Polarization Of Secondary Electrons Research Articles

Oscillations of Negative Polarization in Secondary Electrons Emitted from Au/Fe(110)

To study the oscillatory interlayer magnetic coupling through epitaxial Au thin films, we investigated the polarization oscillation of secondary electrons emitted from a Au(111)/Fe(110) surface. Comparing the polarization of secondary electrons as a function of the Au thickness for Au(111)/Fe(110) and for Au(001)/Fe(001), we found that the polarization of conduction electrons in Au(111) thin films always oscillates with negative polarity. This result shows that the density of quantum-well states for the majority-spin electrons is lower than that for the minority-spin ones independent of the Au film thickness.

An Application of the Small Mott Spin Analyzer to the SP-SEM

A spin-polarized scanning electron microscope (SP-SEM) with a retarding-type Mott analyzer was developed. For SP-SEM applications, we improved the figure of merit of the polarimeter, which had been developed in our laboratory, and employed a newly designed secondary electron collector. To avoid the magnetic stray field generated by the magnetic objective lens, a magnetic shield whose shape was designed by means of the finite element method was attached to the lens. Using the SP-SEM system, we observed spin-dependent SEM images of the domain structure of a single Fe crystal (001) surface, and typically observed spin polarization of secondary electrons of about ±14%.

Spin-resolved photoemission of in situ sputtered iron and iron-yttrium alloys

Spin-resolved photoemission spectra of amorphous Fe-Y alloys have been obtained and compared with both those of pure Fe and with theoretical electronic structure calculations. The thin iron and alloy films were prepared by in situ sputtering, and the spectra were obtained using 110-eV photons from the UK Synchrotron Facility. From secondary electron spin hysteresis loops, ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ starts to order ferromagnetically well before the lowest temperature accessible with our equipment (114 K), but ${\mathrm{Fe}}_{41}$${\mathrm{Y}}_{59}$ shows no polarization down to this temperature. Although the spin-integrated photoemission spectrum of the valence band of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ shows little difference from that of pure Fe, the spin-resolved spectra are significantly different. ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ also exhibits a reduced valence band polarization (14%) compared with iron, and contrary to predictions, the majority spin states of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ dominate at the Fermi edge. No evidence is found for strong ferromagnetism. Unexpectedly, the 1-eV secondary electron polarization of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ (7%) is found to be low compared with the valence band polarization, an effect that may be due to a negative moment on the yttrium.

Primary energy-dependent attenuation of polarization of secondary electrons emitted from [formula omitted

We studied the Au thickness and the primary electron energy-dependence of polarization of secondary electrons emitted from Au thin film on an Fe(110) substrate. By fitting to the experimental data using the exponential function, we found that the magnetic information depth λ m of the secondary electron polarization increases with primary electron energy E p from λ m = 6 A ̊ at E p = 70 eV to λ m = 26 A ̊ at E p = 4 keV. The accuracy of the fit degrades with higher E p. We explain this behavior by considering the difference in secondary yields between Au and Fe.

Imaging magnetic microstructure with SEMPA

By measuring the spin polarization of secondary electrons generated in a scanning electron microscope, scanning electron microscopy with polarization analysis (SEMPA) can directly image the magnitude and direction of a material’s magnetization. Because the escape depth of the secondaries is only on the order of 1 nm, SEMPA is especially well-suited for investigating the magnetization of ultra-thin films and surfaces. We have exploited this feature of SEMPA to study the magnetic microstrcture and magnetic coupling in ferromagnetic multilayers where the layers may only be a few atomic layers thick. For example, we have measured the magnetic coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) trilayers and have found that the coupling oscillates between ferromagnetic and antiferromagnetic as a function of the Cr or Ag spacer thickness.The SEMPA apparatus has been described in detail elsewhere. The sample consisted of a magnetic sandwich structure with a wedge-shaped interlayer as shown in Fig. 1.

Magnetic moments in Cr thin films on Fe(100)

The magnetism at the surface of a Cr film grown epitaxially on a Fe(100) whisker is observed as a function of Cr thickness by scanning electron microscopy with polarization analysis. Use of a wedge-shaped film of linearly increasing thickness allows the magnetism to be followed continuously for 75 Cr layers. Over the temperature range measured from just below the Néel temperature of bulk Cr, TN, to 1.8 TN, the surface magnetic moment is seen to persist and change direction with each additional Cr layer, but there are phase slips in this antiferromagnetic ordering. These are consistent with an incommensurate spin density wave (SDW) in the Cr film having a wavelength of 40 layers at TN. An irregularity in the antiferromagnetic stacking order in the first four layers is discussed and the behavior of the moments at the subsequent phase slip is examined. The limitations which prevent the determination of the surface magnetic moment from the spin polarization of secondary electrons are discussed.

Spin polarization of secondary electrons from Gd(0001)

Secondary electrons from Gd(0001) films grown on W(100) show spin polarization values of 28–34% in the energy range 1.5–7 eV at 150 K. No indication of a polarization enhancement is found. Below 1.5 eV the polarization drops to 20%, which is attributed to the presence of the 4f 8 states just above the vacuum level.

Magnetism in Cr thin films on Fe(100).

The spin polarization of secondary electrons from a Cr film on Fe(100), measured with scanning electron microscopy with polarization analysis (SEMPA), oscillates as a function of Cr thickness with a period near two atomic layers, consistent with incommensurate spin-density-wave antiferromagnetism in the Cr. The position of a phase slip due to incommensurability varies reversibly by 14 layers over the temperature range of 310 to 550 K. The Cr surface magnetic moment persists well above the N\'eel temperature of bulk Cr.

Primary energy dependence of secondary electron polarization

To explain quantitatively the primary electron energy dependence of secondary electron spin polarization, the authors propose a simple model, where spin-dependent net mean absorption length of secondary electrons is assumed.

Spin polarization of secondary electrons from Fe(110) excited by unpolarized primary electrons

We report on angle- and energy-resolved spectroscopy of secondary electrons from Fe(110). Intensity and polarization distributions in normal emission as a function of energy of the primary electrons are studied. For the first time angle-resolved spectra as a function of emission angle are measured and compared to theoretical calculations of the polarization fine structure. The influence of oxygen exposure on the polarization spectra is studied and surface-related features are identified. We conclude that surface resonances as well as the bulk band structure both contribute to the polarization fine structure. Consequences for the analysis of magnetic microstructures in a scanning electron microscope with polarization analysis are pointed out.

Surface magnetoelastic coupling.

The surface magnetoelastic coupling coefficient ${\mathit{B}}^{\mathit{s}}$ is determined for cobalt-rich and iron-rich amorphous alloys by measuring the field and strain dependence of the spin polarization of secondary electrons emitted from their surfaces. ${\mathit{B}}^{\mathit{s}}$ is found to be approximately 3 times the bulk value for the cobalt-rich alloy and half the bulk value for the iron-rich alloy.

EMPA measurements of magnetic microstructure in ultrathin films (invited) (abstract)

Very little is currently known about domains and domain walls in ultrathin ferromagnetic films, i.e., films that are a few atomic layers to a few tens of layers thick. A major experimental difficulty is that these films must be grown and examined in situ with strict control over their atomic structure and elemental composition. Recently, scanning electron microscopy with polarization analysis (SEMPA) has been used to image domains in Fe1and Co2 films that were only a few monolayers thick. The structure and composition of the films were monitored during growth using RHEED (reflection high energy electron diffraction) and Auger electron spectroscopy. The magnetic structure of these films was then investigated in situ using SEMPA. SEMPA directly images the magnetization by measuring the spin polarization of secondary electrons emitted in the scanning electron microscope. This combination of surface sensitive techniques allows one to study the domain structure, anisotropy and magnetization of the films as a function of various growth parameters, such as, thickness, substrate roughness, and temperature.<ks>

Determination of spin-polarized secondary-electron escape depth through oxidation of iron

The decay of the spin polarization of low-energy secondary electrons from Fe+ oxygen is measured as a function of oxygen exposure. We conclude that the spin polarization data can be explained through a simple model in which the spin polarization of secondary electrons generated from the oxide is small. From this, we determine, that the escape depth for spin-polarized electrons is approximately 29±3 Å at 2 eV kinetic energy, decreasing to approximately 22±2 Å at 7 and 12 eV. These results indicate that spin-polarization measurements on Fe and its oxide are less surface sensitive than previously thought.

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 (<50 nm) of high current (>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 this review article we review the important instrumental components characterizing the SEMPA system. Characteristics of the electron probe forming optics, electron spin-polarization analyzers with associated transport optics, and signal processing electronics will be described. Emphasis on the fundamental design requirements will be stressed. Data acquisition, storage, and processing, as it applies specifically to SEMPA, will be reviewed. Instrumental artifacts specific to SEMPA will be outlined and techniques for their correction given. Examples of magnetic images at high spatial resolution will be shown.

Spin-dependence of absorbed and reflected current on Fe(110) and a comparison with secondary electron polarization fine structure

Measurements of the normalized difference in the absorption and reflection of low-energy electrons with opposite spin-polarization, and the spin-polarization of low-energy secondary electrons, from Fe(110) are found to be influenced by the spin-split bulk band structure. Maxima and minima in the absorbed and reflected current asymmetries are interpreted in terms of band structure effects and are shown to be directly related to the spin-polarization fine structure in low-energy secondary electron emission from ferromagnets. The feasibility of using spin-dependent low-energy electron reflection from ferromagnets as an electron spin-polarization detector is discussed.

Characterizing magnetic surfaces with secondary electron spin polarization

The measurement of spin polarization of secondary electrons opens new frontiers for high resolution (of order 10 nm) static and dynamic studies of magnetic surfaces. The techniques involved and their limitations are discussed. Applications to some important technical and scientific problems are described.

Role of rediffused primary electrons in secondary-electron ejection from a Cu(100) surface.

The energy and angle dependence of the spin polarization of secondary electrons ejected from a Cu(100) surface by a polarized incident electron beam of energy 14, 50, and 100 eV has been investigated. At energies approaching the incident beam energy the secondary-electron polarization is comparable to that of the incident beam. The polarization decreases with decreasing secondary-electron energy, however, and at the lowest energies approaches zero. The data are interpreted in terms of the fractional contributions of rediffused primary and true secondary electrons to the total secondary-electron current.

A coaxial electrostatic lens design for a low-energy electron-spin detector

A novel coaxial electrostatic lens is employed to focus low-energy electrons through a thin-film low-energy electron-spin polarization analyzer. The coaxial lens consists of an axial electrode suspended through a cylinder or an aperture. Electron transmission is measured as a function of the voltage applied to the central and outer conductors. Surface magnetic hysteresis loops are obtained by plotting the spin polarization of secondary electrons emitted from a ferromagnetic surface as a function of the applied magnetic field.

Nanomagnetism: High-resolution magnetic surface studies using spin polarization of secondary electrons (invited) (abstract)

Intense scientific curiosity and the drive to increase information storage density have fueled extensive theoretical and experimental work on magnetic surfaces. Symmetry and bonding are reduced at surfaces and in magnetic materials this can cause changes in the moment and magnetic anisotropy over distances too small to probe optically. We have constructed and used a thin-film, spin-orbit scattering polarimeter (based on the NBS design) to measure the polarization of secondary electrons excited from magnetic surfaces. The low-energy secondary electrons come from the outermost atomic layers and carry a polarization proportional to valence polarization of the elements in the surface. Results on amorphous cobalt base alloys indicate a surface anisotropy in the outermost 2–5 nm pulling the magnetization away from the bulk preferred direction. High lateral resolution can be achieved by using the thin-film polarimeter to measure the polarization of secondary electrons excited from a magnetic surface by a high-resolution (25 Å), field-emission source, scanning electron microscope.

The role of inelastic exchange scattering in spin polarization experiments (invited) (abstract)

Inelastic exchange scattering of electrons can result in spin-dependent mean free paths (MFP) for electrons in ferromagnetic transition metals and glasses. The MFP of electrons with spin in the majority-spin direction will be greater than those with spin in the minority direction due to the larger number of empty minority-spin d states into which minority-spin electrons can scatter. This difference in MFP can explain the increase in the spin polarization of secondary electrons in transition metals and glasses that is observed at low energies. In addition, it will be shown that the ratio of the majority to minority spin MFP as a function of energy can be determined directly from measurements of the energy dependence of the secondary electron-spin polarization. A different type of experiment in which exchange is important is the production of Ni Auger electrons by the scattering of metastable He atoms from a Ni surface. The number of Auger electrons produced is found to depend on the relative spin of the He and Ni, parallel or antiparallel. The normalized difference of these two cases is found to increase with increasing energy of the Auger electrons. It is found that this effect is directly related to exchange scattering of the Ni electrons that neutralize the He atoms and that the experimental results can be accounted for by a simple model.