Normal Magnetic Component Research Articles

The magnetoelastic interaction of dislocations and ferromagnetic domain walls in iron and nickel

A theory is developed which permits the magnetoelastic interaction of domain walls and dislocations to be computed as a function of their relative separation. The method corresponds to a two constant theory of magnetostriction and is applied to more than sixty cases for iron and nickel. The calculations indicate that dislocations in iron interact more strongly by as much as an order of magnitude with 180 deg walls than with 90 deg Type B walls,i.e., walls with normal component of magnetization but equal zero. The situation for nickel is just the opposite. The results are discussed in terms of the effects of domain walls on individual dislocations and the effect of extensive arrays of dislocations on magnetic properties.

On One of the Possible Mechanisms of Formation of Induced Anisotropy in Ferromagnetic Films

AbstractWith the assumption that the concentration of nonmagnetic inclusions (lattice defects, impurity atoms, oxides) is higher at the grain boundary than inside the grain, a steady state distribution of inclusions is determined along the grain boundary in a ferromagnetic film. It appears to be nonuniform since inclusions leave those boundary sections where the jump of the normal component of magnetization at the boundary is maximum and go into sections where the jump is minimum. Nonuniformity of the inclusion distribution at the boundary results in a magnetic anisotropy. Expressions are derived for the anisotropy energy. The numerical evaluation shows that the experimental value of anisotropy energy is obtained by such a mechanism in the case when inclusions decrease strongly the magnetization of the boundaries.

Ferromagnetic domain structure of iron whiskers with <111> axes

Iron whiskers, with axes in \langle111\rangle directions and {110} faces separated by {112} faces, have been grown, and the domain patterns present were observed using the powder pattern technique. The structure consists of six main domains magnetized at ±55° to the axis in \langle100\rangle easy directions. The main walls are 90° walls having a zig-zag structure and meet the whisker surfaces in the {112} faces. These walls are normal zig-zag walls but are here seen obliquely, producing a surface zig-zag angle of 147°. At these faces there is a normal component of magnetization, and complex closure structures form to reduce the resulting magnetostatic energy. Two different types of closure structure are observed and the different patterns appear on alternate main domains. In the {110} faces the magnetization is in the surface so that no closure structure is necessary. The effect of applying magnetic fields perpendicular and parallel to the whisker axis was studied: in the former case the changes occurred by displacement of the junction of the walls, while in the latter, bodily movement of the main walls and growth of new walls in {112} planes appeared to take place. Tension was applied along the whisker axis, and the resultant changes in the domain pattern were caused by the interaction of the applied stress with the two different types of closure structure. At high applied stresses, a sudden change occurred to a complex surface structure with the disappearance of the original zig-zag walls; this pattern remained when the stress was removed.

Electron Microscopic Observation of Domain Wall-Inclusion Interactions in Iron

Transmission electron microscopic observations of the dynamic behavior of domain walls in thin polycrystalline foils of iron containing inclusions of varying sizes have been carried out under conditions of known applied field.1 The field component in the plane of the specimen was varied over the range 0→±10 Oe, traversing hysteresis loops in times of the order of 10 sec. The observations show that inclusions affect the mobility of domain walls not only through the formation of individual Néel spike domains, but also by acting as tie points for domain wall intersections and by the formation of ``ladder'' domain structures connecting closely spaced inclusions. The detailed domain structures observed during the cutting of large (about 1 μ) inclusions by 180° domain walls correspond closely to the structures predicted by detailed theoretical models of this process.2 New domains are readily formed at grain boundaries. A frequently observed alternative to the cutting of an inclusion by a domain wall is the formation of one or more new domains in other parts of the same grain. Continuity of the normal component of magnetization across grain boundaries is accomplished either by echelon structures of the sort observed in thin nickel platelets3 or by individual spike domains extending from the boundary. A 16-mm motion picture film illustrating these points has been prepared and was presented at the conference.

Structure of Domain Walls in Multiple Films

We derive an approximate solution of the micromagnetic equations for a Néel-type wall in multiple films. A small normal component of magnetization M permits most of the pole distribution to concentrate on the film surfaces. Thus, the magnetic-field energy resides mostly in the nonmagnetic gaps between magnetic films. The wall structure is found to consist of two ``wings'' having little exchange energy which flank a ``kernel'' in which only exchange and anisotropy energy terms are significantly involved. The kernel makes secondary contributions to the total width and energy of the wall. In first order, the wing shape is given by Mx = M0 exp{−[(K/πDb)½ |x|/M0]}, where K = anisotropy, b = thickness of nonmagnetic layer, x = distance normal to the wall plane, and D = one magnetic layer thickness for a double film or one-half of a thickness for an infinite number of layers. The first-order energy per unit area is 2M0(πKDb)½. This energy is lower than previous results and extends the region of stability for Néel walls (as opposed to Bloch walls) to greater D, especially for small b.

Domain Observations on Iron Whiskers

We have confirmed the appearance in iron whiskers of domain walls which apparently violate the restriction that the normal component of magnetization must remain constant across the wall. These walls sometimes have a dotted or serrated appearance. We have found such walls in a closure domain structure which permits the calculation of an approximate value for the surface energy of the serrated wall; this energy is approximately ten times the energy of a normal 90° wall or five times the energy of a normal 180° wall. We conclude that the serrated wall contains an inner structure which largely eliminates any free poles.

The Theory of the Ferromagnetic Anisotropy of Single Crystals

It is well known that in single crystals of iron and nickel the direction of magnetization is not generally parallel to the direction of the magnetic field, although these crystals belong to the cubic system. When the magnetization in a crystal has a certain value as measured in the direction of the field, there will be also, in general, a component of magnetization measured in the direction at right angles. This paper describes the calculation according to the domain theory, of the normal component of magnetization, using certain assumptions which are almost identical with those used by Heisenberg in his calculation of the magnetostriction of iron crystals. Theoretical curves are shown for a variety of crystallographic directions. Each of these curves shows all of the possible positions and magnitudes of the vector representing the magnetization in iron crystals as the magnetic field parallel to any given direction in the crystal increases in strength from zero to a high value. The theoretical curves are compared with the experimental curves of Honda and Kaya, and show good agreement with them.