Magnetostatic Stray Field Research Articles

Domain structures and domain wall pinning in arrays of elliptical NiFe nanoelements

Stable domain patterns in a chain of NiFe nanoelements have been investigated using finite element micromagnetic simulations. The solution of the Landau-Lifshitz-Gilbert equation provides the hysteresis curves as well as the dynamic response of the system subject to an external field. We have studied elliptical elements with a long axis of 165 nm, a short axis of 55 nm and a thickness of 10 nm. Due to the shape anisotropy and magnetostatic interactions neighboring elements spontaneously align their magnetization antiparallel, if the magnetization of the elements was initially parallel to the short axis. If the initial magnetization is antiparallel, the magnetostatic stray field stabilizes this configuration. If the elements are in contact with each other, the effect of domain wall magnetoresistance can be used for applications. Chains of six elliptic elements with contact faces have been investigated. For sufficiently small contact faces, the antiparallel domain pattern is maintained, even though there is exchange coupling between neighboring elements. For small contact faces the stability of the antiparallel pattern is shown and a switching field of 48 kA/m has been obtained. The switching fields can be tailored by the shape and size of the elements.

Magnetic force microscope study of local pinning effects

A magnetic force microscope is used as a highly localized probe of the magnetic properties of Co-Pd multilayer films. The stray field from the magnetic tip is used to induce magnetization changes in the sample on a scale of the order of ∼100 nm. These changes are characterized by discontinuous jumps in the force or force gradient acting on the tip due to its interaction with the magnetostatic stray field from the sample. Both reversible and irreversible domain-wall motion is seen.

Magnetic properties and noise in cylindrical Permalloy thin-film magnetometers

The magnetic properties and low-frequency noise measurements of thin films of Permalloy are described. The films are obtained by electrodeposition. They are polycrystalline and single domain. The noise due to the film is shown to be an increasing function of the dispersion angle α <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</inf> . A model for the noise mechanism taking into account the magnetostatic stray field arising in the grain boundaries and the thermal agitation energy is given. It is in qualitative agreement with the experimental results.

Blocking and locking during rotational magnetization reversal in ferromagnetic thin films

A model describing quasi-static partial rotation and dynamic noncoherent rotation is presented and shown to agree with experiment. It is shown that, with applied field rise times longer than 1 ns, the ripple can be expected to relax in equilibrium with the rotating means magnetization. Magnetostatic stray fields prevent blocking from strongly affecting the rotational process and permit a stable ripple configuration even beyond the critical astroid. At a defined applied field threshold the inhomogeneous portion of the effective field from uniaxial anisotropy and applied field overcomes the stabilizing effects of the magnetostatic stray fields in alternate half-wavelengths of the ripple causing the magnetization there to switch through a large angle. The predicted large angle switching of the magnetization in alternate half wavelengths of the ripple is in good agreement with experimentally observed bands or stripes of reversed and unreversed magnetization both during quasi-static reversals in inverted Permalloy films and during dynamic non-coherent rotation with applied field rise times longer than 1 ns in normal Permalloy films. The model presents a reasonable fit to the field dependence of the angle between the observed stripes and the uniaxial anisotropy axis. Magnitudes of the local anisotropy causing the ripple are obtained from the fit to the data and shown to agree with the values determined by transverse susceptibility measurements.

Dynamic formation of diffuse boundaries and quasi-static domain growth in magnetic thin films

Using photographs obtained with a 10 nsec exposure time Kerr magneto-optic camera, the dynamic and quasi-static growth of reverse domains from nucleation centers at the edge of magnetic thin films have been studied and compared. The dynamic reverse domains grow wide, jagged ends consisting of segments of transverse diffuse boundaries, while the quasi-static domains are commonly observed to have sharp, well-defined tips. The photographs clearly show that dynamic domain tip propagation does not occur by the lateral movement of the domain walls defining the tip, but rather by the longitudinal propagation of the diffuse boundary at the end. The widening of the tips is shown to be expected theoretically and to be caused by the magnetostatic stray fields arising from the divergence of the magnetization at the tip.

Mechanisms of Reversal with Bias Fields, Deduced from Dynamic Magnetization Configuration Photographs of Thin Films

Flux reversal with a transverse field in Ni–Fe thin films has been studied using a Kerr magnetooptic apparatus which permits 10-nsec exposures to be taken of the dynamic magnetization configuration. Whenever the applied fields just exceed the critical asteroid, noncoherent rotation occurs, and a 50–100 μ wavelength striped configuration appears in the film. Before the stripes form there is a fast relaxation of the ripple as the magnetization rotates coherently to angles greater (5°–20°) than either the critical angle for reversal or the uniform torque minimum. Therefore, previous models of noncoherent rotation based on instabilities in the ripple which were expected to occur before or at the critical asteroid or uniform torque minimum do not explain the observed reversal process. After the stripes form, depending on the magnitude of the applied fields, one of two processes occurs: either (1) the rotation of the magnetization stops until the stripes break up into small nucleated regions or (2) the rotation is slowed by an order of magnitude and the stripes decrease in amplitude until they vanish. With lower pulse fields diffuse boundary propagation occurs. Magnetostatic stray fields from the boundary cause the magnetization in small regions to reverse by noncoherent rotation so that the propagation is predominant in the direction of the stripes.

Domain Wall Width Measurement in Cobalt Films by Lorentz Microscopy

A recent report by Suzuki et al. indicates that the domain wall width of 76% Ni–Fe alloy films with thickness 200 to 1800 Å is much wider than those calculated on the basis of the simple wall models. It was suggested that theoretical calculations might be in serious error due to gross overestimation of the magnetostatic stray field energy. In the present work, similar measurements of domain wall width have been made by the defocused mode of Lorentz microscopy for evaporated polycrystalline Co films with thickness from 200 to 1500 Å. Only the intensity profiles of the divergent wall images were taken into account, and a small out-of-focus distance (1.1 mm) was used, so that classical interpretation of the image would be allowable. The present study indicates (1) the wall width in Co is much greater than predicted by present theories with minimum value of 3000 Å at 500-Å film thickness and (2) the change in wall width with film thickness is much less than in the case of Ni–Fe films.