Tae Hee Kim

Demonstration of all in situ magnesium diboride superconductor thin-film tunnel junctions

High-quality planar tunnel junctions of MgB2 superconductor have been fabricated, all in situ, on Si single crystal substrates with Al2O3 as well as MgO barriers. Two superconducting energy gaps due to the σ and π bands of MgB2 have been unambiguously determined from superconductor-insulator-superconductor tunneling conductance measurements, showing good agreement with the theoretical predictions. Phonon structures have also been observed in tunneling. Our junction fabrication technique and well defined tunneling characteristics show a distinct possibility for the development of superconductive electronics based on MgB2 junctions.

Magnesium diboride superconductor thin film tunnel junctions for superconductive electronics

Based on superconducting MgB2 films with higher critical temperature of 39K and the advantage of the conventional superconductors, those that follow Bardeen-Cooper-Shrieffer theory, fabrication of quasiparticle, and Josephson tunnel junctions have been investigated. To explore the potential of MgB2 for superconductive electronics, the essentials such as clean high quality thin film surfaces and reproducible tunnel junction fabrication are addressed. Our results show clean tunneling characteristics for in situ prepared MgB2 junctions on Si wafer with the measured superconducting energy gap values in good agreement with theory and the feasibility of the technology. The recent results on all-epitaxial MgB2∕MgO∕MgB2 Josephson junctions will be also reported.

Fabrication of multiband MgB2 tunnel junctions for transport measurements

The coexistence of multiple superconducting condensates in a material gives rise to intriguing phenomena, such as the possible presence of number-phase fluctuations. In this work, tunnel junctions were fabricated, aiming for the detection of such phenomena. Planar junctions with normal conducting and superconducting counter-electrodes have been realized. It was found from spectroscopic investigations that the MgO barrier used for the planar junctions is not ideal. To optimize for the two-band behaviour of MgB2, ab-plane ramp-type MgB2 tunnel junctions were fabricated. The use of Al2O3 barriers substantially improved the homogeneity of the junctions, as was concluded from the nearly ideal magnetic field dependence of the critical current and excellent scaling of the junction properties with the junction widths. The current–voltage characteristics show clear Shapiro steps under microwave irradiation. However, the spectrum showed a degradation of the MgB2 in the ramp region and the signature of the σ-gap was absent. For tunnel junction spectroscopy in the ab-direction and the detection of new, two-band related modes, the fabrication process still has to be further optimized.

Molecular-Beam Epitaxially Grown MgB2 Thin Films and Superconducting Tunnel Junctions

Since the discovery of its superconducting properties in 2001, magnesium diboride has generated terrific scientific and engineering research interest around the world. With a 𝑇𝐶 of 39 K and two superconducting gaps, MgB2 has great promise from the fundamental point of view, as well as immediate applications. Several techniques for thin film deposition and heterojunction formation have been established, each with its own advantages and drawbacks. Here, we will present a brief overview of research based on MgB2 thin films grown by molecular beam epitaxy coevaporation of Mg and B. The films are smooth and highly crystalline, and the technique allows for virtually any heterostructure to be formed, including all-MgB2 tunnel junctions. Such devices have been characterized, with both quasiparticle and Josephson tunneling reported. MgB2 remains a material of great potential for a multitude of further characterization and exploration research projects and applications.

Perpendicular Magnetization of FePt Alloy Films Epitaxially Grown on Si(100)

This study examined the structure and perpendicular magnetization of FePt films grown on Pt/Fe/MgO(100) buffered Si(100) substrates by molecular beam epitaxy. The [Fe(0.17㎚)/Pt(0.2㎚)] N multilayers were prepared at room temperature to form a L1?-FePt phase after vacuum annealing. Perpendicular magnetic anisotropy (PMA) was observed in the films after at least 15 repetitions (N = 15) of Fe/Pt deposition and annealing at 300 ℃ for 1 hour. Careful structural analysis of the films was carried out by x-ray diffraction and high-resolution transmission electron microscopy. These results will assist in the development of the low temperature L1?-FePt deposition process, which will be essential for future extremely high density magnetic recording media.

Electrical Characteristics of Magnetic Tunnel Junctions with Different Cu-Phthalocyanine Barrier Thicknesses

V-I characteristics of Fe(100)/MgO(100)/Cu-phthalocyanine (CuPc)/Co hybrid magnetic tunnel junctions were investigated at different temperatures. Fe(100) and Co ferromagnetic layers were separated by an organic-inorganic hybrid barrier consisting of different thickness of CuPc thin film grown on a 2 nm thick epitaxial MgO(100) layer. As the CuPc thickness increases from 0 to 10 nm, a bistable switching behavior due to strong charging effects was observed, while a very large magenetoresistance was shown at 77 K for the junctions without the CuPc barrier. This switching behavior decreases with the increase in temperature, and finally disappears beyond 240 K. In this work, high-potential future applications of the MgO(100)/CuPc bilayer were discussed for hybrid spintronic devices as well as polymer random access memories (PoRAMs).

Electronic and Structural Properties of Interfaces in Fe∖MgO∖Cu-Phthalocyanine Hybrid Structures

The influence of insertion of an ultra-thin Cu-Phthalocyanine (CuPc) between MgO barrier and ferromagnetic layer in magnetic tunnel juctions (MTJs) was investigated. In order to understand the relation between the electronic and structural properties of FeMgOCuPc, the surface (or interface) analysis was carried out systematically by using spin polarized metastable He de-excited spectroscopy for the CuPc films grown on the Si(001)5 nm MgO(001)7 nm Fe(001)1.6 nm MgO(001) multilayer structure as the thickness of CuPc increases from 0 to 5 nm. In particular, for the 1.6 nm CuPc surface, a rather strong spin asymmetry between up- and down-spin band appears while it becomes weaker or disappears for the CuPc films thinner or thicker than ~1.6 nm. Our results emphasize the importance of the interfacial electronic properties of organic layers in the spin transport of the hybrid MTJs.

Interfacial Properties in Cu-phthalocyanine-based Hybrid Inorganic/Organic Multilayers

Interfacial properties of 5 nm MgO(001)/7 nm Fe(001)/1.8 nm MgO(001)/t nm Cu-phthalocyanine (CuPc) hybrid multilayers with t = 0, 1, 7, and 10 were investigated by using x-ray photoemission spectroscopy (XPS). Rather sharp interfacial properties were observed in the CuPc films grown on an epitaxial MgO/Fe/MgO(001) trilayer than a MgO/Fe(001) bilayer. This work suggests a new way to improve device performance of organic spintronic devices by utilizing an artificially grown MgO(001) thin layer.

Perpendicular magnetic anisotropy and spin reorientation transition in L10 FePt films

We investigated the thickness and composition dependence of perpendicular magnetic anisotropy (PMA) in L10 Fe1−xPtx (x = 0.4, 0.5, and 0.55) films. The FePt films with different thicknesses of 35 and 70 A were grown at the substrate temperature Ts = 300 °C by molecular beam epitaxy coevaporation technique. A (001)-oriented epitaxial L10 FePt film was grown on the thin (001)-oriented fcc Pt layer, while a poorly crystallized FePt film was formed on the (111)-textured Pt layer. Our results showed that, at a fixed thickness of 70 A, the PMA of FePt alloy films is enhanced as Pt content increases from 40% to 55%.

Interface Engineering in Superconducting Ultra-thin Film of Ga

Spin polarized tunneling studies were carried out with Al-Ga bilayer as a spin detector, by Meservey-Tedrow technique. The superconductor (SC)/Insulator (I)/Ferromagnet (FM) tunnel junctions were provided by ultra high vacuum molecular beam epitaxy (UHV-MBE) technique. The analysis of interfacial properties in the Al-Ga bilayer was also carried out by Auger electron spectroscopy. It was observed that the superconducting transition temperature and energy gap were raised in comparison with that of bulk Ga and pure ultrathin Al films. Current studies clearly show how one can modify the material properties at the interface just with a few monolayers.