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Spin-polarized scanning tunneling microscopy with quantitative insights into magnetic probes
Phark Soo-hyon,Sander Dirk 나노기술연구협의회 2017 Nano Convergence Vol.4 No.8
Spin-polarized scanning tunneling microscopy and spectroscopy (spin-STM/S) have been successfully applied to magnetic characterizations of individual nanostructures. Spin-STM/S is often performed in magnetic fields of up to some Tesla, which may strongly influence the tip state. In spite of the pivotal role of the tip in spin-STM/S, the contribution of the tip to the differential conductance dI/dV signal in an external field has rarely been investigated in detail. In this review, an advanced analysis of spin-STM/S data measured on magnetic nanoislands, which relies on a quantitative magnetic characterization of tips, is discussed. Taking advantage of the uniaxial out-of-plane magnetic anisotropy of Co bilayer nanoisland on Cu(111), in-field spin-STM on this system has enabled a quantitative determination, and thereby, a categorization of the magnetic states of the tips. The resulting in-depth and conclusive analysis of magnetic characterization of the tip opens new venues for a clear-cut sub-nanometer scale spin ordering and spin-dependent electronic structure of the non-collinear magnetic state in bilayer high Fe nanoislands on Cu(111).
Nucleation and growth of primary nanostructures in SrTiO <sub>3</sub> homoepitaxy
Phark, Soo-hyon,Chang, Young Jun Springer US 2015 Nanoscale research letters Vol.10 No.1
<P>SrTiO<SUB>3</SUB> nanoislands on SrTiO<SUB>3</SUB> (001) in a <I>diffusion-limited growth</I> regime were studied using <I>in situ</I> scanning tunneling microscopy (STM). The STM images revealed two characteristic features of nucleation stages. First, the minimum lateral size of the one-unit-cell (<I>uc</I>)-high SrTiO<SUB>3</SUB> islands was 4 × 4 <I>uc</I><SUP>2</SUP>. Second, one-dimensional SrTiO<SUB>3</SUB> islands of a 4 <I>uc</I> width grew along the crystal symmetry directions. These observations suggest that 4 × 4-<I>uc</I><SUP>2</SUP> islands act as a minimum nucleation seed, and the addition of SrTiO<SUB>3</SUB> molecular species of the same width is the primary and dominant growth process in SrTiO<SUB>3</SUB> homoepitaxy. A close inspection of the surface of the substrate during the deposition process revealed possible connections between surface reconstruction and energetically favorable nucleation of SrTiO<SUB>3</SUB> islands.</P>
Characterization of Pt/a-Plane GaN Schottky Contacts Using Conductive Atomic Force Microscopy
Phark, Soo-Hyon,Kim, Hogyoung,Song, Keun Man,Kang, Phil Geun,Shin, Heung Soo,Kim, Dong-Wook American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.2
<P>We investigated the local electrical properties of Pt Schottky contacts to a-plane n-type GaN using conductive atomic force microscopy (C-AFM). Current-voltage characteristics obtained by C-AFM showed rectifying properties, indicating nano-scale Schottky junction formation. Two-dimensional current maps revealed that the surface microstructures of GaN influenced transport properties of the junctions.</P>
Current transport in Pt Schottky contacts to <i>a</i>-plane n-type GaN
Phark, Soo-Hyon,Kim, Hogyoung,Song, Keun Man,Kang, Phil Geun,Shin, Heung Soo,Kim, Dong-Wook Institute of Physics [etc.] 2010 Journal of physics. D, applied physics Vol.43 No.16
<P>The temperature-dependent electrical properties of Pt Schottky contacts to nonpolar <I>a</I>-plane n-type GaN were investigated. Barrier height and ideality factor, estimated from the conventional thermionic emission model, were highly temperature dependent. A notable deviation from the theoretical Richardson constant value was also observed in the conventional Richardson plot. Analyses using the thermionic field emission model showed that consideration of defect-assisted tunnelling was necessary to explain the observed electrical behaviours.</P>
Chang, Young Jun,Phark, Soo-hyon American Chemical Society 2016 ACS NANO Vol.10 No.5
<P>Revealing growth mechanism of a thin film and properties of its film substrate interface necessarily require microscopic investigations on the initial growth stages in temperature-and thickness-resolved manners. Here we applied in situ scanning tunneling microscopy and atomic force microscopy to investigate the growth dynamics in homo-(SrTiO3) and hetero-(SrRuO3) epitaxies on SrTiO3(001). A comparison of temperature dependent surface structures of SrRuO3 and SrTiO3 films suggests that the peculiar growth mode switching from a 'layer-by-layer' to 'step-flow' type in a SrRuO3 films arises from a reduction of surface migration barrier, caused by the change in the chemical configuration of the interface between the topmost and underlying layers. Island densities in perovskite epitaxies exhibited a clear linear inverse-temperature dependence. A prototypical study on island nucleation stage of SrTiO3 homoepitaxy revealed that classical diffusion model is valid for the perovskite growths.</P>
Chang, Young Jun,Phark, Soo-hyon Elsevier 2017 CURRENT APPLIED PHYSICS Vol.17 No.5
<P>The growth of perovskite oxide films is known to be strongly influenced by both substrate surface lattice symmetry and stoichiometry. However, this has been postulated mainly based on indirect evidences. Scanning tunneling microscopy (STM) is unambiguously capable of identifying the real-space distribution of the structural and electronic properties of solid surfaces. Therefore, oxide film growth technologies combined with STM are strongly desirable for resolving atomic-scale growth mechanisms of perovskite thin films. Here, we review recent advances in STM studies on initial growth stages of perovskite oxides on SrTiO3(001). First, we introduce surface terminations and reconstructions of SrTiO3(001), as well as their influence on the initial growth of perovskite films studied by STM on an atomic-scale, followed by a discussion of a feasible model for the surface atomic structures and chemistries behind such growth behaviors. We then introduce studies on the growth dynamics of perovskite oxides on SrTiO3(001) in terms of temperature (T) and thickness-resolved STM: (1) Layer-by-layer identification of T-dependent surface structures of ultrathin SrRuO3, suggesting a dramatic change in the surface migration barrier caused by switching of the surface terminations and (2) a prototypical study on the surface diffusion dynamics in perovskite growth realized by the application of the classical diffusion model to the island nucleation stage of SrTiO3 homoepitaxy. (C) 2016 Elsevier B.V. All rights reserved.</P>
Kim, Haeri,Kim, Dong-Wook,Phark, Soo-Hyon Institute of Physics [etc.] 2010 Journal of Physics. D, Applied Physics Vol.43 No.50
<P>Using Pt/TiO<SUB>2</SUB>/Ti planar junctions fabricated with micrometre-sized gaps between electrodes, we found that the application of a bias voltage between the electrodes significantly decreased the resistance of the junction. The nanoscopic resistance profile revealed that the electrical stress modified the bulk as well as the contact resistance. Electrostatic force microscopy was used to investigate the charge distribution and its time evolution in local areas scanned by positively biased Pt-coated tips. Comparative investigations of the transport and scanning probe microscopy results suggest that the electrical stress induced a redistribution of ions, which then modified the junctions' transport characteristics.</P>
Fischer, Jeison A.,Sandratskii, Leonid M.,Phark, Soo-hyon,Sander, Dirk,Parkin, Stuart American Physical Society 2017 Physical review. B Vol.96 No.14
<P>We combine spin-polarized scanning tunneling microscopy (SP-STM) and first-principles calculations to demonstrate the control of the wavelength of helical spin textures in Fe nanoislands by varying their atomic structure. We make use of the complexity of submonolayer growth of Fe on Cu(111) to prepare nanoislands characterized by different thickness and in-plane atomic structure. SP-STM results reveal that the magnetic states of different nanoislands are spin helices. The wavelength of the spin helices varies strongly. Calculations performed for Fe films with different thickness and in-plane atomic structure explain the strong variation of the wavelength by a subtle balance in the competition between ferromagnetic and antiferromagnetic exchange interactions. We identify the crucial role of the effectively enhanced weak antiferromagnetic exchange interactions between distant atoms.</P>