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.     

Comparison of magnetic behavior of cylindrical and flat films from Kerr effect probe measurements

The magnetic characteristics of flat and cylindrical permalloy films are compared. 1000 Å-flat films and 10 000-Å cylindrical films electrodeposited onto 5-mil-diameter wires show very similar magnetic properties. Geometrical differences arise largely in the relative ease or difficulty with which fields may be applied to the films and signals extracted. The greater thickness of the cylindrical films described gives rise to enhanced magnetostatic effects apparent in the Kerr effect probe measurements and to strong self-shielding against nonaxial external fields.     

Magnetization in Ni and Ni-Fe thin films

Nickel films have been deposited in conventional high vacuum, ultra-high vacuum, and in controlled contaminant, ultra-high vacuum environments. High field saturation magnetization Msmeasurements on the films made in high vacuum and an oxygen-contaminated ultra-high vacuum system show that they posses low values of Msfor film thicknesses of 1000 Å. The normal moments observed for ultra-high vacuum deposited films lead to the conclusion that the low Msvalues are associated with the oxygen and are caused by a true loss of effective magnetization rather than coupled superparamagnetism or a ripple blocking field. Supplementary torque and ferromagnetic resonance studies on the Ni and two different Ni-Fe compositions also show that the grain boundary perpendicular anisotropy models are not particularly relevant for films made under such nominal technical vacuum conditions.     

Magnetization and remagnetization of anisotrophic magnetically hard ferrites

Magnetization curves and hysteresis loops of anisotropic hard ferrites were analyzed after the specimens had been demagnetized either thermally or by an ac field parallel or perpendicular to the preferred axis (PA) between -196°C and Curie temperature, or by a dc field. Each demagnetization yields a specific magnetization curve; below saturation fields, asymmetrically positioned and noncentrosymmetrical loops develop, except after ac demagnetization at 20°C (=measuring temperature) parallel to PA. This is due to domain configurations differently arranged and of different stability. The asymmetry is caused by "frozen" unstable configurations, which undergo an irreversible change due to the action of the field. Therefore, with reversed field not all of the previous possibilities for remagnetization are still available.     

Magnetization process in Permalloy multilayer films

The magnetization process in positively coupled double and multilayered systems of Permalloy separated by carbon layers has been investigated. Domain patterns appearing in this process have been observed by both Lorentz electron microscopy and high-resolution Kerr effect. The patterns display a microstructure different from that observed in the single films, e.g., twin walls with crossing points, quasi-walls, superimposed cross and circular Bloch lines, and 360 degrees walls. The origin of the observed structures is analyzed. The effects of magnetic flux closure are shown.<>     

Magnetization reversal in sputtered Co-Cr films

The magnetic characteristics of a set of r.f. sputtered Co-Cr films of various composition and thickness are examined in order to evaluate their performance for perpendicular recording. The mechanism of the magnetization reversal is discussed on the bases of: i) the shape of the rotational hysteresis loss as a function of the internal rotating field, ii) the value of the rotational hysteresis integral, iii) the dependence of the coercivity Hc, and the remanence coercivity Hr, on the angle between the easy direction and the field direction. In particular the measurements of rotational hysteresis were performed by monitoring the rate of change of the angular velocity of a top containing the specimen spinning in a magnetic field perpendicular to its axis. The deviation of the experimental behaviour from the ideal models are related to the amount of anomalies in the columnar structure of the film.     

Measurement of energy and direction distribution of neutron and photon fluences in workplace fields

Within the EU Project EVIDOS, a spectrometer with 24 silicon detectors mounted on the surface of a polyethylene sphere is used for the determination of the energy and direction distribution of neutrons and photons. It has been characterized with respect to neutron radiation with energies from thermal up to 15 MeV and to photon radiation with energies from 65 keV to 6 MeV. The first measurements described here were performed in the simulated workplace field, CANEL, at Cadarache, with the purpose of checking the instrument and the unfolding procedures.     

Magnetization distribution in an isolated transition

A large-scale model was used to study the magnetization distribution in an isolated transition by measuring the stray fields near the top and bottom surfaces of the medium with a small Hall probe. It was found that, in an unoriented γFe2O3medium, more flux escapes from the top than from the bottom surface. A simple, four-parameter transition function was found which is consistent with the observed stray-field distribution; Mx(x) is an arctan function, whereas My(x) is an arctan derivative. Expressions for Hxand Hy, inside and outside the medium, were derived and values for the four parameters were calculated by a least-squares fit to 28 experimental points. In a thin, square-loop medium with high saturation magnetization, the isolated transition has a pronounced asymmetry and Myis either zero or an odd function of y.     

Interpretation of Hall effect measurements in nonuniform magnetic fields

The interpretation of Hall effect data obtained with a long thin-film transducer of width a is discussed; the size of Hall probe contact pad and the characteristic dimension of the magnetic field nonuniformity are and l _n respectively. A diagram of local sensitivity of the transducer to the magnetic field has been plotted for l _i and a. Several particular examples of Hall emf measurement in a nonuniform magnetic field are considered.Translated from Izmeritel'naya Tekhnika No. 4, pp. 41–43, April, 1994.The author is grateful to V. B. Sandomirskii for proposing the problem and to A. A. Sukhanov and P. S. Sul'zhenko for a discussion of this work.     

A numerical analysis of flexure induced cylindrical cracks during indentation of thin hard films on soft substrates

In this paper, the fracture behavior of a thin hard film, perfectly bonded to a soft substrate, containing circumferential (cylindrical) cracks subjected to spherical indentation is studied using the finite element method. These cracks emanate upwards from the film-substrate interface and are driven by the flexure of the film over the soft substrate under indentation. The film is taken to be linear elastic while the substrate obeys an elastic-plastic constitutive model with linear strain hardening. Three values for the substrate yield strength are considered in the analysis. The variation of energy release rate and mode mixity are examined as functions of crack length and load, for cracks located near and away from the indentation axis. The results show that, when the crack length is small, predominantly mode I conditions prevail due to tensile radial stresses near the interface. As the crack length increases, the mode mixity gradually changes from mode I to II. For cracks located near the axis, the crack growth process is stable over a range of crack lengths up to about a third of the film thickness and thereafter becomes unstable. The role of the substrate yield strength on the above issues is investigated.     

Deformation and stability of technologically imperfect cylindrical shells in a nonuniform stress state

The authors examined the problem of deformation and stability of thin-walled shells on the basis of technological mechanics and the multifactorial theoretical-experimental approach. The combined effect of the nonplanar form of the end of a smooth cylindrical shell on its subcritical behavior and the process of stability loss is examined. The investigations are carried out by the method of holographic interferometry and multifactorial experiment; the experimental results are used to construct the numerical solution.Deformation of the shell corresponds in a stable manner to the nonplanar form of the end and is close to the state of nonuniformly compressed shells. The form of the contact pressure at high values of the amplitude of the nonplanar form greatly differs from the uniform form. The dependence of the load-carrying capacity on the amplitude and number of waves of the nonplanar form is nonmonotonic and separate examination of the effect of the parameters of the nonplanar form on the level of critical loads is incorrect. It is shown that increasing the load-carrying capacity by increasing the amplitude at a large number of nonplanar waves is possible.Translated from Problemy Prochnosti, No. 12, pp. 28–32, December, 1990.     

The elastic layer with a cylindrical hole subjected to a nonuniform axisymmetric radial displacement

Summary A problem in the linear theory of elasticity is considered wherein a layer with a circular cylindrical hole is subjected to a nonuniform axisymmetric radial displacement. The solution utilizes Navier's equations of elasticity which are sloved by means of extended Henkel transforms. A special case in which the readial displacement is a linear function of the axial coordinate is presented. Numerical results are given in graphical form for the case when hole radius and layer thickness are equal. The inversion integrals were evaluated numerically using Longman's technique for computing infinite integrals of oscillatory functions.

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Die elastische Schicht mit zylindrischem Loch unter nicht-konstanter axialsymmetrischer Radialverschiebung

Zusammenfassung Die Lösung des im Titel genannten Problem der linearen Elastizitätstheorie wird ausgehend von den Navierschen Gleichungen mit Hilfe von erweiterten Henkel-Transformationen gelöst. Eine Spezialfall (Radialverschiebung ist lineare Funktion der Axialkoordinate) wird erkläutert. Numerische Resultate des Falles bei dem Lochradius und Schichtdicke übereinstimmen werden in graphischer Form angegeben. Die Integrale der Rücktransformation werden numerisch, unter Verwendung der vonLongman angegebenen Methode zur Berechnung uneigentlicher Integrale oszillierender Funktionen ausgewertet.

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With 5 Figures     

Vibrations and buckling of composite orthotropic cylindrical shells with nonuniform axial loads

The vibrational response of orthotropic composite cylindrical shells, subjected to circumferentially nonuniform axial loads, is investigated based on Flügge-type field equations. The use of a complex finite Fourier transform provides a simple method for handling any arbitrary nonuniform load but introduces modal coupling between the transformed equations. For simply supported boundaries (conditions SS3) the determination of the critical buckling load reduces to finding the eigenvalues of a finite matrix. Two different nonuniform loads are considered, having forms proportional to (1+2cos θ) and (θ*−θ), where is the Heaviside function, θ is the circumferential coordinate and aθ* is the width of an axial strip of the shell of radius a. Computed results indicate the sensitivity of the critical buckling loads and free vibrational frequency to the type of nonuniform load and the material lay-ups of the cylinders.     

Change in the nature of hydrodynamic cavitation in nonuniform magnetic fields

It is known that in nonuniform magnetic fields the precavitation properties of aqueous media change, leading to an increase in the irreversible physicochemical changes.Notation l length of zone II - D and d diameters of tubes I, III, and II - p_I, p_II, p_III pressures in regions I, II, and III - p_cr critical pressure at which cavitation occurs - p_cr and p _cr ⁰ critical pressures in the magnetic field and when there is no magnetic field - [V_I, V_II, V_III] velocities of the liquid in regions I, II, and III - V_II, lim velocity of the liquid at which breakdown of the hydrated layer occurs for a certain value of the induction - V_cr and V _cr ⁰ critical velocities at which cavitation occurs in the magnetic field and when there is no magnetic field - p_a atmospheric pressure - p_sv saturation-vapor pressure at the given temperature - density of the liquid - kinematic viscosity - Re Reynolds number - Re_cr critical Reynolds number - c_gf and c_gd concentrations of free and dissolved gases in the magnetic field and when there is no magnetic field - c_gf and c_gd, and c _gf ⁰ and c _gd ⁰ concentrations of free and dissolved gases in the magnetic field and when there is no magnetic field - _sc space-charge density - electrical conductivity in the volume of the liquid - _b electrical conductivity in the boundary layer - _l , _g, _d dielectric constants of the liquid in the volume, of the gas in the bubbles, and of the diffusion layer - j, j_b, j_i, and j_T current density of the general, boundary layer, induced and current flow - f_MHD and f_EHD volume forces of magnetohydrodynamic and electrodynamic nature (per unit volume) - p_MHD pressure in the liquid due to the action of the magnetohydrodynamic forces - ₀ limiting shear stress in the liquid - B magnetic induction - E electric field strength in the volume of the liquid Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 5, pp. 842–850, November, 1978.