The performance of magnetic devices correlates with the spatial distribution and time evolution of the magnetization. In recording heads for example, the magnetization components M-x(t), M-y(t), and M-z(t) are very sensitive to the device boundaries, defects, issues of design and processing, and an applied or internal magnetic field or magnetomotive force. With the increasing difficulty of reliably modeling complex magnetic devices ever decreasing in size, efficient experimental Kerr-effect contrast imaging of M-components becomes more important as aid to progress in fundamental understanding, diagnostics and development. This paper discloses innovations which enable imaging of pure in-plane magnetization sensitive Kerr components devoid of polar Kerr and background signals. For the component sensitive to perpendicular magnetization, a means for calibrating pure polar Kerr contrast relative to the pure in-plane contrast is described. These methods operate without the reliance on changing magnetic state for background subtraction as in earlier methods. They are therefore applicable to imaging the three M-components at different stages of magnetization in recording heads, and in magnetically 'hard' materials, for example amorphous magnets and media for perpendicular magnetic recording.

Many useful materials such as Permalloy exhibiting a very small magneto-optic Kerr-effect (MOKE), are in a class called weak phase objects (WPOs). Polarized light microscopy converts a WPO’s phase distribution into a viewable and recordable intensity map. Nearly crossing the polars is necessary to convert a WPO to an amplitude image. Therefore, digital image processing procedures are usually necessary to enhance MOKE images. In order to increase efficiency and throughput, three methods to increase light intensity are described and demonstrated. All three methods conduct laser light to the conventional source spot in the microscope by imaging the output face of a multimode optical fiber onto the objective’s rear focal plane. Dithering the fiber or its image more rapidly than video scan rates, average illumination nonuniformities caused by fiber modes and diminishes short-range artifacts due to laser coherence. With a third method the long-range nonuniformity is avoided by dithering the location of the fiber-face image. Time-averaged distributions of fiber-face output and corresponding patterns of illumination on the air bearing surface of a recording head, are presented together with polar Kerr contrast images as evidence of the quality, resolution, and stability of the improved method. A combination of all three methods synchronized at a multiple of the video scan rate, produced the most uniform, steady and incoherent source, making it suitable for efficient, real-time, Kerr-effect video microscopy of magnetic materials and devices.

Ferrite MiG heads intended for narrow track (≲10 μm) digital recording were recently investigated in the critical pole-tip region at the air-bearing-surface using micro-ellipsometry, Kerr microscopy, and electron back-scatter diffraction from individual grains,1 and using magnetic force microscopy to detect air-gap remanent fields.2 Comparison of these direct observations with readback-after-write waveforms from written test tracks, and consideration of granularity influences on bulk permeability and domain size, indicate that waveform instability and asymmetry from polycrystalline ferrite (PCF) heads would be diminished by suitable size and orientation of the grains.1 The use of single-crystal ferrite3 (SCF) for advanced laser enhanced etch definition3 of narrow pole MiGs can avoid this type of distortion. However, secondary signals4 often appear as weak pulses separated in time from the main gap pulse. We have associated this effect with a zig-zag shaped wall seen nucleated and propagated from the pole tips by a write pulse.4 This wall and its underlying domains lie remanent in the stressed ABS material and evidently react to the bit fields during the read cycle. The secondary read-back response, though similar to the pseudo-gap effect, differs in origin. Its timing depends on the distance of the zig-zag wall to the gap, not the fixed position of the sendust-ferrite interface. Our results indicate that suitable grain oriented ferrite would reduce PCF MiG head read-back asymmetry and instability. For SCF heads, a method for electrically removing zig-zag walls is possible and secondary pulse removal has now been demonstrated on a spin test strand.

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. >

Direct observations using micro-ellipsometry, Kerr microscopy, and electron back-scatter diffraction are made for the first time on individual grains in the leading pole-tip region of polycrystalline ferrite MIG heads intended for narrow track (less than or similar to 10 mu m) data recording. These observations, compared with readback-after-write waveforms, indicate that waveform instabilities and asymmetry which can cause high error rate would be diminished by controlling grain size and orientation in this critical pole-tip region. When large grains (near the gap) exceed the single domain size (similar or equal to 5 mu m) but are oriented so their media-facing surface contains an easy-axis of magnetization, the head produces smaller asymmetry and instability than when missoriented (large) gap grains or multiple single-domain size grains are present. This indication from a limited sampling of MIG heads appears consistent with polarized neutron and permeability measurements on bulk polycrystalline ferrite and an understanding of surface domain behavior in single crystal ferrite MIG heads.

Nonmagnetic ceramic substrates supporting magnetic devices occasionally contain magnetic inclusions or particles, often submicron in size. This report describes a convenient method for locating and characterizing such particles, especially those not visible in direct optical inspection because they lie just beneath the surface or are suboptical in size. We make use of their stray fields which can extend beyond the optical resolution limit (particles this small do not usually demagnetize). A recently reported scheme places a bubble-type magnetic garnet film in contact with the substrate.1 The film is saturated in-plane and its Kerr contrast imaged while the field is reduced. The particle’s stray field causes a visible deviation in an otherwise normal contrast pattern of the critical magnetic phase transformation. Large fields (H0≳Hk) and critical alignment of the field are required. We describe and demonstrate another method not requiring large fields at critical alignment. It relies on rf or pulsed fields to excite fast motions in arrays of bubble domains. The time average of these motions produces concentric rings of Kerr contrast centered on each particle whose stray fields influence the motion. This ‘‘bull’s-eye’’ pattern is useful as it is viewable with modest magnifications, thereby allowing larger fields of optical view. A bull’s-eye Faraday pattern was discovered and applied earlier2 to defect detection in garnet films. The fundamental origin of this pattern and the practical application to substrates for magnetic devices will be discussed.

The magnetic domain structure at the media-facing surfaces of the single-crystal ferrite MIG heads is investigated using the magnetooptical Kerr effect. Domain behavior depends strongly on the stress state and surface treatment of the head. The pole tips display coercive zig-zag domain walls, which are nucleated at the gap by write current pulses. For the samples studied, nucleation and motion of these walls are different between heads which exhibit small secondary pulses in the read-back signal and heads which do not.< >

The addition of Cu to amorphous U-As films lowers the resistivity and extends the compositional range of ferromagnetism. Although the saturation magnetization of these materials is small, they have large coercive fields, anomalous Hall effects and magneto-optical rotations which may be due in part to strong spin-orbit interactions. The uranium f-d transition found in the binary U-As is depressed and broadened by Cu.

The cause of the readback waveform distortion in polycrystalline ferrite metal-in-gap (MIG) heads was investigated by using a magnetic force microscope (MFM) and a polarized optical microscope capable of microellipsometry for grain contrast and Kerr microscopy for domain contrast. It was revealed that, for high-asymmetry MIG heads, relatively large leakage of magnetic flux associated with complex multiple domains exists at remanence in the leading-side ferrite surface adjacent to the gap. The strengths of the magnetic flux and the magnetic domain states change irreproducibly when coil current is repeatedly applied. Such a remanent flux associated with the complex remanent state of magnetization distribution in the ferrite can cause an inhomogeneous flux coupling between the head and the recorded track, resulting in readback waveform asymmetry

Magnetic film laminations containing nonmagnetic spacers have been explored with the hope of eliminating domain walls to diminish Barkhausen instabilities. Such laminates have limitations however, which originate in their ‘‘edge-curling walls’’ (ECWs).1 We have developed a new structure, free of ECWs, in which flux closure at opposing edges occurs via edge-shorting material added to circulate the easy-axis flux of the flat layers. We show experimentally with Kerr-effect imaging that (1) this edge-closed laminated (ECL) structure can support an (ECW-free) ‘‘easy-axis’’ (EA) magnetic state under conditions as modeled recently by Slonczewski,2 and (2) that this EA state is quite robust in the face of imperfect structure fabrication. This is, if the imperfections are not too severe, the resultant states depart minimally from the pure EA state and conduct hard-axis-driven flux nearly as well. Flat-film ECL elements in diamond, stripe, and recording-head-yoke shapes, plus experimental heads with ECL top yokes, were fabricated. Our domain images verify some key predictions from Slonczewski’s static equilibrium modeling; additional results taken in applied magnetic fields extend the micromagnetic understanding. The sketch shows a typical domain pattem for a yoke-shaped element. The most stable state in the open portion of the yoke is the single domain shown. This remanent pattern was stable in the face of (slowly varying) external fields up to the 150 Oe that could be applied. The pole tip region contained a few 180° walls as indicated. On close inspection, these walls were seen to end in vestigial, nontouching, closure domains as predicted by the model when only partial flux closure occurs via the edge shorting material. The wall spacing in the tip varied somewhat following saturation–demagnetization cycles. The dynamic stability of this EA state was investigated in the experimental heads having ECL top yokes. The pseudodynamic LAMOM technique3 was applied using ‘‘write’’ pulsations. The EA state was stable up to twice typical write currents. A movie of dynamic results will be shown.

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. >

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 sh...