B. Petek

Micromagnetics of laminated Permalloy films

The edge curling wall (ECW) is observed in an optical Kerr microscope. Conditions are derived energetically favoring single transverse domains with ECWs over other multiple closure domains or single longitudinal domains that are undesirable because of their low permeability. Computed figures illustrate how the maximum permissible spacer thickness depends on Permalloy sublayer thickness, uniaxial anisotropy, width of the magnetic strip, mismatch of Permalloy thickness, and perpendicular anisotropy. The case with strip width W=100 mu m and vanishing magnetostriction or stress should require only a few sublayers. However, if W is very small, or if stress-induced or some other form of perpendicular anisotropy is excessive, then attainment of single transversely magnetized domains requires many sublayers. Some of these conclusions are supported by microscopic observations. >

Optical imaging of magnetic domains in motion (invited)

We describe a laser magneto‐optic microscope (LAMOM) system, used to observe magnetic domain dynamics in Permalloy thin‐film devices. Optimization of Kerr magneto‐optic contrast is achieved by using laser illumination in a polarized light microscope in combination with video image processing. High magnetic contrast, a diffraction‐limited (0.25 μm) resolution, and motion picture recording capability at up to 30 frames/s are demonstrated by recent results.

Domain conversion under high frequency excitation in inductive thin film heads

A magnetooptic microscope was used to observe the time-averaged magnetization distribution in an inductive thin-film head excited by continuous sine waves. Domain activity in yokes driven with sinusoidal currents (1 to 20 MHz) was observed using the Kerr effect at video frame rates (0 to 30 Hz). Thus, the average location and shape of domains in the top yoke of the head could be recorded. It is shown that the domain pattern generally undergoes significant changes in a slow, repeatable evolution. Some changes lead to abrupt conversions of domain states. Although specific behavior varies from head to head, these conversions follow measurable curves having a common trend in the amplitude versus frequency space. Previous work on analysis of head response has not considered this type of dynamic response, although it appears to be common to many magnetic system. In addition to possible response at the excitation frequency, the wall network can also undergo large changes with a time scale much longer than the excitation period. Three possible mechanisms driving the domain conversions are outlined. >

Time-resolved domain dynamics in thin-film heads

A novel magnetooptic imaging technique was used to investigate the dynamics of magnetization response in thin-film head yokes. Completed head devices were excited with a sinusoidal current applied to the integrated coils. The amplitude (20 to 40 mA pp) and frequency (1 to 50 MHz) of excitation were chosen to simulate the write process. Pulsed laser illumination permitted stroboscopic observation of domain-wall and flux-flow dynamics with a time resolution limited only by the 5-ns pulse width (full width at half maximum). Results suggest that high-frequency write performance is degraded by two mechanisms not considered previously for thin-film heads: first, 180 degrees walls appear to impede the flux-flow across the plane of the wall and, second, the inhomogeneous rotational magnetization response observed is known to be much slower than the typically assumed coherent rotation. The effect of NiFe composition on dynamics was also investigated by comparing responses of two heads, one having positive and the other negative magnetostriction. Flux in the head with positive magnetostriction flows in a constricted path along yoke edges, rather than at its center, which is consistent with the significantly lower efficiency measured electrically for this head. >

Methods for wide-field Kerr imaging of small magnetic devices

Wide-field magneto-optic Kerr observation techniques are described for the imaging of magnetic recording heads and other small magnetic devices. The imaging problems encountered for these samples are quite different from those for continuous films and lead to a different set of experimental approaches. Observation techniques that preserve the magnetic state of the sample allow for the implementation of new image acquisition algorithms based on multiple additions and subtractions that progressively build up a weak Kerr contrast even under unfavorable experimental conditions. Examples of results are presented. >

Bloch line influence on wall motion response in thin‐film heads

We describe a new dynamic response behavior of domain walls in the permalloy of thin‐film inductive heads. A laser magneto‐optic microscope (LAMOM) which enhances the longitudinal Kerr effect, is used to image Bloch lines (BLs) within the 180° walls by aligning the optical plane of incidence perpendicular to these walls. BLs are visible due to the reversal in the Neel‐type surface components of the wall magnetization at the BL position. Current pulses with fast transition times and ac currents within the frequency range of 1–10 MHz are applied to the integrated coil windings. Continuous excitation induces either a continuous flowing of the wall network or a temporary displacement. When individual pulses are applied, displacements of BLs are observed. Correlation of wall displacements with the BL displacements is demonstrated for some pulsed excitations.

Study of field‐driven wall‐configuration conversions for laminated Permalloy in the easy‐axis state

Laminated Permalloy, with edge‐curling walls replacing closure domains, has been proposed to increase permeability and reduce wall noise in recording. However, in structures meeting the criteria for Slonczewski’s ‘‘easy‐axis’’ state, normal walls often coexist with edge‐curling walls. We have used our laser magneto‐optic microscope to study inductive‐head‐yoke shaped elements of two and four Permalloy layers separated by nonmagnetic, metallic spacers. In the four‐magnetic‐layer sample a state with a single wall, terminating at the edge‐curling regions and lying along the easy‐axis direction, is often observed on the top and bottom layers. Some elements may be driven into an easy‐axis state with no observed domain walls. The two‐magnetic‐layer sample also exhibited simultaneous one‐wall structures on the top and bottom layers. The other stable configuration was a no‐wall state on the top layer and a two‐wall (three‐domain) state on the lower layer. These ‘‘coupled’’ states were exceptionally stable in both...

Bloch lines, cross ties, and taffy in permalloy (invited)

Cross ties and their attendant Bloch lines have been imaged optically for the first time in permalloy films. Kerr effect images, recorded with our laser magneto‐optic microscope (LAMOM), on 50‐μm square films (300, 800, and 5800 A thick), are consistent with wall types seen in large sheet films by other methods. In the 300‐ and 800‐A samples 180° domain walls are cross‐tie type; ties do not occur in the 5800‐A sample. We observe several effects of film‐edge proximity on the 180° walls not previously reported. In the thinner films, the density of ties (and Bloch lines) increases as a wall is driven closer to an edge. Magnetization sectoring occurs about the edge side of a wall when it is forced sufficiently close. For all three thicknesses, the 180° walls bow toward the nearest edge, the bow increasing with proximity.   The 180° walls arise from the lowest‐energy demagnetized state (Kittel configuration for a square) by applying a bias field which displaces the cluster knot formed by two intersecting 90° w...

Edge-curling-wall discontinuities and interactions with Bloch walls in easy-axis Permalloy

Laminations of soft-magnetic and nonmagnetic films, which meet the criteria for easy-axis state, have edge-curling walls instead of the triangular closure domains of single sheets. A laser magnetooptic microscope has been used to image and model two types of edge-curling wall discontinuities that are induced by geometry in finite elements. One is a sectored-vortex discontinuity in a single-domain state that was not previously predicted. In addition, laminates with Permalloy layers on the order of 1- mu m thick are usually divided into easy-axis domains by 180 degrees Bloch walls. Several types of interaction of a Bloch wall with an edge-curling wall are described. The existence of coupled (top and bottom layers) single-domain states in two-magnetic-layer elements is confirmed. >

Ion beam sputter deposited Permalloy thin films

Ni/sub 80/Fe/sub 20/ thin films were deposited using a wide range of process parameters in a dual source ion beam sputter deposition system. The films were characterized structurally, chemically, and magnetically. Two modes of deposition were investigated; the first permitted concurrent second source bombardment during film deposition but was limited in net deposition rate to about 300 AA/m; the second provided deposition rates in excess of 1000 AA/m, but did not allow for concurrent ion bombardment from the second ion source. Depending on specific conditions film stress varied from slightly tensile to highly compressive in both deposition modes. This, combined with small variations in magnetostriction, resulted in films with vertical anisotropy and stripe domain patterns as well as conditions where well-formed closure domain patterns were observed in yoke shaped structures. For monolithic films, easy axis coercivities >