Tzay-Ming Hong

Enhancement of ferromagnetic properties in Zn1−xCoxO by additional Cu doping

The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.

Simple mechanism for a positive exchange bias

In recent experiments 1 by Schuller et al. a ferromagnetic ~FM! film is cooled on a compensated antiferromagnetic ~AF! surface under large cooling field, and the hysteresis loop is observed to shift along the positive side of the field axis. This phenomena belongs to the general category of exchange anisotropy, discovered more than 40 years ago by Meiklejohn and Bean. 2 However, different from the original observation and later theories, 2‐4 a compensated ~i.e., no net magnetization! surface was used and the sign of the bias was unexpected. The compensated part 1,5 is resolved by a recent micromagnetic calculation by Koon, 6 but the sign remains only a speculation. 7 Without knowledge of the detail structure at the interface or confirmation of the existence of AF domains, we try to build a simple intuitive theory. It not only can explain the main features of the phenomena, but also gives reasonable quantitative estimations. The quantum aspect of the interface coupling is analyzed in the second half of the paper. Experimentally the exchange bias HE decreases with in

Transport in quantum wells in the presence of interface roughness

The effective Hamiltonian for two-dimensional quantum wells with rough interfaces is formally derived. Two terms are generated. The first term is identified with local energy-level fluctuations, and was introduced phenomenologically in the literature for interface roughness scattering, however, is now shown to be valid only for an infinite potential well or Hamiltonians with one single length scale. The other term is shown to modulate the wave function and cause fluctuations in the charge density. This will further reduce the electron mobility to a magnitude that is close to the experimental result.

Spontaneous Emergence of Ordered Phases in Crumpled Sheets

X-ray tomography is performed to acquire 3D images of crumpled aluminum foils. We develop an algorithm to trace out the labyrinthian paths in the three perpendicular cross sections of the data matrices. The tangent-tangent correlation function along each path is found to decay exponentially with an effective persistence length that shortens as the crumpled ball becomes more compact. In the meantime, we observed ordered domains near the crust, similar to the lamellae phase mixed by the amorphous portion in lyotropic liquid crystals. The size and density of these domains grow with further compaction, and their orientation favors either perpendicular or parallel to the radial direction. Ordering is also identified near the core with an arbitrary orientation, exemplary of the spontaneous symmetry breaking.

Particle-wave duality in quantum tunneling of a bright soliton

One of the most fundamental difference between classical and quantum mechanics is observed in the particle tunneling through a localized potential: the former predicts a discontinuous transmission coefficient (T) as a function in incident velocity between one (complete penetration) and zero (complete reflection); while in the latter T always changes smoothly with a wave nature. Here we report a systematic study of the quantum tunneling property for a bright soliton, which behaves as a classical particle (wave) in the limit of small (large) incident velocity. In the intermediate regime, the classical and quantum properties are combined via a finite (but not full) discontinuity in the tunneling transmission coefficient. We demonstrate that the formation of a localized bound state is essential to describe such inelastic collisions, showing a nontrivial nonlinear effect on the quantum transportation of a bright soliton.

X-ray tomography of a crumpled plastoelastic thin sheet.

Three-dimensional x-ray tomography is performed to investigate the internal structure and its evolution of a crumpled aluminum foil ball. The upper and lower bounds of the internal geometric fractal dimension are determined, which increase with the compression. Contrary to the simulation results, we find that the mass distribution changes from being inhomogeneous to uniform. Corroborated with the evidence from previous experiments, these findings support the physical picture that the elastic property precedes the plastic one at dominating the deformation and mechanical response for all crumpled structures. We show that the interior of a crumpled ball at the plastic regime can be mapped to the compact packing of a granular system.

Crumpling under an ambient pressure.

A pressure chamber is designed to study the crumpling process under an ambient force. The compression force and its resulting radius for the ball obey a power law with an exponent that is independent of the thickness and initial size of the sheet. However, the exponent is found to be material dependent and less than the universal value, 0.25, claimed by the previous simulations. The power-law behavior disappears at high pressure when the compressibility drops discontinuously, which is suggestive of a locked state.

Field effects on electron conduction through self-assembled monolayers

The electronic conduction through the self-assembled monolayer (SAM) can be modulated by the electric potential applied to the silicon gate electrode surrounding the SAM. The dependence of the current through SAM on the gate voltage can be explained that the renormalized molecular energy levels are swept through the window between the Fermi levels of the source and drain electrodes. The effects of the lowest unoccupied molecular orbital and a hybrid energy level near the Fermi level in the transmission spectrum can be identified.

Long-range exchange bias through a metal spacer

Abstract We have theoretically analyzed the long-range exchange bias between a ferromagnet and an antiferromagnet separated by a nonmagnetic metal spacer. The Fermi–Dirac distribution was included in the Ruderman–Kittel–Kasuya–Yosida interaction to study the thermal effects of the conduction electrons, and thus to study the temperature effects and thickness dependence in the trilayer structure. The experimentally observed oscillatory exchange bias through the metal spacer is in good agreement with the calculated results.

Uniaxial to unidirectional transition of perpendicular interlayer coupling in IrMn/CoFe/NiFeO/CoFe quadrilayers

We studied the interlayer coupling in the quadrilayer consisting of IrMn∕CoFe (bottom layer)/NiFeOx∕CoFe (top layer). An in-plane perpendicular interlayer coupling is observed between CoFe layers at room temperature. An anisotropy transition from uniaxial to unidirectional in a perpendicular direction is observed around Tt=55K. The nano-oxide layer NiFeOx shows no distinguishable ferromagnetic signal in the high-temperature (uniaxial) phase, while a strong signal appeared in the low-temperature (unidirectional) phase. A possible two-component scenario, in which the nano-oxide layer may contain both amorphous short-range antiferromagnetic domains and superparamagneitc clusters, is proposed to explain the phase transition.