Seng Ghee Tan - Academia.edu (original) (raw)

Papers by Seng Ghee Tan

Quantum Physics and Modern Applications

Physica B: Condensed Matter, 2009

Perpendicular-magnetized pseudo-spin-valves (PSV) boast the advantages of lower aspect ratio, gre... more Perpendicular-magnetized pseudo-spin-valves (PSV) boast the advantages of lower aspect ratio, greater magnetic stability and thermal stability; and hence constitute a promising candidate for achieving higher integration area density. In this paper, we investigate the spin transport and spin transfer torque in a Co/Cu(111)/Co perpendicular PSV structure where the magnetization of the Co layers are oriented in the out-of-plane direction, in the presence of a spin-polarized current in the perpendicular-to-plane (CPP) geometry. Both ballistic spin-dependent transmission/reflection at the two Co-Cu interfaces and diffusive spin relaxation within the Co and Cu layers are considered in our model. The ballistic calculations predict an absorbed spin current component at the Cu-free Co interface, which constitutes the main source ($90%) of the total spin transfer torque in the free Co layer. The remaining torque arises from the spin relaxation of transmitted spin current within the free Co layer. Our model predicts a lower range of the critical current density of less than 10 7 A/cm 2 for perpendicularly magnetized PSVs, while that for in-plane magnetized PSVs is of the order of several 10 7 A/cm 2. Additionally, perpendicularly magnetized PSVs also possess other practical advantages which make them promising candidates for future MRAM applications.

Journal of Applied Physics, 2005

We present a micromagnetic model which incorporates eddy current and spin transfer torque effects... more We present a micromagnetic model which incorporates eddy current and spin transfer torque effects due to the passage of a spin-polarized current in the current-perpendicular-to-plane (CPP) geometry. Eddy current effects are modeled by solving Faraday’s and Poisson’s equations self-consistently with the Landau-Lifshitz-Gilbert (LLG) equation, whereas spin transfer torque is modeled by including the Slonczewski spin torque term in the LLG equation. We consider a 50 nm cubic Co element, which forms the free layer of a pseudo-spin-valve structure. With a typical damping parameter of α=0.5, the eddy currents act to accelerate the magnetic reversal process. The spin torque effect can also assist the reversal process but at high current densities exceeding Jext=108A∕cm2 onwards. At these current densities, spin transfer torque also causes a substantial reduction in the coercivity.

IEEE Transactions on Magnetics, 2008

A model is proposed to study the spin dependent transport of a quantum dot coupled to two noncoll... more A model is proposed to study the spin dependent transport of a quantum dot coupled to two noncollinear ferromagnetic leads configuration, in the sequential tunneling regime. The angular deviation () and lead polarization () dependence of the tunneling current and tunneling magnetoresistance (TMR) are investigated for both singly occupied and doubly occupied dot, in the presence of spin flip effect. The current and TMR are found to be more sensitive to and the spin flip rate in the singly-occupied bias regime.

Applied Physics Letters, 2010

The Klein tunneling of charge pairs in an electrostatically created p-n junction of monolayer gra... more The Klein tunneling of charge pairs in an electrostatically created p-n junction of monolayer graphene is shown to occur at an observable rate for moderate fields. The pairs undergo zitterbewegung (ZBW) in opposite directions leading to their separation and transverse dipole moment, since the valleys contribute constructively. The dipole moment depends critically on the exponential collimation characteristic of Klein tunneling and serves as a diagnostic signature of ZBW.

Physical Review B, 2016

We propose a multilayer device comprising of a thin-film-based ferromagnetic hetero-structure (FM... more We propose a multilayer device comprising of a thin-film-based ferromagnetic hetero-structure (FMH) deposited on a diamond layer doped with nitrogen vacancy centers (NVC's). We find that when the NVC's are in close proximity (1-2 nm) with the FMH, the exchange energy is comparable to, and may even surpass the magnetostatic interaction energy. This calls for the need to consider and utilize both effects in magnetometry based on NVC's in diamond. As the distance between the FMH and NVC is decreased to the sub-nanometer scale, the exponential increase in the exchange energy suggests spintronic applications of NVC beyond magnetometry, such as detection of spin-Hall effect or spin currents.

Journal of Applied Physics, May 6, 2005

We propose a ballistic device model which harnesses the Dresselhaus spin-orbit coupling effect to... more We propose a ballistic device model which harnesses the Dresselhaus spin-orbit coupling effect to induce spin polarization ∣P∣ by constraining the transverse electron wave vector kx to specific, quantized values and aligning the crystalline c axis along the electron conduction path z. ∣P∣ induced in this magnetoelectronic device is further enhanced by applying a periodic system of delta magnetoelectric barriers that constitute a net “zero-A” periodic unit. Calculations for GaAs, GaSb, and InSb show a field enhancement of ∣P∣ from peak 2%–3% to 10%–80% near the conduction band. Magnetic and electric potentials have been shown to modulate ∣P∣ with few gate elements required.

Journal of Applied Physics, Apr 28, 2005

We present a device model that couples electron spin to the crystal field, external magnetic, and... more We present a device model that couples electron spin to the crystal field, external magnetic, and transverse electric field Ey, such that current jx transport in the ballistic regime is produced. As Ey modulates jx and its polarization P at a particular distance y, our model shows that the Landau spin-orbit system can implement the functions of a spin-field-effect transistor. We have found that a net P can be obtained in the z axis, thus allowing resistance detection to be performed in that axis. Further calculations show spatial variation of P across the cyclotron diameter that increases with higher Landau levels.

Journal of Superconductivity, Jul 12, 2005

ABSTRACT We propose a ballistic magneto-electric device that permits conductance modulation with ... more ABSTRACT We propose a ballistic magneto-electric device that permits conductance modulation with both electric and magnetic fields applied perpendicular to its current conduction channel. Fields are applied through the ferromagnetic gates deposited on top of a HEMT heterostructure that contains a 2DEG for current conduction. The minimal-coupling Hamiltonian with spatially uniform electrical potentials, and delta Zeeman splitting is solved in the weak-coupling limit for which the Rashba spin orbit coupling is not considered. Ballistic transmission of electrons through a periodic system of zero-gauge double-pair magnetoelectric barriers is studied. Manipulation of barriers’ geometrical symmetry and configuration leads to the conception of a spin-FET for non-volatile storage and digital logic operations. The linear modulation of electron spin polarization (|P|) is also studied for its relevance to electrical signal amplification. Perpendicular magnetization of the ferromagnetic gates permits modulation of both |P| and electron transmission (T) threshold, the latter is particularly useful for spin logic design.

Journal of Applied Physics, Jun 1, 2004

A periodic array of magnetoelectric barriers is modeled to achieve maximum spin polarization (P) ... more A periodic array of magnetoelectric barriers is modeled to achieve maximum spin polarization (P) at high transmission probability (T). Each double-pair unit of the array consists of four magnetic barriers designed in several ways, such that an electron passing through, in the Landau gauge A=(0,Ay(x),0), acquires zero gain in kinetic energy. This enables multiple double-pairs to be used to enhance P without sacrificing T. By tuning the magnetoelectric barrier heights, a high P of 75%–100% is obtained at 0.8–1.0EF, for a 27 unit array. For antisymmetrical arrays, electrical barriers act as a switch to the polarization capability.

Journal of Magnetism and Magnetic Materials, Aug 1, 2006

We present a theoretical model to study spin injection (Z) through a quantum dot system sandwiche... more We present a theoretical model to study spin injection (Z) through a quantum dot system sandwiched by two ferromagnetic contacts. The effect of contact magnetization on Z was studied using Green's function descriptions of the density of states. Green's function models have the advantages that coherent effects of temperature, electron occupation in the QD, and lead perturbation on the state wave function and hence the current can be formally included in the calculations. In addition, self-consistent treatment of current with applied electrochemical potential or lead conductivity, a necessary step which has not been considered in previous works, has also been implemented in our model.

Physical Review B, Nov 28, 2005

We model the ballistic spin current within a two-dimensional electron gas ͑2DEG͒ under the influe... more We model the ballistic spin current within a two-dimensional electron gas ͑2DEG͒ under the influence of magnetoelectric barriers and Rashba spin-orbit coupling. The magnetic field B y is applied in the 2DEG in-plane direction rather than in the perpendicular direction B z , as considered previously. The use of an in-plane field induces a spin current which is more resistant to the D'yakonov type of spin relaxation. It is shown theoretically that the electron energy dispersion is independent of the magnetic vector potential arising from in-plane fields. This enables electron conduction to maintain high conductance, even when multiple barriers are used to enhance the spin polarization of current. The polarization of current is also derived as a function of the Rashba spin-orbit coupling strength, the electric potential, and the magnetic field strength. The magnitude and direction of spin polarization can be modulated externally by a B y field, which is useful for spintronics applications.

The spin orbit coupling spin torque consists of the field-like [REF: S.G. Tan et al., arXiv:0705.... more The spin orbit coupling spin torque consists of the field-like [REF: S.G. Tan et al., arXiv:0705.3502, (2007).] and the damping-like terms [REF: H. Kurebayashi et al., Nature Nanotechnology 9, 211 (2014).] that have been widely studied for applications in magnetic memory. We focus, in this article, not on the spin orbit effect producing the above spin torques, but on its magnifying the damping constant of all field like spin torques. As first order precession leads to second order damping, the Rashba constant is naturally co-opted, producing a magnified field-like damping effect. The Landau-Liftshitz-Gilbert equations are written separately for the local magnetization and the itinerant spin, allowing the progression of magnetization to be self-consistently locked to the spin. PACS: 03.65.Vf, 73.63.-b, 73.43.-f † Correspondence author: Seng Ghee Tan Email: Tan_Seng_Ghee@dsi.a-star.edu.sg Magnified Damping under Rashba Spin Orbit Coupling November 1, 2015

Annalen der Physik, 2020

The existence of spontaneous magnetization in low dimensional magnetic systems has attracted inte... more The existence of spontaneous magnetization in low dimensional magnetic systems has attracted intensive studies since the early 60s and research remains very active even now. Only recently, magnetic van der Waals (vdW) systems down to a few layers have been broadly discussed for their magnetic order ground states at finite temperature. The naturally inherited layered structure of the vdW magnetic systems possessing onsite magnetic anisotropy from band electrons can suppress the long‐range fluctuations. This provides an excellent vehicle to study the transition of magnetism to 2D limits both theoretically and experimentally. Here the current status of 2D vdW magnetic system and its potential applications are briefly summarized and discussed.

Scientific Reports, 2018

In this work, we study the effect of introducing a periodic curvature on nanostructures, and demo... more In this work, we study the effect of introducing a periodic curvature on nanostructures, and demonstrate that the curvature can lead to a transition from a topologically trivial state to a non-trivial state. We first present the Hamiltonian for an arbitrarily curved nanostructure, and introduce a numerical scheme for calculating the bandstructure of a periodically curved nanostructure. Using this scheme, we calculate the bandstructure for a sinusoidally curved two-dimensional electron gas. We show that the curvature can lead to a partner switching reminiscent of a topological phase transition at the time reversal invariant momenta. We then study the Bernevig-Hughes-Zhang (BHZ) Hamiltonian for a two-dimensional quantum well. We show that introducing a curvature can lead to the emergence of topological surface states.

INTERMAG Asia 2005. Digests of the IEEE International Magnetics Conference, 2005., 2005

ABSTRACT A magnetic device that enables the realization of various logic functions in a single tr... more ABSTRACT A magnetic device that enables the realization of various logic functions in a single transistor is proposed. The device consists of a current-conducting 2DEG layer, and a set of four ferromagnetic (FM) gates and two non-magnetic (NM) gates deposited on top of a HEMT heterostructure. FM gates are magnetized perpendicular-to-plane so that magnetic fields (B) extend vertically through the 2DEG plane. Electrical voltages are applied to the NM gates so that a periodic series of magneto-electric barriers is established across the 2DEG conduction path. This device selects the functions of AND, OR, NAND, NOR, XOR, NOT, YES, according to the magnetizations of FM1, FM3, and the electrical potentials (U) of NM1, NM2. FM2 and FM4 function as the two inputs to the logic gate depending on the selection. Electron motion is described by the minimal coupling Hamiltonian with the Zeeman perturbation term at the delta B field region.

ABSTRACT Periodic delta magnetic barriers are applied in the plane of a III-V semiconductor 2DEG ... more ABSTRACT Periodic delta magnetic barriers are applied in the plane of a III-V semiconductor 2DEG across the electron conduction path of a HEMT device. Because of structural inversion asymmetry in the 2DEG, electron spin is coupled to both the crystal field and the in-plane magnetic field, producing highly spin-polarized current in the device. To maximize spin polarization |P|, an array of repeating zero-gauge type of magneto-electric barriers is used. Our calculation for electron transmission T for 10 repeating barrier units shows resonant-like peaks for different electron spin, resulting in ideal |P| of 100% over a wide energy range of 0.5-0.9 EF. It was also found that a perpendicular E field can also modulate |P| in the 2DEG channel.

Physical Review B, 2005

Spin dependent interfacial resistance ͑R I ͒ is crucial for achieving high spin injection efficie... more Spin dependent interfacial resistance ͑R I ͒ is crucial for achieving high spin injection efficiency from a ferromagnetic ͑FM͒ metal into a semiconductor ͑SC͒. We present a self-consistent model of spin transport across interfacial resistances at the FM-SC junctions of a FM-SC-FM trilayer structure. The SC layer consists of a highly doped n ++ AlGaAsu GaAs 2DEG while the interfacial resistance at the FM-SC junction is modeled as delta potential ͑␦͒ barriers. The self-consistent scheme consists of a ballistic model of spindependent transmission across the ␦ barriers to evaluate R I , and a drift-diffusion model to obtain the spin-split ⌬ in the electrochemical potentials. The R I values of the two junctions were found to be asymmetric despite the symmetry of the trilayer structure. This asymmetry arises from the finite biasing voltage which causes a difference in electrochemical potentials and spin accumulation at the two interfaces. The effect of R I on the spin-injection efficiency and magnetoresistance is studied over a range of ␦-barrier heights. Significant spininjection efficiency ͑50% ͒ requires high ␦-barrier heights approaching 1 eV. Even higher barrier heights are required to obtain equivalent magnetoresistive effect.

Journal of Superconductivity, 2005