Chae Ok Kim

Room temperature formation of half-metallic Fe3O4 thin films for the application of spintronic devices

Half-metallic Fe3O4 films were prepared at room temperature using a rf sputtering system specially integrated with an external rf source. Primary emphasis was placed on obtaining a large amount of active oxygen radicals through an external electrode for efficient deposition. The insertion of an external electrode was found to be critical for room temperature growth of Fe3O4 thin films. The structural and electrical properties gave shift and broadening effects to the Verwey temperature at various powers. The magnetization could only be saturated when a 300 Oe field was applied along an easy axis of magnetization during growth. However, there was no sign of saturation up to 5 T under zero-field growth.

Performance of Co/Al2O3/NiFe magnetic tunnel junctions prepared by a two-step rf plasma oxidation method

A two-step rf plasma oxidation technique of an insulating layer has been performed to enhance electrical and structural properties of magnetic tunnel junction (MTJ) devices. Comparison was made by analyzing properties of the MTJ oxidized by conventional rf and two-step rf plasma oxidation methods. Experimentally observed results give improved surface imaging and sufficient oxygen contents of the insulating layer under the two-step oxidation method. In addition, electrical breakdown voltage and magnetoresistance of the MTJ were increased from 0.7 to 1.8 V and from 4.5% to 6.8%, respectively, correlated with improved structural information.

Enhanced electrical characteristics of Au nanoparticles embedded in high-k HfO2 matrix

We present experimental results for laser-induced Au nanoparticle (NP) embedded in a HfO2 high-k dielectric matrix. Cross-sectional transmission electron microscopy images showed that the Au NPs of 8nm in diameter were clearly embedded in HfO2 matrix. Capacitance-voltage measurements of Pt∕HfO2∕AuNPs∕HfO2 on p-type Si substrate reliably exhibited metal-oxide-semiconductor behavior with a large flatband shift of 4.7V. In addition, the charge retention time at room temperature was found to exceed 105h. This longer time was attributed to the higher electron barrier height via high work function of the Au NP.

Influence of grain boundaries and voids on the saturated magnetization in Fe/sub 3/O/sub 4/ films at a low magnetic field

We report the magnetic behaviors of Fe/sub 3/O/sub 4/ thin films grown by zero field growth (ZFG) and field growth (FG) techniques during the sputtering process. In FG conditions, an in situ 300 Oe field during growth is applied to a substrate, inducing an easy axis of magnetization. Structural observations obtained by high-resolution transmission electron microscopy measurements clearly depicted a significant reduction of the grain boundaries and voids in the Fe/sub 3/O/sub 4/ films grown under FG conditions, thus explaining the saturated magnetization of the Fe/sub 3/O/sub 4/ films at about 0.01 T. This behavior was expected due to a remarkable reduction of the antiferromagnetic exchange couplings between grains for FG conditions. In addition, the zero-field-cooled magnetization of the ZFG samples showed an abrupt change at about 285 K, confirming the existence of defects or other phases in the ZFG films.

Room temperature operation of magnetic tunnel transistors with half-metallic Fe3O4 emitter source

Magnetic tunnel transistors (MTTs) based on a half-metallic Fe3O4 emitter source were investigated to obtain a high efficiency of spin-dependent hot electron transport. The MTTs include magnetic tunnel junctions with an AlOx insulating barrier. The insulating barrier formed by a remote rf plasma oxidation method was used to enhance electrical and structural properties of MTTs. The highest magnetocurrent ratio of the MTTs was experimentally observed to be about 51% at room temperature. Especially, the transfer ratio of about 10−2 was obtained by systematically varying the heights of emitter and collector barrier.Magnetic tunnel transistors (MTTs) based on a half-metallic Fe3O4 emitter source were investigated to obtain a high efficiency of spin-dependent hot electron transport. The MTTs include magnetic tunnel junctions with an AlOx insulating barrier. The insulating barrier formed by a remote rf plasma oxidation method was used to enhance electrical and structural properties of MTTs. The highest magnetocurrent ratio of the MTTs was experimentally observed to be about 51% at room temperature. Especially, the transfer ratio of about 10−2 was obtained by systematically varying the heights of emitter and collector barrier.

Simultaneous measurements of the magnetostrictive coefficient, Young’s modulus, and Poisson ratio of thin films

A method, which determines simultaneously the magnetostrictive coefficient, Young’s modulus, and Poisson ratio of a thin film utilizing the minimization of the total elastic energy of a cantilever film–substrate system, is suggested. An inaccuracy for the magnetostrictive coefficient, caused by assuming the elastic properties of the film as those of the corresponding bulk material, could be avoided and only a single elastic isotropic substrate is employed in the present method. The experimental data of an Fe-based amorphous thin film were analyzed by using the model. The calculated dependence of the magnetostrictive coefficient on the external magnetic field was compared with the experiment, and the discrepancy between both results were explained. The elastic properties of the film were also obtained.

Efficient fabrication and characterization of cobalt nanoparticles embedded in metal∕oxide∕semiconductor structures for the application of nonvolatile memory

Metal-oxide-semiconductor (MOS) capacitors with Co nanoparticles (Co NPs) were successfully fabricated by utilizing an external laser irradiation method for the application of nonvolatile memory. Experimental images of cross-sectional transmission electron microscopy showed that the Co NPs of 5nm in diameter were clearly embedded in SiO2 layer. Capacitance-voltage measurements of Pt∕SiO2∕Co NPs∕SiO2 on p-type Si (100) substrate certainly exhibited typical MOS behavior with a flatband voltage shift of 1.1V. In addition, the charge retention characteristics of MOS capacitors with the Co NP were investigated using capacitance-time measurements. The present results indicate that their unique laser process gives rise to a possible promise for the efficient formation or insertion of metal NPs inside the MOS structures.

Ferromagnetic resonance in CoZr/Ag/CoZr trilayer films

A series of CoZr(200 /spl Aring/)/Ag(t/sub Ag/)/CoZr(200 /spl Aring/) trilayers with Ag interlayer thickness t/sub Ag/ in the range of 10-160 /spl Aring/ was deposited on Si[100] wafers at room temperature by using a magnetron sputtering system. In order to understand the dependence of the magnetic exchange interaction between ferromagnetic Co/sub 84/Zr/sub 16/(at.%) layers separated by nonmagnetic Ag layers on the t/sub Ag/, we investigated the FMR spectra. The coupling strength increases with increasing t/sub Ag/ up to 20 /spl Aring/ with a maximum value of 2.0 kOe, but decreases rapidly with increasing t/sub Ag/ in the range of 40-100 /spl Aring/. Finally, the value approaches zero above 120 /spl Aring/. The coupling strength is positive for all samples.

Surface plasmon resonance analysis of insulating AlO/sub x/ thin film for magnetic tunnel junctions prepared by natural oxidation method

The AlO/sub x/ insulating barrier in MTJ was fabricated and analyzed by utilizing a natural oxidation and surface plasmon resonance spectroscope (SPRS) technique. The basic structure of MTJ was Ta/CoFe/AlO/sub x/:natural oxidation/NiFe/Ta. SPRS was used to investigate optimum thickness and dielectric properties of the AlO/sub x/ layers. The SPRS results exhibited changes in the oxidation state of the barrier, depending on the oxidation time. Natural oxidation depth and speed were calculated by comparing SPRS simulation results with experimental ones. It was also found that 8 /spl Aring/ of Al layer is the optimum thickness when MTJ was formed using natural oxidation method.

Enhanced thermal stability of magnetic tunnel junctions formed by in situ radiation annealing process on AlOx insulating barriers

An in situ direct radiation annealing (IDRA) technique on the AlOx insulating barrier was performed to enhance the thermal stability of magnetic tunneling junctions (MTJs). Our method was found to improve the dielectric and structural properties of AlOx insulating barrier in MTJs. After the proper IDRA process, the conventional ex situ annealing process exhibited an additional enhancement of thermal stability by significantly reducing the interdiffusion process between Co80Fe20 electrode and AlOx insulating barriers. Experimentally observed tunneling magnetoresistance of MTJs after the IDRA process showed about 55% up to 350°C. Finally, a surface-plamon resonance spectroscope measurement observed that the dielectric constant of insulating barriers was increased from 2.7 to 3.2 after the IDRA process.