Takeshi Kawagoe

Abstract We investigate the room-temperature growth of ultrathin Mn films on Au(001) surface for Mn thicknesses (dMn) up to 1.5 nm. The surface structure and morphology at various dMn are investigated via scanning tunneling microscopy and low-energy electron diffraction (LEED). Although the formation of surface Au-Mn alloy is observed at dMn = 0.3 nm, body centered-tetragonal (bct) Mn(001) films grow epitaxially in a layer-by-layer mode at dMn = 0.6 and 1.0 nm, showing a p(1×1) LEED pattern with the in-plane lattice constant of Au(001). The surface morphology comprises primarily atomically flat terraces with spiral growth patterns at dMn = 1.0 nm. At dMn = 1.5 nm, three-dimensional islands are observed, thus indicating growth-mode change from layer-by-layer to layer-plus-island. The differential-conductance spectrum for dMn = 1.0 nm shows distinct peaks at -0.07 and -0.45 V, which may correspond to the spin-polarized surface states of the bct Mn(001) film surface.

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.

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.

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.