Ernst Feldtkeller

Coupled Walls in Multilayer Films

If the thickness of an intermediate nonmagnetic layer separating two ferromagnetic layers is larger than the ferromagnetic wall thickness (order of 1 μ), only a magnetostatic interaction at the film edges exists. If the intermediate‐film thickness is smaller than the wall thickness, the magnetostatic interaction of the domain walls is essential, yielding special multilayer wall configurations. If the intermediate film is thinner than that necessary for a complete exchange isolation (order of 10 nm), the wall configurations are modified by an exchange coupling between the local magnetizations of the layers. The wall configurations and their influence on the wall motion and switching behavior are reviewed in this paper.

Struktur und Energie von Blochlinien in dünnen ferromagnetischen Schichten

ZusammenfassungDer Radius des Gebietes in einer Kreisblochlinie, in dem die Magnetisierungsrichtung wesentlich von der Schichtebene abweicht, hängt nur wenig von der Schichtdicke ab und liegt für 80/20-Nickeleisenschichten in der Größenordnung 100 Å. Das Gebiet ist umgeben von einem zirkular magnetisierten Gebiet, das erst bei wesentlich größeren Radien in die nicht mehr zirkulare Konfiguration der Wände und Domänen übergeht.RésuméOn a calculé, pour des couches minces ferromagnétiques, la structure (Fig. 2) et l’énergie (Fig. 5) d’une ligne de Bloch circulaire (∘ dans Fig. 1). Le rayon du cercle, où le vecteur d’aimantation émerge nettement du plan de la couche, ne dépend guère de l’épaisseur de la couche. Il est de l’ordre de 100 Å pour les couches de 20/80 fer-nickel (Fig. 4). Ce cercle est entouré d’une zone magnétisée circulairement, d’un rayon beaucoup plus grand, qui s’étend jusqu’à la zone de transition où la configuration des parois et des domaines cesse d’être circulaire (Fig. 6).AbstractFor circular Bloch lines (∘ of Fig. 1) in ferromagnetic thin films the structure (Fig. 2) and the energy (Fig. 5) have been calculated. The radius of the region in which the magnetization is tilted considerably out of the film plane, depends only weakly on the film thickness and has the order of 100 Å for 80/20 nickel-iron films (Fig. 4). This region is surrounded by a circularly magnetized region. The transition to the noncircular configuration of the walls and domains takes place at an essentially higher radius (Fig. 6).

Suszeptibilitätsmessungen an anisotropen Nickeleisen-Schichten

A d. c. field was applied along one of the anisotropy axes of evaporated nickel-iron films with uniaxial anisotropy, and a small a. c. field was applied perpendicular to the d. c. field. The magnetization amplitude parallel to the a. c. field was measured. The dependence of the susceptibility, measured like this, on the d. c. field strength corresponds to the single domain theory only for films with a smallHc/HK ratio or for large d. c. fields. The deviations from the single domain theory and the losses occurring can be explained by the ripple structure of the magnetization in polycrystalline films.

Ripple Hysteresis in Thin Magnetic Films

Hysteresis measurements and electron micrographs have shown that the rotation in films with uniaxial anisotropy in low fields is not reversible, in contradiction to the single domain theory. A film with Hc/HK=2.3, for instance, has a remanence of 0.04 Ms in an alternating field of the amplitude Ĥ=0.4 HK parallel to the hard axis. The remanence depends on Hc/HK, on the dispersion of the anisotropy, and on the amplitude and the direction of the applied field.This irreversibility is caused by the magnetization ripple. If the magnetization is rotated, the ripple small‐angle walls rearrange themselves by wall motion normal to the new magnetization direction. This wall motion requires a certain excess of energy which results in hysteretic losses even if the applied field strength is lower than the Stoner‐Wohlfarth critical field for irreversible rotations.

Wall Streaming: A Gyromagnetically Induced Kind of Wall Motion

A field applied perpendicular to a Bloch wall does not favor one of the adjacent domains. Nevertheless, the gyromagnetic behavior of the wall magnetization, i.e., the precession around the field instead of a rotation towards the field direction, may induce a wall displacement. This has been observed experimentally in Permalloy films with a thickness larger than 0.1 μ when a pulse field has been applied along the hard axis with a pulse risetime smaller than 20 nsec. The effect (called wall streaming) is compared, from a theoretical point of view, with the wall behavior in a pulse field applied parallel to the wall.