Jung Mee Park

STRUCTURE, VERTICAL ELECTRON-DETACHMENT ENERGY, AND O-H STRETCHING FREQUENCIES OF E+(H2O)12

The first comprehensive ab initio study is performed on an excess electron bound to the water dodecamer to find out if this wet electron can be regarded as a precursor of the fully solvated electron. Various structures of the wet electron are explored using ab initio calculations. Among a number of possible geometries categorized as unbounded, surface, internal, and partially internal excess-electron states, the lowest-energy conformer is predicted to be a structure of a partially internal state. The predicted vertical electron-detachment energy of this structure is in good agreement with the experimental value of Coe et al. [J. Chem. Phys. 92, 3980 (1990)]. This indicates that in the experiment the partially internal excess-electron state structure would have been detected. The electronic structure, interactions between the excess electron and dangling H atoms (e⋯ H interaction), and red-shifts of the O–H stretching frequencies with strong IR intensities are discussed.

Nature of the interaction of paramagnetic atoms (A=4N,4P,3O,3S) with π systems and C60: A theoretical investigation of A⋅⋅⋅C6H6 and endohedral fullerenes A@C60

The nature of the interaction of paramagnetic atoms A (=4N,4P,3O,3S) with π systems and C60 in the A⋅⋅⋅C6H6 complexes and endohedral fullerenes (A@C60) has been investigated employing second-order Moller–Plesset perturbation level of theory and density functional theory calculations. The coupled cluster theory with single and double excitations, and with perturbed triplet excitations have also been carried out for the A⋅⋅⋅C6H6 complexes. The calculated geometries indicate that the paramagnetic N and P atoms lie on the C6 axis of benzene in the A⋅⋅⋅C6H6 complex and at the center of the C60 cage in A@C60. On the other hand, the O and S atoms are slightly shifted towards the C–C bond of benzene in the A⋅⋅⋅C6H6 complex. A comparison of the calculated binding energies (BEs) of these paramagnetic complexes and the corresponding rare gas complexes like He⋅⋅⋅C6H6 and He@C60 indicate that the BEs of the former are much larger than those of the latter. For both the rare gas and paramagnetic atom complexes dispersiv...

Non-resonant and resonant X-ray scattering studies on multiferroic TbMn2O5

Comprehensive x-ray scattering studies, including resonant scattering at Mn L, Tb L, and M edges, were performed on single crystals of TbMn2O5 for crystallographic data to elucidate the nature of its commensurate and incommensurate phases. The scattering results provide direct evidence of symmetry lowering to the ferroelectric phase driven by magnetically induced lattice modulations and show the presence of multiple magnetic orders. The competing orders under spin-frustrated geometry are believed to cause discommensuration and result in the commensurate-to-incommensurate phase transition around 24 K. It is proposed that the low temperature incommensurate phase consists of commensurate domains separated by antiphase domain walls which change both signs of spontaneous polarizations and x-ray scattering amplitudes for forbidden reflections.

Magnetic tunnel junctions with a tunnel barrier formed by N2O plasma

We investigated a magnetic tunnel junction (MTJ) with a tunnel barrier formed by N2O plasma. Compared with a MTJ with a tunnel barrier formed by conventional O2 plasma, the MTJ fabricated with N2O plasma shows much lower specific junction resistance and a comparably high tunneling magnetoresistance ratio. In particular, it was found that N2O plasma oxidation is quite important in the junction with a thin tunnel barrier, for which O2 plasma cannot be used. From x-ray photoelectron spectroscopy, we observed that N2O plasma oxidation leads to the slight nitridation of the Al2O3 layer and significantly reduces the oxidation of the bottom electrode, especially for a thin tunnel barrier. Thus, we conclude that the use of N2O plasma in forming the tunnel barrier is effective for achieving a low junction resistance and for minimizing the oxidation of the bottom electrode during plasma oxidation in MTJs.

Dependence of magnetic tunnel junction's reliability on oxidation condition

Time-dependent dielectric breakdown measurements under constant voltage stress were carried out for magnetic tunnel junctions (MTJs), prepared by different oxidation techniques. Insulating barriers in MTJs were fabricated by oxidation of a predeposited Al layer with different oxidation techniques, such as conventional O2 plasma, off-axis O2 plasma, Ar-mixed O2 plasma, and off-axis Ar-mixed O2 plasma oxidations. The time to breakdown (TBD) of the MTJs was estimated from Weibull failure distribution plot. The estimated TBD when 63% of the junctions formed by conventional O2 plasma oxidation failed was estimated to be about 55 s. The TBD increased up to about 250, 750, and 2,500 s for the junctions of off-axis, Ar-mixed, and off-axis Ar-mixed O2 plasma oxidation, respectively. We believe that the enhanced reliability of MTJs is due to the reduction of plasma damage during oxidation process, resulting in low electron trap density in the Al2O3 tunneling barrier. Low-frequency 1/f noise power density was measur...