S. G. Tan

Gauge fields in spintronics

We present an overview of gauge fields in spintronics, focusing on their origin and physical consequences. Important topics, such as the Berry gauge field associated with adiabatic quantum evolution as well as gauge fields arising from other non-adiabatic considerations, are discussed. We examine the appearance and effects of gauge fields across three spaces, namely real-space, momentum-space, and time, taking on a largely semiclassical approach. We seize the opportunity to study other “spin-like” systems, including graphene, topological insulators, magnonics, and photonics, which emphasize the ubiquity and importance of gauge fields. We aim to provide an intuitive and pedagogical insight into the role played by gauge fields in spin transport.

Spin transverse separation in a two-dimensional electron-gas using an external magnetic field with a topological chirality

We propose a two-dimensional electron gas (2DEG) system in which an external magnetic (B) field with a small chirality is applied to provide a topological U(1) U(1) gauge field that separates conduction electrons of opposite spins in the transverse direction. Additionally, the vertical electric (E) field in the 2DEG, together with spin-orbit coupling, produces a SU(2) gauge field which reinforces / opposes the effect of the topological gauge. The system thus provides a tunable spin Hall effect, where an applied gate voltage on the 2DEG can be used to modulate the transverse spin current. As this method leads to the enhancement or cancellation of intrinsic spin Hall, it naturally distinguishes the extrinsic from the intrinsic effect.

MR Enhancement in a Current Perpendicular-to-Plane Spin Valve by Insertion of a Ferromagnetic Layer Within the Spacer Layer

In this paper, we propose an alternative method to improve magnetoresistance by inserting a thin ferromagnetic (FM) layer into the nonmagnetic (NM) spacer of a basic spin-valve (SV) trilayer (FM1-NM-FM1), thus creating a penta-layer SV structure (FM1-NM-FM2-NM-FM1). We investigated the effect of increasing the resistivity (rhoF2) of the FM2 on overall magnetoresistance (MR). We performed both analytical and numerical studies on the MR of the current perpendicular-to-plane (CPP) structure using the phenomenological spin drift-diffusion models. For finite rhoF2 , the MR profile is dependent on the intrinsic conductance polarization (alphaF2) of FM2. We found that inserting FM2 enhances MR when alphaF2 exceeds a critical value of alpha2C. It is found that MR can be doubled by inserting a FM layer with high alphaF2, such as the half-metallic Cr 2O. We have numerically calculated MRmax and the corresponding rhoF20 values for different alphaF2 values. Finally, we studied the effect of spin relaxation on the MR of the CPP SV

High magnetoresistance in graphene nanoribbon heterojunction

We show a large magnetoresistance (MR) effect in a graphene heterostructure consisting of a metallic and semiconductor-type armchair-graphene-nanoribbon. In the heterostructure, the transmission across the first subband of the semiconducting armchair-graphene-nanoribbon and metallic armchair-graphene-nanoribbon is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and electron energy. Finally, we design a magnetic-field-effect-transistor which yields a MR of close to 100% (85%) at low (room) temperature.

Simulation for a shingled magnetic recording disk

Shingled magnetic recording (SMR) is a new technology based on partially overlapping, or shingling, adjacent tracks, which is capable of increasing the density of a disk drive up to 10

Klein tunneling in graphene systems under the influence of magnetic field

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Topological state transport in topological insulators under the influence of hexagonal warping and exchange coupling to in-plane magnetizations

. To write data on overlapping tracks may erase data being written previously. Therefore, without any data translation layer to redirect updates to different locations to avoid unintended data erasures, the SMR disk cannot be used as a conventional disk to perform unrestricted reads/writes directly. Various data translation/mapping strategies have been proposed to allow the disk to perform unrestricted reads/writes. However, these strategies have tradeoffs in terms of system performance and cost, such as disk space utilization. Nevertheless, system performance varies for different applications and under different workload conditions.

Magnetoresistance in Ferromagnetically Coupled Three-Dimensional Topological Insulator Strips

We study Klein tunneling across a PN junction in monolayer graphene (MLG) and the AB-bilayer graphene (BLG) under the effect of a perpendicular magnetic-field (B-field). In the Klein tunneling process, normally incident electrons in MLG (BLG) are fully transmissive (reflective) upon hitting the junction barrier. When a finite B-field is applied, transmission of the normally incident electrons is suppressed in MLG over an energy range around the PN barrier height, effectively opening an energy gap. This suppression may be attributed to the magnetic deflection arising from Lorentz force, which shifts the transmission profile of the device in the transverse direction.

Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer

A hexagonal warping term has been proposed recently to explain the experimentally observed 2D equal energy contours of the surface states of the topological insulator Bi2Te3. Differing from the Dirac fermion Hamiltonian, the hexagonal warping term leads to the opening up of a band gap by an in-plane magnetization. We study the transmission between two Bi2Te3 segments subjected to different in-plane magnetizations and potentials. The opening up of a bandgap, and the accompanying displacement and distortion of the constant energy surfaces from their usual circular shapes by the in-plane magnetizations, modify the transverse momentum overlap between the two Bi2Te3 segments, and strongly modulate the transmission profile. The strong dependence of the TI surface state transport of Bi2Te3 on the magnetization orientation of an adjacent ferromagnetic layer may potentially be utilized in, e.g., a memory readout application.

Optical Faraday rotation with graphene

In this work, we calculate the eigenstates and dispersion relations for the 2-D surface states of 3-D topological insulator (TI) planar nanostrips of finite width coupled to a proximate insulating ferromagnetic layer. The magnetization of the ferromagnetic layer in the transverse, longitudinal and out-of-plane directions have differing effects on the dispersion relations. We also calculated the transmission through a short central TI strip sandwiched between TI source and drain leads of semi-infinite length. We investigate the transport through the setup when the magnetization of the ferromagnetic layer on top of the central region differs from that of the leads. We found that when the lead magnetization is in the longitudinal direction, i.e., along the current direction, the transmission is maximal for both the parallel and anti-parallel alignment of the lead and central magnetizations. However, for the case where the lead magnetization is in the transverse direction, the transmission is maximal for parallel alignment and minimal for the antiparallel alignment.