Michael H. Kelley

Scanning electron microscopy with polarization analysis (SEMPA)

The high spatial resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. One technique for imaging surface magnetic microstructure on the 10‐nm‐length scale is scanning electron microscopy with polarization analysis (SEMPA). This technique employs a scanning electron microscope (SEM) electron optical column to form a medium energy (10–50 keV), small probe ( 1 nA) on a ferromagnetic specimen. Secondary electrons excited in the ferromagnet by the high spatial resolution probe retain their spin‐polarization orientation as they leave the sample surface. The spin polarization of the emitted secondary electrons can be related directly to the local magnetization orientation. A surface magnetization map is generated when the spin polarization of the secondary electrons is analyzed as the electron beam is rastered point‐by‐point across the ferromagnet’s surface. In th...

Improved low‐energy diffuse scattering electron‐spin polarization analyzer

An improved low‐energy diffuse scattering electron‐spin polarization analyzer is described. It is based on the low‐energy (150 eV) diffuse scattering of polarized electrons from polycrystalline evaporated Au targets. By collecting large solid angles and efficiently energy filtering the scattered electrons, a maximum figure of merit, FOM=S2I/I0=2.3×10−4 is achieved. Maximum measured values of the Sherman function were S=0.15. Further, the instrumental (false) asymmetry due to changes in the trajectory of the incident electron beam has been minimized by balancing the angular and displacement asymmetries. A total residual scan asymmetry as low as 0.0035/mm has been measured over 4‐mm scan fields at the Au target in the detector. This instrumental asymmetry would produce a maximum error in the polarization in a SEMPA experiment of less than 0.3% for a 100‐μm full‐field scan. Details of the design and performance of the new detector are given.

Domain Structures in Magnetoresistive Granular Metals

We have imaged the magnetic domain structure of several heterogeneous CoxAg1−x alloys by using scanning electron microscopy with polarization analysis. These images show that extended domain structures exist in both the as‐deposited samples and in samples annealed at moderate temperatures. This suggests that a significant fraction of the cobalt in these materials does not contribute to the giant magnetoresistance. Only those samples annealed at 600 °C and containing less than 40% cobalt by volume show no domain structure.

Electron spin polarization analyzers for use with synchrotron radiation

Abstract The measurement of the spin polarization of photoelectrons emitted from a magnetic material is discussed. An important consideration is the acceptance phase space of the spin analyzer relative to the phase space of the photoemitted electrons to be measured. Other considerations include the magnetization direction relative to the extracted beam and whether the measurements are angle integrated or angle resolved. In the longitudinal geometry where the magnetization is normal to the sample surface and along the extracted photoelectron beam, conservation of canonical angular momentum adds an additional magnetic term to the beam emittance which is absent when the magnetization is in the sample plane and transverse to the extracted beam. For angle resolved measurements in the transverse geometry, the advantages of a new, low-energy (∼ 100 eV) spin analyzer which is easily movable, compact and efficient are discussed. Different spin analyzers are described and compared, and an analysis of their application to different spin polarized photoemission measurement configurations is given.

Fabrication and domain imaging of iron magnetic nanowire arrays

Arrays of magnetic nanowires are fabricated by using a corrugated surface, produced by chromium atoms laser-focused in a one-dimensional standing wave, as a shadow mask for an iron evaporator. The deposited iron forms a periodic array consisting of thousands of 20–40 nm high ferromagnetic lines with width of ∼100 nm, spaced every 213 nm, and having aspect ratios (width:length) of 1:1500. Images of the magnetic domain structures of these nanowires are obtained by scanning electron microscopy with polarization analysis. Elongated domains ∼100 nm wide and ∼16 μm long are observed.

Magnetic domain percolation in granular Co‐Ag

The most dramatic changes in the properties of granular metals occur near the percolation threshold xc, which generally occurs at a volume fraction of approximately 0.50. There has been much speculation concerning the evolution of magnetic domains at xc, however, no direct observations have been produced. Using scanning electron microscopy with polarization analysis, we investigate the formation of domains in a series of granular Co‐Ag samples. We find xc to lie in the range [0.50, 0.55], and we report on the size and morphology of the observed domains. Below xc, a domain pattern appears which may be due either to interparticle correlations or to residual Co in the silver matrix; we have previously discussed the ramifications of this observation on an understanding of GMR. In this paper, we place limits on the latter of these two models. Above xc, the domains resemble ‘‘stripe domains’’ which have been observed in amorphous materials. This observation suggests the presence of anisotropy perpendicular to t...

Analysis of collisional alignment and orientation studied by scattering of spin-polarized electrons from laser excited atoms

A general framework using density matrices is developed for the analysis of atomic excitation by spin-polarized electrons. This framework is applied to the specific case of the 3S1/2→3P3/2 transition in Na, as studied by the time-reversed, superelastic process. The scattering is characterized in terms of physical parameters describing the collisionally excitedp-state, i.e., its angular momentum (L⊥), linear polarization (Plin), and alignment angle (γ), with these parameters defined separately for singlet and triplet excitation. An expression for the scattering intensity is derived which is valid for arbitrary electron polarization and atomic state preparation. Specific examples are discussed with a view toward complete determination of the relevant scattering amplitudes and phases. Recent experimental results are reevaluated for comparison with theoretical calculations, and suggestions are made for future experiments.

Optimizing The Nist Magnetic Imaging Reference Sample.

We have further developed the NIST magnetic imaging reference sample to include a magnetic pattern which can indicate the magnetic polarity of a magnetic force microscope tip. Several samples cut from the same disk were measured with a single tip. We have also measured a single transition with several tips. Both measurements have shown the variability in images taken with different tips and different instrument configuration which underscores the need for a well calibrated sample.

Magnetic depth profiling Co/Cu multilayers to investigate magnetoresistance (invited)

The magnetic microstructure responsible for the metastable high resistance state of weakly coupled, as-prepared [Co(6 nm)/Cu(6 nm)]20 multilayers was analyzed using polarized neutron reflectivity and scanning electron microscopy with polarization analysis (SEMPA). This article focuses and expands on the SEMPA measurements. In multilayer structures such as these, SEMPA can be combined with ion milling to directly image the layer-by-layer magnetization and quantitatively depth profile the interlayer magnetic domain correlations. We found that in the as-prepared Co/Cu multilayer, the domains are about 1 μm in size and the magnetizations in adjacent layers are almost completely oppositely aligned. The relative magnetoresistance derived from this measured degree of anticorrelation is in agreement with the measured magnetoresistance.

Spin sensitivity of a channel electron multiplier

We report direct measurements of the sensitivity of a channel electron multiplier to electrons with different spin orientations. Four regions of the multiplier cone were examined using polarized electrons at 100‐eV incident energy. Pulse counting and analog modes of operation were both investigated and in each case the observed spin effects were less than 0.5%.