• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.303-303
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• 2011

Orderings and electronic structures of organic molecules on metal substrates have been studied due to possible applications in electronic devices. In molecular systems, delocalized pi-electrons play important roles in the adsorption behaviors and electronic structures. We studied the adsorption and electronic structures of Co-Porphyrin molecules on Au(111) using scanning tunneling microscopy (STM) and spectroscopy (STS) at low temperature. Molecules form closely packed two-dimensional islands on Au(111) surface with two different types, having different shape evolutions in our energy-dependent STM observations. The Kondo resonance state, occurred by spin exchange interaction between the Co center atom and conduction electrons in the metal substrate, was observed in one type, while it was absent in the other type in scanning tunneling spectroscopy measurements. Possible origins of two molecular shapes will be discussed.

• Journal of Magnetics
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• v.7 no.2
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• pp.59-62
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• 2002

Magnetic tunnel junctions (MTJs) were fabricated with high bias for plasma oxidation and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As-grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, $T_{max}$, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain this abnormal temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of $T_{max}$ due to annealing process are described phenomenologically with spin-dependent transfer rates of electrons tunnel through the barrier.

• Journal of the Korean Magnetics Society
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• v.25 no.2
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• pp.47-51
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• 2015

To achieve a high tunneling magneto resistance (TMR) of sputtered magnetic tunnel junctions (MTJs) with an MgO barrier, the annealing process is indispensable. The structural and compositional changes as consequences of the annealing greatly affect the spin-dependent transport properties of MTJs. Higher TMR could be obtained for MTJs annealed at higher annealing temperature. The diffusion of Ru, Mn and/or Ta in the MTJs may occur during annealing process, which is known to be detrimental to spin-dependent tunneling effect. The rapid thermal annealing (RTA) process was used for annealing the MTJs with synthetic antiferromagnets. To suppress the diffusion of Mn, Ru and/or Ta in the MTJs, the process time and temperature of RTA were minutely controlled.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.60-60
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• 2011

Giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and magnetic random-access memory (MRAM) are currently active research areas in spintronics. The high magnetoresistance and the high spin polarization (P) of electrons in the ferromagnetic electrodes of tunnel junction or intermediate layers are required. Magnetite, Fe3O4, is predicted to possess as half-metallic nature, P ~ 100% spin polarization, and has a high Curie temperature (TC~850 K). Experiments demonstrated that the P~($80{\pm}5$)%, ~($60{\pm}5$)%, and ~40-55% for epitaxial (111), (110) and (001)-oriented Fe3O4 thin films, respectively. Epitaxial Fe3O4 films may enable us to investigate the effects of half metals on the spin transport without grain-boundary scattering.In addition, it has been reported that the Verwey transition (TV, a first order metal-insulator transition) of 120 K in bulk Fe3O4 is strongly affected by many parameters such as stoichiometry and stress, etc. Here we report that the growth modes, magnetism and transport properties of Fe3O4 thin films were strongly dependent on the oxygen pressure during film growth. The average roughness decreases from 1.021 to 0.263 nm for the oxygen pressure increase from $2.3{\times}10-7$ to $8.2{\times}10^{-6}$ Torr, respectively. The 120 K Verwey transition in Fe3O4 was disappeared for the sample grown under high oxygen pressure.

• Journal of Magnetics
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• v.8 no.1
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• pp.24-31
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• 2003

The discoveries of antiferromagnetic coupling in Fe/Cr multilayers by Grunberg, the Giant Magneto Resistance by Fert and Grunberg and a large tunneling magnetoresistance at room temperature by Moodera have triggered enormous research on magnetic thin films and magnetoelectronic devices. Large opportunities are especially opened by the spin dependent tunneling resistance, where a strong dependence of the tunneling current on an external magnetic field can be found. We will briefly address important basic properties of these junctions like thermal, magnetic and dielectric stability and discuss scaling issues down to junction sizes below 0.01 $\mu\textrm{m}$$^2$with respect to single domain behavior, switching properties and edge coupling effects. The second part will give an overview on applications beyond the use of the tunneling elements as storage cells in MRAMs. This concerns mainly field programmable logic circuits, where we demonstrate the clocked operation of a programmed AND gate. The second 'unconventional' feature is the use as sensing elements in DNA or protein biochips, where molecules marked magnetically with commercial beads can be detected via the dipole stray field in a highly sensitive and relatively simple way.

• Proceedings of the Korean Magnestics Society Conference
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• 2003.12a
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• pp.240-240
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• 2003

• Journal of Magnetics
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• v.6 no.1
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• pp.1-4
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• 2001

We have used acetic acids ethanol and distilled water as a solvent to synthesize $La_{2/3}Sr_{1/3}Mn_{0.99}{^{57}}Fe_{0.01}O_3$(LSMFO) precursor. Crack-free LSMFO granular polycrystalline thin films have been deposited on thermally oxidized silicon substrates by spin coaling. The dependence of crystallization, surface morphology, magnetic and transport properties on annealing temperature was investigated. With increasing annealing temperature, the metal-semiconductor (insulator) transition temperature and the magnetic moment decrease while the resistivity increases. The lattice constants remain almost unchanged. For LSMFO thin films, spin-dependent interfacial tunneling and/or scattering magnetoresistance were observed. Our results indicate that the annealing temperature is very important in determining the intrinsic and extrinsic magnetotransport properties.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.57-57
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• 2010

Sb(111) is a spin textured surface due to the strong spin-orbit coupling, often viewed as a proto-type topological insulator. We used scanning tunneling microscopy (STM) to characterize various Mn-induced subsurface defects existing at the surface of Mn-doped Sb at 50 K. Our STM images show that every defect exhibits 3-fold symmetry with a single rotational orientation and can be categorized by their shapes and sizes. We found more than 10 types of subsurface defects with distinctive orders, which allows the resolution of the vertical positions of the magnetic dopants lying more than 10 layers down from the surface. We will discuss about our findings in comparison with theoretical results.

• Journal of the Korean Magnetics Society
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• v.10 no.3
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• pp.118-122
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• 2000

MR characteristics of $Al_2$ $O_3$ based magnetic tunneling juction with various $Al_2$ $O_3$ thicknesses were investigated. Spin-dependent tunneling junctions, in which the tunneling barrier $Al_2$ $O_3$ is formed by depositing a 1-3 nm thick Al layer, followed by thermal oxidation at room temperature in an $O_2$atmosphere, were fabricated on 4$^{\circ}$tilt(111)Si substrate in 3-gun magnetron sputtering system. The top and bottom ferromagnetic electrodes were Ni$_{80}$Fe$_{20}$ and Co. A maximum Tunneling MR ratio of 14% was obtained in the junction of which insulating barrier thickness was 2 nm. By increasing the tunneling voltage across the junction, maximum MR ratio reduced and finally showed no MR characteristics.s.

• Progress in Superconductivity
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• v.2 no.1
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• pp.43-46
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• 2000

We report on the tunneling spectroscopy of tunnel junctions using d-wave superconductor in relation to Andreev reflection. The zero bias conductance peak (ZBCP) which has maximum on [110] direction of ab-plane is observed on metal $Au/YBa_2Cu_3O_y$ tunnel junctions while it is suppressed on the ferromagnetic $Co/Au/YBa_2Cu_3O_y$ tunnel junctions. The effects of Andreev reflection on the differential conductance of each junction are dependent on the tunnel direction. For the $Co/Au/YBa_2Cu_3O_y$ junction, the suppression of Andreev reflection takes place by spin-polarized quasiparticles tunneling from a ferromagnetic material to a d-wave superconductor. By comparing these experimental results with recent theoretical works on Andreev reflection, the existence of Andreev bound state due to the d-wave symmetry of the pair potential is verified in high-$T_c$ superconductor.