Jae-Geun Ha

Composition dependence of particle size distribution and giant magnetoresistance in Co–Al–O granular films

Abstract We have investigated the particle size distributions in Co–Al–O insulating granular films with various Co concentrations ( x Co ) by analyzing the superparamagnetic behavior of magnetization. The comparison between the particle size distributions estimated from the magnetization analysis and the TEM observation has made it possible to distinguish between the topological size and the magnetic size of particles. It is suggested that some Co particles couple ferromagnetically to form larger magnetic clusters as x Co increases. The magnetoresistance (MR) ratio decreases slightly with x Co for x Co >36 at%, which is different from the result for metallic granular systems where the MR ratio decreases drastically with the coarsening of magnetic clusters.

Enhanced tunneling magnetoresistance and thermal stability of magnetic tunnel junction by rapid thermal anneal

Abstract The effect of rapid thermal anneal (RTA) has been investigated on the properties of an FeMn exchange-biased magnetic tunnel junction (MTJ) using magnetoresistance and I – V measurements and transmission electron microscopy (TEM). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be ∼27%, while the TMR in MTJs annealed by RTA increases with annealing temperature up to 300°C, reaching ∼46%. TEM images reveal that the interface of Al 2 O 3 layer for the annealed MTJ has changed into a relatively clear morphology, as compared to that for the as-grown MTJ. The oxide barrier parameters are found to vary abruptly with annealing time within a few ten seconds. Our results demonstrate that the present RTA enhances the thermal stability of MTJs.

Temperature dependence of magnetoresistance for tunnel junctions with high-power plasma-oxidized barriers: Effects of annealing

Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power 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, Tmax, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the 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 Tmax due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier.

Effect of Ti seed layers on structure of self-organized epitaxial face-centered-cubic-Ag(001) oriented nanodots

The influence of Ti seed layers on the structure of self-organized Ag nanodots, obtained with a Ti seed-layer-assisted thermal agglomeration method, has been investigated. The samples were grown on MgO(001) single crystal substrates by RF magnetron sputter deposition. The samples were deposited at room temperature and post-annealed at 350 °C for 4 h while maintaining the chamber vacuum conditions. The results of atomic force microscopy (AFM) observations indicated that the insertion of the Ti seed layer (0.6–5.0 nm) between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot array. Comparisons between the AFM images revealed that the size of the Ag nanodots was increased with an increase in the Ti seed layer thickness. The atomic concentration of the film surface was confirmed by X-ray photoelectron spectroscopy (XPS). The XPS result suggested that the nanodot surface mainly consisted of Ag. Moreover, X-ray diffraction results proved that the initial deposition of the Ti seed layer (0.6–5.0 nm) onto MgO(001) prior to the Ag deposition yielded high-quality fcc-Ag(001) oriented epitaxial nanodots. The optical absorbance spectra of the fabricated Ag nanodots with various Ti seed layer thicknesses were obtained in the visible light range.

Temperature dependence of the tunneling magnetoresistance for tunnel junctions

Abstract The temperature dependence of the tunneling magnetoresistance (TMR) for magnetic tunneling junctions is investigated experimentally before and after the sample is annealed. As grown, the TMR is observed to increase with temperature from 80 to 160xa0K. A modified Julliere model in conjunction with a spin-dependent two-step tunneling is suggested to describe this temperature dependence.

Annealing effect of tunnel-type GMR in Co-Al-O granular thin films

Abstract We have investigated the transport and GMR behavior for Co–Al–O granular thin films on annealing, in conjunction with the change in microstructure. The samples were annealed at 300°C for 10, 60 and 360xa0min to change the microstructure. The temperature dependence of electrical resistivity ( ρ ) shows the relationship of the log ρ∝T −1/2 for all the annealed samples. The tunneling activation energy decreases with increasing annealing time. The magnitude of GMR decreases with increasing annealing time.

Atomic force microscopy and x-ray diffraction studies on agglomeration phenomena of ultrathin Au/Fe bilayers

The agglomeration phenomena of a few nanometer thick Au/Fe bilayers, grown on an MgO(100) substrate, were studied by using atomic force microscopy and x ray diffraction (XRD). The authors found that the insertion of an Fe ultrathin layer between an MgO(100) substrate and a 4 nm thick Au layer promotes the agglomeration process of the Au layer, in which the bilayer structure changes into large Fe/Au islands of ∼200 nm in diameter. In addition, XRD results revealed that the Au in the agglomerated islands has only a (111)-crystallographic orientation, presumably caused by reducing the large surface energy of Au on the MgO(001) substrate. These findings are quite different from cases in which structural stabilization is achieved by inserting an Fe seeding layer of a few nanometers on an MgO(001) substrate.

Epitaxial growth of fcc-Ag(0 0 1) nanodots on MgO(0 0 1) substrates via Ti seed layer-assisted agglomeration

We have analysed the influence of Ti seed layer (2.0 nm thick) on the agglomeration of Ag films (4.0 nm thick) grown onto MgO(0 0 1) single crystal substrates by RF magnetron sputtering. The samples were deposited at room temperature and post-annealed at 200–450 °C for 4 h while still maintaining the chamber vacuum condition. The surface profile of the sample, as analysed using atomic force microscopy, confirms that the insertion of a Ti seed layer between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot. Furthermore, the atomic concentration depth profile of the Ag/Ti/MgO film, as estimated by using angle-resolved x-ray photoelectron spectroscopy, suggests that the nanodot surface mainly consists of Ag. Moreover, x-ray diffraction studies prove that the initial deposition of the Ti seed layer onto MgO(0 0 1) prior to the Ag deposition yields high-quality face-centred cubic (fcc)-Ag(0 0 1) oriented epitaxial nanodots. Based on these results, it can be concluded that the Ti thin film acts as a seed layer, assisting the epitaxial growth of the Ag nanodot onto the MgO substrate.

Improved piezoelectric properties of Ag doped 0.94(K0.5–βNa0.5–δ)NbO3–0.06Li1–γNbO3 ceramics by templated grain growth method

In this work, Ag doped 0.94(K0.5–βNa0.5–δ)NbO3–0.06Li1–γNbO3 lead-free piezoelectric ceramics have been prepared by templated grain growth (TGG) using cubic (K,Na)NbO3 single crystal seeds as templates. Specimens were prepared by the conventional mixed method and sintered in controlled atmosphere. TGG method was well known, because that this method can improve piezoelectric properties by increasing the grain size during the sintering process. Sintering caused densification and grain growth of ceramics by the expense of matrix particles. Densification prior to grain growth was found to be necessary to obtain highly textured ceramics. Crystalline properties were analyzed by the XRD method. The effects of TGG on phase structure, microstructure, piezoelectric and dielectric properties of Ag doped 0.94(K0.5–βNa0.5–δ)NbO3–0.06Li1–γNbO3 ceramics were investigated.

Analysis and improvement of interfacial adhesion of growth-dominant Ge-doped SbTe phase change materials

A semiquantitative characterization of the interfacial adhesion of phase change materials is developed, which consists of determining critical adhesion temperature (TCA) via measuring the probability of adhesion failure with temperature using patterned films. By comparison of TCA values, Ge-doped SbTe (Ge-ST) is shown to have weaker adhesion than Ge2Sb2Te5 (GST), which results from its limited ability in relaxation of crystallization-induced stress. Nitrogen or oxygen doping in Ge-ST produces significant increase in TCA, close to that of GST. This improvement is due to smaller grain size of N-/O-doped Ge-ST, which facilitates the relaxation of the stress via grain boundary diffusion or sliding.