Yu-Hsin Su

Planarization, Fabrication, and Characterization of Three-Dimensional Magnetic Field Sensors

Nanomagnetism deals with magnetic phenomena in nanoscale structures, involving processes at the atomic level. Magnetic sensors, which exhibit the surprising giant magnetoresistance (GMR) effect, are some of the first real applications of nanotechnology, and have become very important in the last two decades. In addition, high-performance magnetoresistance (MR) measurement is a critical technique in modern electrical applications, including electronic compasses, aviation navigation, motion tracking, noncontact current sensing, rotation sensing, and vehicle detection. Both GMR and tunneling magnetoresistance (TMR) sensors have been used in the state-of-art electronic compasses. A new planar design layout of a vector magnetometer is proposed in this report. It can sense variations in three-dimensional (3-D) magnetic fields. The planarization of a vector magnetometer is carried out with consideration of materials, magnetic schematics, as well as transducer circuit designs. The optimization of an advanced magnetic material for use in GMR and TMR sensors and its planarization in a 3-D design are crucial practical issues. This paper presents an overview of the planarization of vector magnetometers and the development of its applications. It focuses on recent works, covers an analytic model of magnetoresistive sensors, and methods of thin film fabrication. It also addresses the planar vector magnetometer with a flux-guide, the chopping technique, and techniques for microfabrication of substrates. Planarization in magnetic sensors will become increasingly exploited as nanomagnetism grows in importance.

Schwarz–Christoffel Transformation for Cladding Conducting Lines

We present an application of the conformal mapping methods to determine the magnetic field from the magnetic clad line. Making use of the Schwarz-Christoffel (S-C) transformation, we derive the expression for the magnetic field in such configuration, conveniently used to study the spatial feature of the magnetic field. The calculated results show that there is a threshold ratio for appearance of the plateau and the effect of the finite poles and structure parameters on the field distribution. Finally, an optimal geometry for the magnetic clad line is qualitatively concluded.

Competition between spin–orbit interaction and exchange coupling within a honeycomb lattice ribbon

Spin density patterns of a pinned magnetic impurity that is embedded in a honeycomb lattice with zigzag edges are investigated by employing a mean-field assisted Landauer–Keldysh formalism. Both the intrinsic spin–orbit coupling and the extrinsic localized magnetic moments are considered, and the effects of the pinning directions and the species of the sublattice on the electron spins are analyzed. A local time-reversal symmetry breaking cannot destroy the equilibrium edge-state spin accumulation that is induced by intrinsic spin–orbit coupling when the pinning field lies in the plane of the ribbon and the embedding position is the A-site at the edge. The induced local spin can be either parallel or antiparallel to the localized impurity moment, depending on the location of the pinned impurity, because itinerant electrons are found only at the B-site of the edge, but not at the A-site.

Spin flip of a single anisotropic magnetic impurity in four-terminal Landauer setup with Rashba spin-orbit coupling

The mean-field-assisted Landauer–Keldysh formalism is employed to study the flip of an impurity spin under a uniaxial anisotropic field in two-dimensional electron gas with Rashba spin-orbit coupling. The spin-flip process with uniaxial anisotropic axis set to three different directions is investigated in a four-terminal Landauer setup. We show that the spin flip follows a three-dimensional trajectory rather than in-plane motion. As bias voltage changes sign, the impurity spin will flip from one saturated state to another and move along a different trajectory, depending on the chosen initial saturated state.

Magnetoresistance fluctuations in a weak disorder indium nitride nanowire

We report measurements of magnetoresistance (MR) fluctuations in a weak disorder indium nitride nanowire. The MR fluctuations are reproducible, aperiodic and symmetric in magnetic field but are asymmetric upon reversal of bias direction. The fluctuations are analysed for both perpendicular and parallel external magnetic field configurations in the light of tunnel magnetoresistance at low field and impurity scattering at higher field. The asymmetry in bias reversal is caused by breakdown of time reversal symmetry.

Coulomb blockade behavior in an indium nitride nanowire with disordered surface states

We present electron transport phenomena in a single electron transistor based on an individual indium nitride nanowire. Meticulous Coulomb oscillations are observed at low temperatures. While the device shows single period Coulomb oscillation at high temperatures or at high bias voltages, additional satellite peaks along with the main Coulomb peak appear at low temperatures and low bias voltages. The quasiperiodic structure is attributed to the mixing of dissimilar Coulomb oscillations arising from two serially coupled islands embedded inadvertently in the surface metallic states of the nanowire. The proposed model is numerically simulated with good agreement with the experimental data.

Non-equilibrium study of spin wave interference in systems with both Rashba and Dresselhaus (001) spin-orbit coupling

We study the electron spin transport in two dimensional electron gas (2DEG) system with both Rashba and Dresselhaus (001) spin-orbital coupling (SOC). We assume spatial behavior of spin precession in the non-equilibrium transport regime, and study also quantum interference induced by non-Abelian spin-orbit gauge field. The method we adopt in this article is the non-equilibrium Greens function within a tight binding framework. We consider one ferromagnetic lead which injects spin polarized electron to a system with equal strength of Rashba and Dresselhaus (001) SOC, and we observe the persistent spin helix property. We also consider two ferromagnetic leads injecting spin polarized electrons into a pure Dresselhaus SOC system, and we observe the resultant spin wave interference pattern.

Spin stability and magnetic screening of a magnetic impurity in four-terminal Landauer setup with Rashba spin-orbit coupling

The mean-field-assisted Landauer-Keldysh formalism is employed to study the orientation of a magnetic impurity embedded in two-dimensional electron gas with Rashba spin-orbit coupling. Interesting physics arises about the stability of the impurity spin. The spin of Rashba electrons interacting with magnetic impurity spin has two steady states, while the stable or unstable state depends on the local charge current and the exchange coupling strength between the local impurity spin and itinerant electrons. Furthermore, the stable steady state of impurity spin gives rise to an interesting phenomenon, magnetic screening effect in a 2DEG system.

Magnetic-field and temperature dependence of the energy gap in InN nanobelt

We present tunneling measurements on an InN nanobelt which shows signatures of superconductivity. Superconducting transition takes place at temperature of 1.3K and the critical magnetic field is measured to be about 5.5kGs. The energy gap extrapolated to absolute temperature is about 110μeV. As the magnetic field is decreased to cross the critical magnetic field, the device shows a huge zero-bias magnetoresistance ratio of about 400%. This is attributed to the suppression of quasiparticle subgap tunneling in the presence of superconductivity. The measured magnetic-field and temperature dependence of the superconducting gap agree well with the reported dependences for conventional metallic superconductors.

Enhanced spin Hall accumulation with two charge current vortices in the Landauer setup with four terminals

We investigate the enhancement of intrinsic spin Hall accumulation in a two-dimensional electron gas with Rashba and Dresselhaus (001) spin-orbit coupling. Employing the real-space Landauer–Keldysh Green’s function formalism, we show that intrinsic spin Hall accumulation can be strongly enhanced at the specific energy point where the system develops two charge current vortices with certain chirality. We find that the localized vortex state is accompanied by a conductance dip arising from the Fano–Rashba effect. The spin-orbit force picture is used to analyze the enhanced spin accumulation patterns.