Dean A. Herman

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. >

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...

Prospects of silicon-germanium-based technology for very high-speed circuits

Silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) BiCMOS technology has developed into a production manufacturing technology that replaces and extends the performance of silicon-based BiCMOS technology. The market impetus for this development has been the insatiable requirement for bandwidth in network communication at speeds up to 40 Gbit/s and the rapid growth of the global cellular and wireless LAN markets. There has also been much work on the use of silicon-based structures for microwave frequencies. This paper focuses on a review of the status of our SiGe BiCMOS technology, based on four generations of scaling CMOS-compatible SiGe. We also show in principle how techniques commonly used in Ill-V semiconductor technology and microwave systems can also be applied to SiGe chips, along with silicon-on-insulator (SOI) and other existing technology, to provide a possible further extension in SiGe performance, for 50+ GHz circuits.

Using silicon-germanium mainstream BiCMOS technology for X-band and LMDS (25-30 GHz) microwave applications

The 0.18 /spl mu/m SiGe HBT BiCMOS technology we have developed has found a variety of uses in high-speed digital applications, up to 50 Gb/s (Freeman et al, 2001). This paper focuses on the use and applicability of this mainstream HBT BiCMOS technology for microwave applications, particularly X-band, satellite, and LMDS (20-30 GHz). We discuss the pros and cons relative to the well-known III-V MMIC technology, as well as Si microwave circuits on high-resistivity substrates (SIMMWICs) (Russer, 1998). It is shown that the SiGe BiCMOS technology is widely applicable to microwave technology, with examples such as filters, switches, and VCOs. We also review new technology developments that can be applied to the SiGe BiCMOS technology.

50-200 GHz silicon-germanium heterojunction bipolar transistor BICMOS technology and a computer-aided design environment for 2-50+ GHz very large-scale integration mixed-signal ICs

Silicon–germanium (SiGe) heterojunction bipolar transistor (HBT) BICMOS technology is a stable, ultra-high performance, semiconductor technology capable of supporting mixed-signal, very large-scale integration (VLSI) circuit designs for a variety of emerging communication applications. This technology is supported by a computer-aided design (CAD) system that supports a variety of high-performance circuit designs, mixed-signal circuit block reuse, and the ability to accurately predict circuit performance at the highest frequencies. This paper summarizes the progress this technology has made in recent years in moving from the research laboratory to a production environment. We also specifically address performance, operating voltage, reliability and integration considerations for using 100–200 GHz SiGe HBTs in high-speed (10–40 Gb/s) network ICs, an application space previously only addressed by InP technology. All indications are that SiGe will be very successful at addressing this new application space, and all facets of the networking IC market.

Edge‐closed laminated structures for thin‐film heads

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, ...

Domain conversion under high frequency excitation in thin-film inductive heads

Introduction The impact of domains, particularly closure domains, on the flux-carrying capability of thin-film inductive heads has been considered by a number of authors143*4. We have recently shownS that specific wall displacements can be induced by applying pulses, bursts of pulses or continuous sine waves to the drive coils of an inductive head. These wall displacements are associated with changes in the wall structure. Wall displacements can be so large that the walls are swept through the yoke, resulting in a continuous flowing of the wall network through the head. In this study, we systematically investigate quasi-static changes in domain configuration when an inductive head is subjected to high-frequency sinusoidal excitations.