G. H. Ho

Soft X-ray magnetic circular dichroism and magnetic films

Abstract Soft X-ray magnetic circular dichroism is the difference in the total absorption cross-section of positive and negative helicity circularly-polarized soft X-rays at the magnetically interesting L 2 and L 3 absorption edges of 3d transition metals or the M 4 and M 5 absorption edges of the rare earths. The absorption edges can be measured using a variety of techniques including transmission, partial electron yield, total electron yield, partial fluorescence yield, sample current, and reflection. The different measurement schemes represent different probing depths, allowing depth dependent studies, and are complementary to each other. A description of the relative merits of the various measurement schemes is presented. In addition, complications such as beam instability, saturation effect, and incident beam Bragg scattering into the detectors are discussed.

Multidomain and incomplete alignment effects in giant magnetoresistance trilayers

The element specific magnetic hysteresis curves of Fe/Cu/Co trilayer structures can be used in conjunction with the measured magnetoresistance curves to extract the coefficient of the giant magnetoresistance (GMR) independent of magnetic domain effects and incomplete alignment effects, allowing for a measure of the maximum attainable GMR for that trilayer. Information concerning the details of the trilayer switching can be extracted showing that sputtered polycrystalline films of Fe/Cu/Co deposited on Si switch their magnetization directions like multidomain Ising magnets.

EXTRACTION OF THE COEFFICIENT OF GIANT MAGNETORESISTANCE

Element specific magnetic hysteresis (ESMH) loops have been used in conjunction with measured magnetoresistance curves to extract the coefficient of giant magnetoresistance (GMR), which is the maximum achievable value of the GMR independent of magnetic domain effects and incomplete moment alignment effects, for sputtered Fe/Cu/Co sandwiches on silicon substrates. Using the ESMH loops to calculate GMR curves, comparison with the measured GMR curves shows that the magnetization reversal process is best described by multidomain Ising behavior.

MPEGTool: an X window based MPEG encoder and statistics tool

In this paper, we describe MPEGTool, an X window-based tool that can be used to generate a motion picture expert group (MPEG) encoded bit stream for video sequences and to study the statistical properties of the encoded data. The tool was designed to study the characteristics of variable bit rate video sources for transmission over asynchronous transfer mode (ATM) based broadband integrated services digital network (B-ISDN). The tool, which has a window-based graphic user interface, allows a user to specify several of the MPEG parameters such as the intraframe-to-interframe ratio and the quantizer scale. The tool also includes a statistical package that allows the user to plot graphs of various statistics including bit distributions, ATM cell distributions, time se ries, autocorrelation functions and cell interarrival times.

Element-specific magnetic hysteresis measurements: a new application of circularly polarized soft x rays

Element-specific magnetic hysteresis measurements on heteromagnetic materials have been achieved by using circularly polarized soft-x-rays. Dramatically different Fe and Co hysteresis curves of Fe/Cu/Co trilayers were obtained by recording the magnetic circular dichroism at their respective L3 white lines as a function of applied magnetic field. The data resolve the complicated hysteresis curves, observed by conventional magnetometry, and determine the individual magnetic moments for the Fe and Co layers. The data show a two-step-like hysteresis loop for all three elements, suggesting that the magnetic properties near the interfaces are very different from those away from the interfaces. The Mn layer was found to be ferromagnetic and aligned with the magnetic direction of Fe and Co. These measurements demonstrate a new application of circularly polarized soft-x-rays in the investigation of magnetic systems.

Element specific magnetic hysteresis curves of Fe/Cu/Co multilayers (invited) (abstract)

We report the first results employing a new technique for studying heteromagnetic multilayers by element specific magnetic hysteresis measurements. Dramatically different Fe and Co hysteresis curves of Fe/Cu/Co trilayers were obtained by recording the soft x‐ray magnetic dichroism1 at their respective L3 absorption edges as a function of the applied magnetic field. The complicated conventional hysteresis curves, obtained by a vibrating sample magnetometer, were resolved into linear combinations of the two individual elemental hysteresis curves, thus determining the average atomic magnetic moments of each element within the multilayer structure (Fe is 2.0±0.08 μB and Co is 1.1±0.04 μB). Furthermore, fine hysteresis features associated with one element, imperceptible in the conventional curves, were also observed, and demonstrate a new and powerful means for studying heteromagnetic multilayer systems.

Elemental determination of the magnetic moment vector (abstract)

Recently, we have shown that magnetic circular dichroism (MCD) can be used to obtain element specific magnetic hysteresis (ESMH) loops of heteromagnetic systems. By using magnetic circular dichroism (MCD) we map the three orthogonal components of the magnetic moment vector of each element of a strongly (90°) coupled Fe30Co70/Mn/Fe30Co70 single crystal trilayer heterostructure as a function of applied magnetic field. The intensity of the MCD of the Mn L2,3 absorption edges clearly shows that the Mn possesses a ferromagnetically aligned net magnetic moment. Furthermore, the individual element specific hystersis curves, when followed through the magnetic reversal process, reveals a detailed description of the magnetization reversal in the presence of both cubic anisotropy and strong exchange. This vividly demonstrates the capability of this technique to analyze the magnetic moment reversal process in the presence of strongly competing interactions.

Experimental investigation of the magnetic circular dichroism sum rules (abstract)

High quality L2,3 edges magnetic circular dichroism (MCD) data of in situ grown Fe, Co, and Ni ferromagnetic thin films were measured using transmission x‐ray absorption technique. These data were utilized to investigate the validity of the recently proposed x‐ray MCD sum‐rules that offer a new means for deducing orbital‐ and spin‐magnetic moments from MCD data. We find that the orbital to spin magnetic moment ratios of Fe, Co, and Ni deduced from MCD data according to the sum‐rules are at ±30% discrepancy with those deduced from neutron scattering data. This inconsistency may be attributed to the neglecting of multi‐electron excitations and band structure effects in the derivation of the sum‐rules or due to uncertainties in the neutron scattering results. Caveats for MCD data taken with the fluorescence yield x‐ray absorption technique will also be discussed.