Takeshi Kawagoe

Abstract We investigate the growth of ultrathin Cr films on a Au(001) surface and observe that the growth of 1.5 nm thick Cr layers at 290 K, followed by post-annealing at 520 K, results in high-quality epitaxial Cr(001) films with atomically flat large terraces and distinct surface states. Subsequently, these optimized growth conditions are successfully applied to the growth of 1 nm and 3 nm thick Cr films. Magnetic imaging of 1 and 1.5 nm thick Cr(001) films prepared under the optimized growth conditions is performed using spin-polarized scanning tunneling microscopy. Distinct magnetic contrasts featuring a topological antiferromagnetic (TAF) order are observed in both films; however, spin frustration originating from the density of screw dislocations for both films shows a significant difference. The 1.0 nm thick Cr film, which exhibits a clear TAF order with the suppression of a large spin-frustrated area, is suitable for application to spin-electronic devices.

Abstract We investigated the growth and surface morphology of 10 monolayer (ML)-thick Cr(001) films on clean Au(001) surfaces. High quality epitaxial Cr(001) films with large atomically flat terraces and distinct surface states were successfully fabricated through growth at 300 K and subsequent post-annealing at 520 K. At 300 K, spin-polarized scanning tunneling microscopy images of both the topological and magnetic structures of this Cr film were obtained. The magnetic images exhibited the following features: (1) The layered antiferromagnetic (AF) order appeared in adjacent terraces and one ML-depth shallow hole in the terraces; (2) significant spin frustrations induced by adjacent paired screw dislocations caused the AF domain formation with 90 degrees quantum axis rotation and a large spin frustration area, not always limited in the vicinity of screw dislocations. The feature (2) was qualitatively reproduced by the micromagnetic simulation. These findings may be essential for the further development of spin-electronics utilizing thin AF films.

We studied magnetism and morphology of metastable bcc Co nanostructures on Au(001) by x-ray magnetic circular dichroism in combination with scanning tunneling microscopy and photoelectron emission microscopy. While room-temperature deposition of Co onto Au(001) leads to the formation of bcc Co thin films with pure in-plane magnetization, postannealing of these thin films at 500 K drastically changes the morphology to bcc Co islands embedded in Au. In accordance with this morphological change, we find that an out-of-plane magnetization emerges additionally and the coexistence of in-plane and out-of-plane remanent magnetizations is observed for the islands. The nanostructure-size dependence of magnetic moments suggests that the magnetization easy axis of the island changes from the in-plane to the out-of-plane direction with decreasing nanostructure size. Such a spin reorientation transition is likely due to the increased fractional population of rim atoms generating the out-of-plane magnetization in the smaller islands. The observed out-of-plane remanent magnetization of the smaller islands indicates their large magnetic anisotropy.

Barrier-height (BH) imaging using scanning tunneling microscopy (STM) was used to study the growth of Co films on Au(001) surfaces. We have observed BH of metastable bcc Co film (>1 ML) for the first time, and that showed a large BH value (similar to 6 eV), whereas the observed BH of the Au(001) surface (similar to 3.5 eV) was consistent with the previous results. The origin of the large BH was qualitatively understood by considering that 3d electrons for tunneling are dominant for the Co(001) surface. We have observed numerous islands with different sizes and heights after 0.15 ML Co coverage and successfully obtained, from the BH imaging, an element-specific contrast, i.e. recognizing aggregated Au islands and Co islands, and information about inhomogenities of BH with proper consideration of the artifacts near the step edges. The height modification by the large BH difference is discussed. STM/BH studies of FePt films revealed two kinds of monolayer heights, the sum which was equal to the c axis lattice constant of L1(O) FePt. Two different dl/dz signal levels were observed on atomically flat terraces. (C) 2011 Elsevier B.V. All rights reserved.

Spin−resolved surface images and surface nanoscale structures can be observed simultaneously by spin−polarized scanning tunneling spectroscopy using a ferromagnetic tip. By means of this technique, a magnetic image of the ultrathin Cr(001) film epitaxially grown on a Au(001) surface was observed at room temperature. The Cr(001) film examined shows the topological (layered) antiferromagnetic order in spite of there being a high screw dislocation density. A narrow domain wall width (about 10 nm) induced by screw dislocations was observed. On the Fe(001) surfaces, we observed an electronic state that may be related to the scattering of the spin−polarized surface state electrons at the atomic steps.

The distributions of the switching field (H_SW) for single magnetic wires (150 nm width) were investigated as a function of temperature between 5 and 300 K. One end of each wire was connected to a square pad (large area), and magnetization reversal phenomena were very sensitively detected by using the giant magnetoresistance effect. While the distribution width of the H_SW for the Co layer in NiFe/Cu/Co drastically broadened below 100 K, that of the H_SW for the NiFe layer in NiFe/Cu/NiFe had three narrow peaks at each temperaure down to 5 K. The origin of these three peaks can be attributed to the existence of three different kind of magnetic domain structures in the pad area, which was suggested by MFM observation.

We fabricated magnetic rectangular block arrays of 130 and 260 Gblock/in² by using focused ion beam lithography. The ratios R of the total block area to the array area are 0.73 and 0.64, respectively. The magnetic domain boundaries of the arrays run along the grooves between the blocks. Each block has a single domain structure and perpendicular magnetic anisotropy. The pattening increased the squareness from 0.2 of the continuous film to 0.87 of the array with 130 Gblock/in². Application of the block arrays to high-density patterned magnetic recording media is discussed.

The tunneling magnetoresistance (TMR) in ferromagnet-vacuum-ferromagnet junction was studied by using an STM setup. After in situ cleaning of both of two ferromagnetic electrodes, up to 20% of TMR was observed. Such a signal was not observed for air gap nor nonmagnetic electrode. This technique is suitable for basic research of the TMR effect. (C) 2002 Elsevier Science B.V. All rights reserved.

The influence of steps on the surface states of the magnetic Fe(001) surface was studied at room temperature by scanning tunneling microscopy (STM). We observed the distinct bias-dependent differential conductivity (dI/dV) contrasts at step edges from those on flat terraces. The observed dI/dV line profiles across the step edge exhibit the strong reduction of the empty-state local density of states up to 1 eV above the Fermi level. That can be attributed to the scattering of the minority-spin surface states by the step edge, thus, a sizable spin-dependent contrast at the step edge is predicted in spin-polarized STM.

Magnetic domain structure related to a coercivity mechanism in Fe/FeRh(0 0 1) exchange coupled ferromagnet/antiferromagnet bilayers was studied as a function of Fe layer thickness, t(Fe), by magnetic force microscopy (MFM). Magnetooptic magnetometry revealed coercive fields reaching a value of 250 Oe at t(Fe) = 5 nm and decreasing down to 80 Oe at t(Fe) = 20 nm. In as-grown specimens with t(Fe) > 20 nm, the MFM images show micron-scale magnetic domains with rather straight domain walls parallel or perpendicular to [1 0 1] crystallographic direction of the Fe film. For t(Fe) < 20 mn, fine and random magnetic patterns composed of small elements with a lateral size of 200-300 nm are observed. For t(Fe) < 4 nm these elements are separated. With increasing t(Fe) they start to gather and form complicated elongated island objects. Mechanism to form such very small elements in the Fe layer is thought to be the fine magnetic structure of the underlying FeRh-Ir layer, i.e. to the excessive magnetic charges at terrace edges and screw dislocations or to a related domain structure of the antiferromagnetic layer. (C) 2001 Elsevier Science B.V. All rights reserved.

The growth of Fe submonolayer films on Au(001) at various substrate temperatures (300-500 K) was investigated by scanning tunneling microscopy (STM). STM topographic and barrier height images show that the growth mode and morphology of the films markedly depend on growth temperature. Fe islands with a monolayer height nucleate and grow on hat terraces of Au(001) at 300 K. However, Fe islands with double-layer height and small pinholes were observed on terraces at 353 and 393 K, and step edges were decorated by Fe dendrites. A clear element-specific contrast between Fe islands and Au substrate was observed below 400 K. At elevated growth temperature, above 400 K, a segregated Au layer was observed. The mechanism of the surface segregation of Au is also discussed in terms of the surface diffusion through the pinholes. (C) 2000 Elsevier Science B.V. All rights reserved.

Magnetization processes in microfabricated NiFe wires were observed by using a Kerr microscope equipped with an oil-immersion lens (NA = 1.3) and an Hg lamp. NiFe wires 20 nm in thickness were prepared by using lift-off techniques. The width (W) of the wires was designed as 0.5, 1.0 and 2.0 μm and the length (L) as 50 μm. One end of the wire was connected to a square-shaped head with a side of 2 W, which was designed to act as a domain wall source. In each wire, necks of different widths were introduced as artificial pinning sites of a domain wall. Magnetization reversals in very narrow wires with 0.5 μm width were clearly observed. It was confirmed that domain wall penetration, pinning, depinning, and also the direction of wall motion can be controlled by using square-shaped head and necks with optimized width. The Kerr microscope image with the domain wall near the neck is almost consistent with the Kerr effect image obtained by micromagnetic calculation.

The complex reflectance ratio, rho=R(p)/R(s), for p- and s-polarized lights of copper island films with the mean mass thickness d(w) of 4 to 16 nm on glass substrates was measured in the visible region by means of in-situ ellipsometry. The observed wavelength dependence of rho was interpreted with the effective anisotropic continuous film model. The clear resonance peak of plasma oscillation in epsilon(x)'' was observed at E congruent-to 1.8 eV, which corresponded to the mean distance between the particles of 7.5 nm in the film of d(w)=4 nm. The estimated particle radius r was smaller by a factor of 10(-1) or 10(-2) than the island size directly observed by means of a scanning electron microscope.

The complex reflectance ratio, R(p)/R(s), and the optical thickness, d(o), of thin films of Cu deposited on Pyrex glass were measured at various substrate temperatures (T(sub)) by means of in-situ ellipsometry. It is revealed, from the observed growth curves, that the films are continuous and homogeneous only when the thickness d is larger than a critical thickness d(c), which is about 40 angstrom at T(sub) = 20-degrees-C, and larger for higher substrate temperatures. The dependence of the optical constant of thick copper films on the substrate temperature was interpreted by the model of surface oxidation and surface roughness.

Optical constants, n and k, and both polar Kerr rotation angle, theta(k), and ellipticity, eta(k), of Co/Pd, Fe/Pd bilayered films with overcoating layer thickness of 50 to 200 angstrom were measured in visible region. The absolute values of theta(k) of bilayered films of 200 angstrom Fe or Co overcoating Co or Fe on Pd exceed those of pure Fe or Co with maximum enhancement of 29% at 1.9 eV for Fe/Pd and 19% at 1.8 eV for Co/Pd, respectively. The diagonal component of effective dielectric tensor of these bilayered films can be roughly interpreted with the optical constants of pure elements, Fe or Co, and Pd, when the multiple reflections are taken into account, but the observed theta(k) and eta(k) cannot be explained quantitatively. An effective off-diagonal component of the underlying layer was deduced from observed theta(k) and eta(k), Which must be affected by the spin polarization effect of Pd and mixing of ferromagnetic Fe or Co atoms with Pd at the interface.