Ji Ung Cho

Effect of external magnetic field on anisotropy of Co∕Cu multilayer nanowires

We have investigated the effect of external magnetic field exercised during the fabrication of the Co and Co∕Cu multilayer nanowires in anodic aluminum oxide (AAO) templates using pulse electrodeposition. It is found that the effect becomes significant when the pore size is small. Deterioration occurs in the magnetic properties for the Co nanowires studied, whereas a distinct enhancement in the magnetic properties for the Cu∕Co multilayer nanowires is achieved by applying an external magnetic field during nanowire growth using the AAO template comprising pores of 50nm diameter.

The pH dependence of Co-Cu alloy thin films fabricated on amorphous substrate by DC electrodeposition

Bulk Co-Cu alloy thin films were fabricated on amorphous substrates, instead of the commonly employed metallic substrates, by dc electrodeposition. The stoichiometric distributions of thin films on both sides was estimated using bath parameters, e.g., current pulse amplitude and electrolyte pH by energy dispersive spectroscopy (EDS). The results obtained revealed that film Cu contents were much higher on the solution/film interface than at the substrate/film interface. This indicates that the metals exchange between Co and Cu occurred immediately after the end of cathodic pulse until the films completely separated from electrolyte contribution to the asymmetry of Cu distribution. On increasing the cathodic current density, the films became more and more Co rich in a low pH bath, whereas at above 80 mA/cm/sup 2/ Co content was as high as 90 wt.%. The significant variations in the compositions of thin films formed using conditions of identical current density were also observed when the pH value was varied, which indicates the presence of a strong relation between bath pH value and composition dependence for the Co/Cu electrodeposition process. Moreover, some films deposited from higher pH baths showed oxygen incorporation in films at the solution side of the deposited film. The coercive fields of films obtained under fixed current, e.g., 80 mA/cm/sup 2/, at varying thicknesses were measured by vibrating sample magnetometer, and a correlation between coercivity and film thickness was obtained.

Effects of Cu doping on the microstructure and magnetic properties of CoPt nanowires

We studied the effects of copper doping on the microstructure and magnetic properties of CoPt nanowires fabricated in anodized aluminium oxide (AAO) nanotemplates by electrodeposition. The morphological observation indicates well-formed CoPt and CoPtCu nanowires, while the structural analysis shows that fcc-Co3Pt phase exists prior to annealing and is retained in the doped nanowires, with minor hcp-CoPt phase postannealing. The doping causes shrinkage of the lattice spacing prior to annealing due to the insertion of smaller copper atoms. Compared to the undoped CoPt nanowires, the magnetic measurements reveal significant enhancement in the magnetic properties of the CoPtCu nanowire postannealing, resulting in the more rapid increase in the coercivity with annealing temperature.

Substrate effects on microstructure and magnetic properties of electrodeposited Co nanowire arrays

The effect of substrates on the microstructure and magnetic properties of electrodeposited Co nanowire arrays in porous anodic aluminum oxide membrane was investigated. It is found that the nanowires possess dominant (002) and (110) textures on the Co-coated nanocrystalline Finemet™ and Au substrates, respectively. Magnetic measurements show that the easy axis of the nanowire arrays switches from perpendicular to the wire axis on Au to parallel to the wire axis on the Co-coated nanocrystalline Finemet™ substrate.

Fabrication of Nano-Porous Structure on Silicon Substrate Using Nanoimprint Lithography with an Anodic Aluminum Oxide Nano-Template

A polymer template, which has an array of 300 nm diameter pillar patterns, was fabricated by hot embossing method using anodic aluminum oxide (AAO) template as an embossing stamp. After depositing the thin layer of silicon oxide and coating of anti-adhesion monolayer of organic film on silicon oxide, UV nanoimprint lithography was carried out with the polymer template. As a result, nano-pore array pattern, identical to anodic aluminum oxide pattern, was fabricated on silicon substrate. Residual layer of imprinted nano-pore array pattern was removed by oxygen plasma etch and thin film of Au/Ti was deposited. After lift-off process, Au/Ti dot array was also fabricated on silicon substrate.

Magnetoresistance Variation of Magnetic Tunnel Junctions with NiFeSiB/CoFeB Free Layers Depending on MgO Tunnel Barrier Thickness

We developed NiFeSiB/CoFeB hybrid free-layers for magnetic tunnel junctions (MTJs) with MgO tunnel barrier layers. These junctions show tunneling magnetoresistance (TMR) ratios and resistance-area (RA) values ranging from 118-209% and 36-2380 Omega mum2 , respectively, obtained at room temperature. Compared to the CoFeB single free-layer case, the NiFeSiB/CoFeB hybrid free-layer approach has the advantage of lowering saturation magnetization. The low magnetization material would be effective to decrease the switching current in spin transfer torque (STT) switching. The experimental results show that the RA value depends not only on the thickness of the MgO barrier but also on the structure of the free layer used. Tunable in the TMR ratio and RA value using the design of the hybrid free-layer, our hybrid free-layered MTJs demonstrate a desirable lower RA value but a similar TMR ratio in comparison to the CoFeB free-layered ones.

Composition-Dependent Crystal Structure and Magnetism in Nanocrystalline Co-Rich Alloy

We report the magnetism of a ferromagnetic two phase mixture system by investigating the microstructural evolution and relevant property changes as a function of alloy composition in CoFeSiB film. The crystal structure evolved from an amorphous to hcp Co nanocrystal embedded two-phase mixture when Co concentration exceeded the critical amount (~75 at.%) confirmed through TEM and XRD. Very low coercivity value was observed in the amorphous samples. The soft magnetic properties are due to the fact that the magnetic moments of Co-rich phase are exchange-coupled via the amorphous matrix, as a consequence the magnetic anisotropies are averaged out in the amorphous system. Meanwhile, beyond 75 at.% of Co, an abrupt increase in coercivity was observed in the nanocrystalline samples where highly anisotropic Co nanocrystals were precipitated. The hard magnetic properties resulted from the magnetic decoupling (breaking of the nanocrystal-amorphous matrix-nanocrystal coupling chain) because the exchange forces are not able to overcome the magnetocrystaline anisotropy. We also found that antiferromagnetic exchange coupling existed at the nanocrystal-amorphous matrix interfaces.

Transport Properties of Magnetic Tunnel Junctions Comprising NiFeSiB/CoFeB Hybrid Free Layers

We report on the magneto-transport measurements of MgO magnetic tunnel junctions (MTJs) composed of NiFeSiB/CoFeB as the free layer for two different structures (top-type and bottom-type pinning). The magneto-transport properties of these MTJs were investigated by varying the thickness of the amorphous NiFeSiB layer for a fixed CoFeB thickness. The tunnel magnetoresistance (TMR), measured in both type of structures, exhibit the same or a higher amplitude (up to 230% measured at room temperature in the case of top-type device), comparing to the case of a single CoFeB free layer. These results suggest that hybrids free layers can be used as good candidates for MTJs with reduced saturation magnetization while keeping a high TMR ratio.

Giant Diamagnetism in AuFe Nanoparticles

Multifunctional Au_0.5Fe_0.5 alloy nanoparticles from a polyol process were investigated, showing the alloy nanostructures with the formation of a new phase and twin microstructures. Giant diamagnetism, on the order of -10^- ² emu g^-1 Oe^-1 , is observed in the zero-field cooled process of the nanoparticles, which is modeled in terms of the spin-orbital interactions tuned by the modified potential energy as a result of the shrinking dimension, peculiar disordered microstructure, and heterogeneous elemental composition.