Zuxin Ye

Thickness dependence of the microstructures and magnetic properties of electroplated Co nanowires.

The correlation between the crystal structure and the magnetic properties of Co nanowires of diameter 65 and 200 nm fabricated by electroplating Co into the pores of anodic aluminum oxide membranes has been investigated. Strikingly different microstructures have been observed in these Co nanowires by means of x-ray diffraction and selected area electron diffraction measurements. The 65 nm thick Co nanowires are composed of long and ordered hexagonal close-packed Co grains (>5 microm), while the 200 nm thick Co nanowires are composed of submicron-long hexagonal close-packed and face-centered cubic Co grains. Correspondingly, different magnetic properties have been observed for these Co nanowires. Magnetization measurements have found that the 65 nm thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm thick Co nanowires. Spontaneous magnetic moments of the nanowires are parallel to the nanowires in the 65 nm thick Co nanowires, but they are transverse to the nanowires in the 200 nm thick Co nanowires, as observed by the magnetic force microscopy. The correlation between their different magnetic properties and microstructures is discussed.

Observation of coherent acoustic and optical phonons in bismuth nanowires by a femtosecond pump-probe technique

Coherent acoustic and optical phonon oscillations in Bi nanowire samples were studied with a femtosecond pump-probe technique. Laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the terahertz range. The transmission signal of nanowires on a glass substrate and the signal of light scattered from freestanding nanowires were measured. The acoustic velocity in nanowires was found to be close to that of bulk polycrystalline material. The changes in the optical phonon frequency at different laser fluences were simulated taking into account excitation inhomogeneity, lattice anharmonicity, diffusion, and recombination of the carriers and gave good agreement with experimental results.

Strongly anisotropic flux pinning in superconducting Pb82Bi18 thin films covered by periodic ferromagnet stripes

We have studied the vortex pinning via their magnetic interaction with a periodic structure of parallel magnetic stripes. Superconducting Pb82Bi18 alloy films were covered by an insulating Ge layer and a periodic Ni array of magnetic stripes was fabricated on the top of the Ge layer by electron-beam lithography and thermal evaporation. The critical current density was significantly stronger when the current was applied parallel to the stripes than when the current was perpendicular to the stripes. This is attributed to the barrier to the vortex motion provided by the magnetic interaction with the magnetic stripes. The enhancement in critical current was most significant at temperatures close to the superconducting transition temperature.

Changes in the crystalline structure of electroplated Co nanowires induced by small template pore size

Co nanowires have been electroplated into porous Al2O3 templates. The crystalline structure and magnetic properties of Co nanowires of diameters 65 and 200 nm have been studied. Long single crystal Co segments with a hexagonal closed-packed (hcp) structure have been found in the 65 nm-thick nanowires. A mixture of face-centered cubic and hcp single crystal Co segments was observed in the 200 nm-thick nanowires. This finding suggests that the small template pore size favors the growth of hcp Co phase. Magnetic force microscopy measurements showed longitudinal spontaneous magnetic moments in the 65 nm-thick nanowires and transverse moments in the 200 nm-thick nanowires.

Superconducting properties of Pb82Bi18 films controlled by ferromagnetic nanowire arrays

The superconducting properties of Pb82Bi18 alloy films deposited on ferromagnetic nanowire arrays have been investigated. Ferromagnetic Co or Ni nanowires are first electroplated into the columnar pores of anodic aluminum oxide (AAO) membranes. Superconducting Pb82Bi18 films are then quench condensed onto the polished surface of the AAO membranes filled with magnetic nanowires. A strong dependence of the Pb82Bi18 superconducting properties on the ratio of the superconducting film thickness to the magnetic nanowire diameter and material variety was observed.

Long-range superconducting proximity effect in template-fabricated single-crystal nanowires

A long-range superconducting proximity effect is investigated in single-crystal nanowires of Zn, Sn, and Pb, of length up to 60 μm, electrochemically deposited into the pores of anodic aluminum oxide membranes and polycarbonate membranes. Using an in situ self-contacting method, single nanowires are electrically contacted on both ends to a pair of macroscopic film electrodes of Au, Sn, or Pb pre-fabricated on both surfaces of the membranes. Superconductivity in the nanowires is strongly suppressed when Au electrodes are used. When electrodes having higher superconducting transition temperatures are used, the nanowires become superconducting at the transition temperatures of the electrodes. Microscopy analyses of the structure and the chemical composition of the nanowires are presented, which demonstrate a sharp interface between the nanowires and the macroscopic film electrodes. Measurements of the I-V characteristics provide evidence that this proximity effect occurs along the length of the nanowires.

Dynamics of coherent acoustic and optical phonon oscillations in nanostructures studied by a femtosecond pump-probe technique

Coherent phononic oscillations in Bi and Ag nanowires were studied with a femtosecond pump-probe technique. In Bi nanowires laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the THz range. The transmission of nanowires on a glass substrate and the light scattered from free standing nanowires were measured. In Ag nanowires laser-induced acoustic oscillations at different excitation levels were studied. The observed reduction of the oscillation frequency at higher pump energy was related to a transient softening of the material. This was directly confirmed for optical phonons by experiments with a pre-pump pulse of variable energy, producing different excitation densities.

Localized phase-slip centers in proximity-induced long superconducting nanowires

Current-voltage (I‐V) curves have been measured on 60‐μm-long Zn nanowires in contact with bulk film electrodes of Sn or Pb. The Zn nanowires become superconducting at the transition temperatures of the respectively electrodes, which are well above the transition temperature of bulk Zn. At temperatures well above the transition temperature of bulk Zn but below the transition temperatures of the electrodes, the I‐V curves display a reproducible steplike pattern characteristic of the establishment of multiple localized phase-slip centers. The typical quasiparticle diffusion length associated with a phase-slip center is estimated to be 5–10μm. These results demonstrate that the observed proximity effect occurs along the entire length of a nanowire, rather than only at the nanowire-electrode interfaces.