Kap Soo Yoon

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.

Correlation between Pd metal thickness and thermally stable perpendicular magnetic anisotropy features in [Co/Pd]n multilayers at annealing temperatures up to 500 °C

We examine highly stable perpendicular magnetic anisotropy (PMA) features of [Co/Pd]10 multilayers (MLs) versus Pd thickness at various ex-situ annealing temperatures. Thermally stable PMA characteristics were observed up to 500 °C, confirming the suitability of these systems for industrial applications at this temperature. Experimental observations suggest that the choice of equivalent Co and Pd layer thicknesses in a ML configuration ensures thermally stable PMA features, even at higher annealing temperatures. X-ray diffraction patterns and cross-sectional transmission electron microscopy images were obtained to determine thickness, post-annealing PMA behavior, and to explore the structural features that govern these findings.

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.

Observation of giant magnetoresistance in CoFeN/AlOx/CoFeN magnetic tunneling junctions employing a nitrogen-doped amorphous CoFeN free layer electrode

We examine the crystallographic and magnetic features of nitrogen-doped amorphous CoFe (CoFeN) electrodes for application as ferromagnetic free layers in magnetic tunnel junctions, in which precise control of the nitrogen content is crucial for achieving the desirable magnetic features. Incorporating nitrogen into the CoFe layer during growth provides numerous benefits including a remarkably reduced coercivity of 5 Oe, a phase transition from polycrystalline to amorphous, a low magnetization of 294 emu/cm3, and an enhanced thermal stability up to 400 °C. A high magnetic resistance ratio of about 220% was also obtained for annealed in-plane CoFeN/AlOx/CoFeN magnetic tunneling junctions containing a 1.2-nm-thick amorphous AlOx tunnel barrier. We anticipate that our experimental findings will aid in the development of a variety of future spintronic devices.

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.

Exploring oxygen-affinity-controlled TaN electrodes for thermally advanced TaO x bipolar resistive switching

Recent advances in oxide-based resistive switching devices have made these devices very promising candidates for future nonvolatile memory applications. However, several key issues remain that affect resistive switching. One is the need for generic alternative electrodes with thermally robust resistive switching characteristics in as-grown and high-temperature annealed states. Here, we studied the electrical characteristics of Ta2O5−x oxide-based bipolar resistive frames for various TaNx bottoms. Control of the nitrogen content of the TaNx electrode is a key factor that governs variations in its oxygen affinity and structural phase. We analyzed the composition and chemical bonding states of as-grown and annealed Ta2O5−x and TaNx layers and characterized the TaNx electrode-dependent switching behavior in terms of the electrode’s oxygen affinity. Our experimental findings can aid the development of advanced resistive switching devices with thermal stability up to 400 °C.

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.

Magnetic properties of laser-induced ferromagnetic cobalt quantum dots passivated by ultrathin Ta film

The ferromagnetic quantum dots (FMQDs) are fabricated by exposing a pulsed Nd:YAG laser (wavelength=355 nm) on ultrathin cobalt (Co) films under an external magnetic field of 1000 Oe. Various dot diameters and densities of more than 3.5×1010/cm2 are ultimately realized by changing laser power, scan condition and initial film thickness. In addition, both AlN and Ta thin films as capping layers are used to protect the FMQDs from natural oxidation effect. The observed coercivity values of uncapped and capped FMQDs are changed from 600 up to 200 Oe with the initial Co thickness of 2 nm, respectively. The observation of magnetic force microscopy and hysteresis loops clearly confirms in-plane single domain state for the Ta-capped FMQDs.