在模拟计算中,分别改变基座与进口的相对角度(Φ)、反应器顶壁倾斜角度(Ψ),得出反应器中流场的相应变化.根据对模拟结果的分析,发现当基座平面与进气口轴线之间的夹角Φ=22.5°及反应器顶壁倾角ψ=30°时,基片表面气流速度大小合适,分布均...     

Superconducting to normal transition of type-II Ta by a rectangular current pulse

The superconducting to normal transition by a rectangular pulse current in type-II Ta has been investigated. The potential drop along the specimen is observed as a function of time. The voltage pattern and the magnitude of the potential drop are obtained as functions of current amplitude, temperature, externally applied magnetic field, the angle between magnetic field and specimen axis, and values. The transition mechanisms are discussed. It is also shown in this experiment that the flux flow or flux creep and flux jump occur. A support to the paramagnetic current flow in the presence of longitudinal magnetic field is given.     

Deformation stability of a flat domain wall in magnetic films

The forces determining the orientation of domain walls in films of magnetooptic materials with a figure of merit of order unity are studied. The behavior of small perturbations of the position of a flat domain wall in the presence of an in-plane component of the anisotropy vector is analyzed. The forces arising when the orientation of the domain wall deviates from the easy-magnetization axis and striving to return the wall to its initial state are conventionally represented as a gradient “effective magnetic field.” The forces exerted by the “effective field, ” due to the in-plane component of the anisotropy vector, on the perturbed domain wall are calculated. The orientational stability conditions for a planar domain wall are found. An explanation is given for the experimentally observed predominant orientation of striped domains. Pis’ma Zh. Tekh. Fiz. 25, 49–56 (September 12, 1999)     

Effect of Critical Current Density and Critical Temperature on ac Susceptibility

Based on the flux creep equation, the effect of critical current density and critical temperature on ac susceptibility is investigated numerically in a superconducting slab immersed in an ac magnetic field. The current density dependence of the flux creep activation barrier is employed as the logarithmic law. The fundamental ac susceptibilities of the slab as a function of temperature for the same ac field have been derived in a unified picture. The results show that ac susceptibility in flux creep regime is affected by critical current density and critical temperature.     

Analysis of creep and modulus loss of the wood cell wall

This paper proposes a nonlinear constitutive model for the wood cell wall based on the nonequilibrium thermodynamics. The wood cell wall is modeled as a long fiber-reinforced composite with cellulose microfibril enclosed by hemicellulose and lignin. An internal variable is introduced into the Helmholtz free energy of the cell wall system, to describe the modulus loss of hemicellulose due to moisture absorption. The viscoelastic behavior of the wood cell wall changes with its moisture content, which leads to different creep evolutions even under the same loading level. To account for this phenomenon, another internal variable is introduced to depict the creep behavior of the wood cell wall, which is correlated with the irreversible energy dissipation processes such as stick–slip mechanism in the wood cell wall. Based on five elastic coefficients of transverse isotropy predicted by the present model, the creep behaviors of the wood cell wall with different microfibril angles are theoretically analyzed and show good agreements with experiment results.     

聚甲基丙烯酰亚胺泡沫压缩失效寿命预测及模型验证

聚甲基丙烯酸酰亚胺(PMI)泡沫因其性能优越,在航空航天领域广泛应用。本文主要针队PMI泡沫在弹筒适配器领域的功能特性开展研究,主要研究了其在常温条件下的压缩蠕变特性。依据使用工况及高分子材料的蠕变特性,采用“时间强化”模型设计实验,分别对密度为0.075 g/cm³和0.110 g/cm³的PMI泡沫进行了为期180天的常温压缩蠕变实验。通过对实验数据分析、拟合,预测了两种不同密度的PMI泡沫常温、1 250 N条件下的压缩蠕变寿命,密度0.075 g/cm³ PMI泡沫压缩蠕变失效寿命约为11年;而密度0.110 g/cm³PMI泡沫约为53年,同时对模型的可靠性进行了验证分析。     

Magnetic measurements with Lorentz microscopy

The electron microscope may be operated in a mode which permits the exploration of the magnetization configurations in thin magnetic films. This mode of operation, known as Lorentz microscopy, is a powerful technique for investigating thin NiFe films because it offers high resolution, because it provides an unequivocal identification of the local magnetization direction, and because it permits correlations to be made between the magnetic structure and the underlying physical (crystallographic) structure of the film. In the past, Lorentz microscopy has found fruitful employment in the analysis of the magnetization configurations of domain walls, in studies of various magnetization reversal processes, and in specialized investigations of unusual magnetic structure. Besides these primarily qualitative investigations, however, some quantitative measurements may be made with this instrument. Such measurements are useful not only because they permit direct evaluation of basic magnetic parameters of films being studied by Lorentz microscopy, but because they afford insight into the fundamental processes which occur in the standard macroscopic magnetic measurements of NiFe films deposited on glass substrates. The following measurements are discussed: 1) determination of the Curie temperature; 2) measurement of the anisotropy field Hkby the standard hysteresigraph and the Feldtkeller techniques; 3) quantitative studies of wall motion by labyrinth propagation and by wall creep; 4) the investigation of anisotropy dispersion by the Crowther and Torok techniques. The accuracy of these measurements is, in general, lower than that of the analogous measurements made by macroscopic methods on films deposited on glass substrates. Nevertheless, macroscopic measurements performed on a film on a glass substrate showed good agreement with Lorentz measurements performed on a simultaneously-deposited film which was suitable for Lorentz microscopy.     

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimens for creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composites, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinations from 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behavior based on microstructural observations are discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 125–128, January–February, 2008.     

On the formation of the diamond grain configuration during high temperature creep and fatigue

A study has been made of the influence of test variables on the formation of the diamond grain configuration during high temperature creep and fatigue deformation of a wide variety of metals. The proposed mechanisms for the formation of this interesting grain morphology are reviewed. It is concluded that the diamond grain configuration arises from a balance between grain-boundary sliding, grain-boundary mobility, intragranular deformation and defect imbalance across the grain boundaries and that it tends to be stabilized by intergranular cavitation. While the phenomenon occurs during high temperature fatigue in a variety of metals irrespective of their crystal structure, during creep it has been observed only in to h c p metals. It is surmised that the occurrence of the diamond array of grain boundaries during creep deformation in h c p metals is aided by the limited number of slip systems which leads to high defect imbalances in adjacent grains and consequently high driving forces for grain-boundary migration. On the basis of quantitative metallography involving measurements of the number of edges per grain section, the number of grains meeting at vertices, angular distribution histograms and grain-boundary lengths in different angular orientations with respect to the stress axis in "annealed" and "diamond" microstructures, it is concluded that the shape of the "diamond" grain is essentially the same as that of the "annealed" grain but in a distorted form.     

Coupling phase field with creep damage to study evolution and creep deformation of single crystal superalloys

A phase-field model coupling with elastoplastic deformation and creep damage has been built to study the microstructural evolution and deformation behavior for Ni-Al single crystal alloy during the whole creep processing.The relevant experiments were conducted to verify the model validity.The simulation results show that under the tensile creep at 1223 K/100 MPa,cubic γ'phases coarsen along the direction parallel to the axis of tensile stress during the first two creep stages;and spindle-shaped and wavy γ'phases are formed during tertiary creep,similar to the experimental results.The evolution mechanism ofγ'phases is analyzed from the perspective of changes of stress and strain fields.The "island-like" γ phase is observed and its formation mechanism is discussed.With the increase of creep stress,the directional coarsening of γ'phase is accelerated,the steady-state creep rate is increased and the creep life is decreased.The comparison between simulated and experimental creep curves shows that this phase-field model can effectively simulate the performance changes during the first two creep stages and predict the influence of creep stresses on creep properties.Our work provides a potential approach to synchronously simulate the creep microstructure and property of superalloys strengthened by γ'precipitates.     

Creep of clay masonry exhibiting cryptoflorescence

To determine creep of clay masonry in the laboratory, total time-dependent strain readings are recorded from a wall subjected to a constant load and from a corresponding control wall not subjected to load. Creep is then calculated by subtracting the moisture movement strain of the control wall plus the elastic strain from the loaded wall, from the total strain recorded on the loaded wall. It is assumed that the moisture movement behaviour is the same in both the loaded and the control walls. The paper questions that assumption for certain types of clay brickwork. Small control walls built from low strength bricks having high water absorption can undergo an enlarged moisture expansion. The enlarged expansion is greater than the irreversible moisture expansion of the unbonded clay brick and is thought to be the result of crystallization of salts at the brick-mortar interface, a process known as cryptoflorescence. In a loaded wall, cryptoflorescence is suppressed or restrained so that the effect of an enlarged expansion of the control wall is to overestimate creep and considerably so in some cases. The paper demonstrates the problem by examining the effect of enlarged expansions when ascertaining creep of masonry built from different types of clay bricks. Experimental and design recommendations are made for more reliable methods of determining creep.     

Nonlinear domain-wall velocity and coercive force in permalloy films

Domain wall velocities are measured for 2700 Å and 2200 Å thick Permalloy films of several coercive forces. The velocity-drive field curves are characterized by two mobilities, as often reported. It is found that these low- and high-field mobilities are strongly affected by coercive force of the films. The low-field mobility is nearly constant for low-coercive-force films, and decreases rapidly with increasing coercive force. The high-field mobility is inversely proportional to the coercive force. The measured results suggest that the nonlinear dependence of wall velocity on drive field would disappear in a zero-coercive-force film. None of the mechanisms proposed up to now can explain this behavior of wall velocity in Permalloy films. The nonlinearity is attributed to the transition of material inhomogeneities from pinning centers in low field to dissipation centers in high field.