Theoretical study on stress-induced demagnetization in magnetic recording media

A simple and fundamental model of stress-induced demagnetization, which has scarcely been analyzed theoretically, is presented. Stress-induced demagnetization in a magnetic recording medium results from the magnetization reversal of individual magnetic particles dispersed in the magnetic layer. The magnetoelastic energy due to the application of three-dimensional stresses and the magnetostatic energy due to the demagnetizing field must be taken into account for an investigation of the magnetization reversal of each particle. It is found that the magnetization reversal can take place even during small stresses if a sufficient demagnetizing field exists. The sign of the magnetostrictive coefficient λ100has a significant relation to the susceptibility of magnetization reversal.     

Theory of magnetization ripple

A review is given about the assumptions the development, and the results of the ripple theory. The limits of the linear theory are pointed out. The result of this theory shows a very strong coupling of the magnetization dispersion perpendicular to the mean magnetization, which is caused by the internal demagnetizing field. The exchange constant of this magnetostatic coupling exceeds the quantum mechanical exchange constantAby orders of magnitude. The nonlinear ripple theory includes the component of the internal demagnetizing field parallel to the mean magnetization. This field becomes important when the rms values of the magnetization dispersion aregapprox 2deg. The nonlinear theory explains all deviations from a single domain behavior without introducing any special model.     

Dynamics of domains and walls in soft magnetic films

General equations are derived to describe the simultaneous nonuniform planar rotations of the magnetization vector and displacements of curved domain walls and their junctions in soft magnetic films. These equations take into account effects of exchange stiffness, magnetic anisotropy, external and either long- or short-range demagnetizing fields, wall energy, and dissipation. The case of a matched film pair using the capacitor or transmission-surface approximation for its short-range demagnetizing energy is considered. The theory is founded on energy and dissipation functionals including domain and wall terms. The constraint of wall-normal magnetization continuity across a domain wall is handled by a method of implementing d'Alembert's virtual work principle without introducing Lagrange multipliers. The result is a set of coupled equations expressing the dynamic torque balance at points inside domains, the wall-domain constraint due to wall-normal magnetization continuity, an additional boundary condition coupling domain magnetization and wall curves, and the wall velocity.<>     

Dynamical interpretation of magnetic recording process

The demagnetization effect in magnetic recording must be evaluated, not as static self-demagnetization, but as a dynamic demagnetizing field at an instant when the head field is applied to the medium. From this fact it becomes necessary to obtain a self-consistent magnetization distribution in the medium. A method of calculation and its results are described. The relation between the longitudinal and the vector magnetization is clarified. The experimental results of the recording demagnetization in sinusoidal recording and the pulse width and the peak shift in digital recording are interpreted as the new phenomena that is related to the dynamical behavior of the demagnetizing field in the recording process.     

Micromagnetic investigation by a simplified approach on the demagnetization field of permanent magnets with nonmagnetic phase inside

A simplified analysis method based on micromagnetic simulation is proposed to investigate effects of nonmagnetic particles on the demagnetizing field of a permanent magnet. By applying the additivity law of the demagnetizing field, the complicated demagnetizing field of the real magnet could be analyzed by only focusing on the stray field of the reserved magnet. For a magnet with nonmagnetic particles inside, the particle size has no significant effect on the maximum value of the demagnetization field, but the area of the affected region by the particle is proportional to the particle size. A large particle produces a large affected area overlapped with those influenced by other particles, which leads to the large demagnetization field. With increasing the length of the particle along the magnetization direction, the demagnetization field on the pole surface increases. The pole surface with a convex shape will increase the demagnetization field. The demagnetizing field near the nonmagnetic particle will be further increased by the large macroscopic demagnetizing field near the pole surface. This work suggests some practical approaches to optimize the microstructure of permanent magnets.     

Magnetostatics of anisotropic ellipsoidal bodies

The demagnetizing tensor formalism for the evaluation of magnetization and internal field in ellipsoidal bodies of isotropic materials is extended to the case of anisotropic materials and to the calculation of the field outside the body.     

Comments on "An analysis of tilted magnetic transition in magnetic transitions in magnetic recording media"

Expressions for the demagnetizing field caused by the surface magnetic charge produced by vertical magnetization in a tilted transition are presented. This field should be added to that used by Chang and Perez in their analysis of tilted transitions.     

Perpendicular magnetization structure of Co-Cr films

In a perpendicular recording system, a Co-Cr film as a medium is capable of storing very high density signals. Lorentz microscopy of 1000 kV TEM was used to observe the structure of recorded magnetizations in Co-Cr films having perpendicular anisotropy. A composite medium of a Co-Cr film with a soft magnetic back layer was shown by Lorentz microscopy to have a horseshoe magnetization structure. The stable antiparallel magnetization of transition in the Co-Cr layer determined the head-on magnetization structure of the soft magnetic back layer, which consists of a new straw-rope domain structure. The perpendicular magnetization structure of the Co-Cr film was found to consist of small domains magnetized through the film thickness which correspond to the columnar microstructure of the film. Since the intrinsic hysteresis loop of a Co-Cr film was shown to essentially have an ideal rectangular shape, it can be concluded that the Co-Cr layer of a composite film can be recorded by an ideal magnetizing process with negligible demagnetizing field at the transition.     

Demagnetizing field computation for dynamic simulation of themagnetization reversal process

The magnetic-field distribution for a thin magnetic film is computed using the fast Fourier transform technique. The method is quite general and accommodates any two-dimensional magnetization distribution. It allows the computation of fields both inside the film (demagnetizing fields) and outside (stray fields and leakage)     

Effects of skew and demagnetizing fields on NDRO performance

Limitations placed upon the NDRO performance of a magnetic film memory device by the detrimental effects of skew and demagnetizing fields are investigated. An expression has been derived that relates these effects to the NDRO critical magnetization angle for devices whose rotational switching can be characterized by a Stoner-Wohlfarth switching astroid. Experimental results for typical devices are presented that substantiate the theory.     

Magnetic properties of thin ferromagnetic films in relation to their structure

The influence of film structure on magnetic properties is primarily due to the creation of spatially fluctuating local anisotropies which make the magnetization direction inhomogeneous. The deviation from homogeneous magnetization of domains, the so-called ripple, influences a great number of static, quasistatic and dynamic effects via the intrinsic demagnetizing field. The ripple theory is expressed in terms of only one phenomenological constant, the structure constant S, which covers completely the influence of the film structure on the deviations of real thin film behaviour from that of ideal single domains.     

Pt Content Dependence of the Switching Field Distributions of CoPtCr-SiO2 Perpendicular Media Characterized by Subtracting the Effect of Thermal Agitation

The switching field distributions (SFD) of CoPtCr-SiO₂ perpendicular media as a function of Pt content were characterized by subtracting the effect of thermal agitation, and discussed in relation to the microstructure. DC demagnetizing (DCD) magnetization curves and minor dc demagnetizing (M-DCD) magnetization curves were measured at applied field sweep rates of ~10 Oe/s and ~10⁸ Oe/s. We estimated the width of SFD, DeltaSFD, from the difference between the DCD and M-DCD curves, and defined them as DeltaH_r/H_r (at ~ 10 Oe/s) and DeltaH_r ^P/H_r ^P (at ~10⁸ Oe/s). The values of DeltaSFD characterized by subtracting the effect of thermal agitation, DeltaH ₀/H₀, were nearly half those of DeltaH_r /H_r for 10-nm-thick media. DeltaH₀/H₀ was about 0.10 at 10 at%Pt content, and increased as the Pt content increased, reaching 0.17 at 30 at%Pt content. The increase in DeltaH₀/H₀ was probably caused by an increase in the stacking fault density and the formation of fcc layers in the hcp CoPtCr lattice. A simple calculation based on the coherent switching of magnetization revealed that the c-axis distribution results in DeltaH ₀/H₀ of about 0.08, independent of Pt content. These results suggest that the DeltaSFD due to the grain-to-grain anisotropy field variation was small, only 0.02-0.03     

On the microstructure and magnetization process of γ-Fe2O3

Experimental evidence is reported for γ-Fe2O3single particles being made up of several microcrystals chained together and their boundaries being sources of demagnetizing fields. This leads to a very defined picture of the magnetization reversal in partially alligned γ-Fe2O3particles: rotation is incoherent but the magnetization vectors are scattered in such a way that Hcivs. angle Ψ (between the orientation direction and the applied field) does not follow the known magnetization reversal modes. The importance of the reduction temperature in the process: α-FeOOHunderrightarrow{red.}Fe3O4underrightarrow{ox.}γ-Fe2O3becomes extremely important if considered as the parameter which rules the pore closure within the particles, i.e. the size and number of microcrystals chained in a single particle.     

Coupling between flat films

The magnetization distribution in a rectangular film, magnetized parallel to one side, is approximated by assuming that the magnetizationoverrightarrow{M}has the same direction everywhere in the film and that its magnitude rises from zero at the edge to the saturation magnetization Msat a distancecfrom the edge, maintaining this magnitude throughout the remainder of the film area. The function representing the magnitude ofoverrightarrow{M}in the edge region is chosen in such a way that the resulting field has no singularities,cis determined by the coercive force, which is set equal to the sum of the maximum demagnetizing field and an applied reversing field. This model lends itself better to the discussion of demagnetizing effects in single or coupled films than either the ellipsoid or the line-charge model. It takes account of the fact that the width of the edge region, where curling and domain walls occur, varies with the applied field, and, in this respect, it agrees fairly well with experiment. It, furthermore, permits the definition of a disturb sensitivity for memory elements consisting of single or coupled films.     

基于元胞自动机的薄膜磁滞回线的计算机模拟

利用Stoner-Wohlfarth模型,概率型元胞自动机的模型和算法,在考虑了退磁场和交换作用的情况下对薄膜的磁滞回线进行了模拟,比较了温度以及和易轴取向变化时磁化曲线以及剩磁矩形比和矫顽力的不同及变化趋势.退磁场和交换作用的引入对磁化翻转过程有显著的影响,剩磁矩形比和矫顽力都有不同程度的提高.     

Magnetization distributions in bulk magnetic material

Knowledge of the internal magnetization distributionsM(r)of polarizable media allows straightforward calculation of fields for any configuration which includes these media. Although useful calculations have been made using an assumedM(r), little work directed at calculatingM(r)has been done. This paper includes assumptions that, in addition to Maxwell's equations, are sufficient for determining the resultant integral equation. A method for the solution of this equation forM(r)is also presented. As an example, the nonuniform demagnetizing fields and magnetization distributions are calculated for cylinders of isotropic magnetic material. The results for these samples are compared with experimentally determined values and show average errors less than 9 percent forM(r).     

Approximate theory of microwave generation in a current-driven magnetic nanocontact magnetized in an arbitrary direction

We present an approximate nonlinear theory of microwave generation by spin-polarized direct current in a magnetic nanocontact magnetized in an arbitrary direction. We argue that, when the spin-transfer torque caused by spin-polarized current compensates the natural magnetic dissipation in a "free" layer of the nanocontact, a nonlinear quasi-uniform precession of magnetization about the direction of the internal bias magnetic field is excited. With the increase of the current magnitude the angle of precession increases, making precession strongly nonlinear and reducing the projection M/sub z/ of the precessing magnetization vector on the precession axis (z axis). This reduction of M/sub z/ is responsible for the nonlinear limitation of the precession amplitude and for the nonlinear frequency shifts of the generated microwave oscillations. Because of the influence of demagnetizing fields in the "free" layer, the nonlinear frequency shifts have different magnitudes and signs for different orientations of the external bias field H/sub e/. The theory gives a good qualitative, and even partly quantitative, explanation of the main part of microwave magnetization dynamics experimentally observed in magnetic nanocontacts.     

Phenomenology of ferromagnetism. I. Effects of magnetostatics onsusceptibility

Magnetic measurements, with particular emphasis on finite cylinders of iron in the form of whiskers, are interpreted with regard to magnetostatic effects. Demagnetizing fields are calculated using various degrees of approximation. In the cases considered the magnetostatic effects are dominant over other contributions to the micromagnetic energies. A particular approximation in which the demagnetizing field is assumed to be proportional to the second derivative of the magnetization is used to describe the sharp changes in response with field that occur near the Curie temperature