M. R. Scheinfein

Photoemission electron microscope for the study of magnetic materials

The design of a high resolution photoemission electron microscope (PEEM) for the study of magnetic materials is described. PEEM is based on imaging the photoemitted (secondary) electrons from a sample irradiated by x rays. This microscope is permanently installed at the Advanced Light Source at a bending magnet that delivers linearly polarized, and left and right circularly polarized radiation in the soft x-ray range. The microscope can utilize several contrast mechanisms to study the surface and subsurface properties of materials. A wide range of contrast mechanisms can be utilized with this instrument to form topographical, elemental, chemical, magnetic circular and linear dichroism, and polarization contrast high resolution images. The electron optical properties of the microscope are described, and some first results are presented.

PRINCIPLES OF X-RAY MAGNETIC DICHROISM SPECTROMICROSCOPY

A review is given of the principles underlying X-ray magnetic circular (XMCD) and linear (XMLD) dichroism spectromicroscopies consisting of polarized X-ray absorption spectroscopy in conjunction with scanning or imaging microscopy. The techniques are shown to have many useful and important capabilities for the study of complex magnetic materials. They offer elemental specificity, chemical specificity and variable depth sensitivity, among others. XMCD microscopy is best suited for the study of ferromagnets and ferrimagnets, and it allows a quantitative determination of the size and direction of spin and orbital moments. XMLD microscopy promises to become a powerful tool for the study of antiferromagnets which are difficult to study by conventional microscopy techniques.

Switching asymmetries in closely coupled magnetic nanostructure arrays

Cobalt nanostructures (220 and 300u200anm×275u200anm×30u200anm) were fabricated using electron beam lithography into ordered, close proximity (170 nm) arrays. Domain configurations with accompanying hysteresis loops were measured using off-axis electron holography. Measurements were compared to solutions of the Landau–Lifshitz–Gilbert equations. Both exhibit switching asymmetries due to strong intercell coupling and the presence of a field normal to the cell surface. Magnetic domain configurations during switching depended strongly on the initial conditions, as well as the direction of the perpendicular field relative to the in-plane hysteresis-field direction.

Magnetic force microscopy utilizing an ultrasensitive vertical cantilever geometry

We have developed a novel magnetic force microscope (MFM) utilizing a vertically cantilevered microtip probe. This new geometry provides maximum sensitivity while inhibiting uncontrolled vertical deflections (tip crashes). We demonstrate the capability of our MFM by imaging domain structure in prerecorded magnetic tape and domain walls in single‐crystal iron whiskers. Good agreement is obtained between the observed magnetic contrast and predictions of a micromagnetic model.

A transmission x-ray microscope based on secondary-electron imaging

A design for a transmission x-ray microscope with 20 nm transverse spatial resolution is presented. The microscope, which is based on the electron-optical imaging of the photoemitted electrons from an x-ray shadowgraph, consists of a transmission x-ray photocathode coupled to a photoelectron emission microscope (PEEM—also called a PEM for photoelectron microscope). Unlike the conventional PEEM, which produces a surface map of photoelectron yield, this microscope can provide information on the subsurface properties of thin samples. The analysis of the microscope’s electron-optical performance is based on the evaluation of Gaussian focusing properties and third-order aberration coefficients computed using several complementary methods. The electron optical properties of the microscope are examined with an emphasis on issues affecting overall performance and achieving the best possible resolution. Preliminary experimental results using a cesium iodide photocathode are shown.

Time aberrations of uniform fields: An improved reflectron mass spectrometer for an atom‐probe field‐ion microscope

The mass resolution of the atom‐probe field‐ion microscope is limited by the time resolution of the ion‐separating spectrometer. Time aberrations of uniform fields are investigated in general in order to characterize the optimal performance of high‐transmission, high‐mass resolution, multistage reflectron lenses. Correction of higher order time aberrations greatly improves the mass resolution. For an ion beam with an energy distributed uniformly about some nominal energy, E0±dE, mass resolutions (base width) of m/dm=848, 1344, 2151, 3571 can be achieved for single‐, double‐, triple‐ and quadruple‐stage reflectron mass spectrometers when dE=0.1E0. A unique design example employing both second‐ and third‐order time correction is given for an atom‐probe field‐ion microscope.

Slow Magnetic Relaxation in Iron: A Ferromagnetic Liquid

The remanent magnetization of single-crystal iron whiskers has been measured from 10-5 to 104 seconds after the removal of an applied field. The observed response is accurately modeled by localized magnon relaxation on a Gaussian size distribution of dynamically correlated domains, virtually identical to the distribution of excitations in glass-forming liquids. When fields of less than 1 oersted are removed, some relaxation occurs before 10-5 second has elapsed; but when larger fields are removed, essentially all of the response can be accounted for by magnon relaxation over the available time window. The model provides a physical picture for the mechanism and observed distribution of Landau-Lifshitz damping parameters.

Slow relaxation in magnetic materials

Magnetic aftereffects in magnetic materials have previously been attributed to rotation of individual domains and wall motion between domains. We have measured the relaxation of the remanent magnetization as a function of time, in several magnetic materials including random magnetic systems and single-crystal ferromagnets, from 10-5 to 104s after removing an applied field. The observed behavior is accurately described by a thermodynamic model for the relaxation of finite-sized dispersive excitations (magnons) on specific distributions of dynamically correlated domains. At low fields (H < 10 Oe) and long times (t 10 μs), aftereffects in magnetic materials are dominated by the relaxation of magnons on finite-sized domains. At higher fields, in addition to the low-energy magnon relaxation, single-crystal iron exhibits evidence for wall motion, which usually occurs on very short time-scales.

A LOW COST PREAMPLIFIER FOR FAST PULSES FROM MICROCHANNEL PLATES

A simple, low cost 3 GHz bandwidth preamplifier suitable for pulse‐counting applications utilizing chevron microchannel plate (MCP) detectors has been built with monolithic microwave integrated circuit (MMIC) amplifiers in multiple stages. Its bandwidth matches the rise time of MCP output pulses (<300 ps), thereby maintaining leading edge timing information. The use of MMICs made this device inexpensive and easy to construct.

Magnetic force microscope utilizing an ultra-small-spring-constant vertically cantilevered tip

Abstract We have developed a novel vertical cantilever geometry for scanning force microscopy. The vertical cantilever has several advantages over horizontal cantilever designs. The vertical cantilever is sensitive to vertical forces and horizontal force gradients; its lateral response is inherently directional; and the vertical geometry inhibits uncontrolled vertical deflections, thereby reducing the incidence of tip crashes and facilitating the use of ultra-small-spring-constant cantilevers for superior force sensitivity. We have constructed a magnetic force microscope (MFM) utilizing the vertical cantilever geometry. Images of magnetic recording media are used to illustrate the capabilities of our MFM.