Perpendicular And Parallel Field Magnetoresistance In Molecular Beam Epitaxy Grown Bi2Te3

textThe topological insulator Bi2Te3 has been grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. Reflection high energy electron diffraction, in situ scanning tunnelling microscopy, x-ray photoelectron spectroscopy and ex situ x-ray diffraction studies have been performed to analyze the quality of the growth. These analyses suggest a very good layer-by-layer epitaxial growth of Bi2Te3 on the atomically at Si surface. The magnetoresistance of the samples has been studied with magnetic field perpendicular and parallel to the sample surface, up to 9 T, over a temperature range of 2 K to 20 K. A sharp dip at low fields (0 T - 1 T) and near-linear behavior for high fields (> 4 T) have been observed in the perpendicular field magnetoresistance. The low field dip is due to weak antilocalization that agrees well with the simplified Hikami-Larkin-Nagaoka model. It has been demonstrated that both the low field dip and the high field near-linear behavior can be explained by the original Hikami-Larkin-Nagaoka formula alone in a system with strong spin-orbit coupling. From the fitting of the perpendicular field magnetoresistance the phase coherence length, the mean free path and the spin-orbit relaxation time have been estimated. The phase coherence length shows power law dependence with temperature indicating two dimensional nature of the transport. The power law also suggests electron electron interaction as the prominent dephasing mechanism. The out-of-plane spin-orbit relaxation time is determined to be small and the in-plane spin-orbit relaxation time is found to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters is useful for topological insulator based magneto electric device applications. It also has been shown that the strong spin-orbit coupling suppresses the Zeeman contribution in perpendicular field magnetoresistance. The logarithmic divergence of perpendicular field magnetoresistance with temperature for low temperature range (2 K - 20 K) at high fields shows the presence of Coulomb interaction in the spin singlet channel. For magnetoresistance with the field parallel to the sample surface, the observed magnetoresistance has parabolic dependence for small fields (0 T - 0.6 T) and logarithmic dependence for large fields (> 3 T), which is due to the Zeeman effect. It is found that the data are inconsistent with only the Maekawa and Fukuyama theory of non interacting electrons with Zeeman contributions to the transport, but are consistent with theory if one also takes into account the electron electron interaction and the Zeeman splitting term in the electron electron interaction theory of Lee and Ramakrishnan. The Zeeman g-factor and the strength of Coulomb scattering due to electron electron interaction have been estimated from fitting of the parallel field magnetoresistance. The magnetoresistance also shows anisotropy with respect to the field directions. The angle dependent anisotropic magnetoresistance can be fitted well by the original HLN theory alone. The anisotropy can have potential application in anisotropic magnetic sensors.Electrical and Computer Engineerin

Theoretical and experimental studies on topological insulators and topological insulator based spintronic devices

Three dimensional (3D) topological insulators (TIs) are unique materials with insulating bulk and two dimensional (2D) metallic surface states having spin-momentum locked Dirac-band dispersion. The remarkable property of spin-momentum locking of the 2D surface states provides an opportunity for manipulating the coupled spin and charge degrees of freedom of electrons on the surface of a 3D TI by controlling one or the other. The charge current-induced spin polarization of the 2D surface states of a 3D TI and subsequent diffusion or tunneling of spin current in an adjacent material, or conversion of spin current to charge current on the surface of a 3D TI, are a few among many effects of this spin-momentum locking, which renders TIs as promising candidates for spintronic applications. In this dissertation, we provide a theoretical description of the electronic transport of the TI surface states in proximity to a non-magnetic (NM) or a ferromagnetic (FM) material, and derive the transport equations based on quantum kinetic equation of non-equilibrium Green’s function. The transport equations are solved for appropriate boundary conditions to obtain the efficiency of the spin current-to-charge current conversion in TI/NM/FM or TI/FM heterostructure, or to calculate the efficiency of the detection of charge current induced spin polarization on the surface of a TI with FM tunnel contacts. We find that these efficiencies strongly depend on the tunnel conductance of the interface and decreases with increasing tunnel conductance, implying the necessity of design optimization of the tunnel interface in actual devices. Here, we also describe low-temperature magnetotransport measurements on an epitaxial Bi2Se3 thin film, and identify the contribution of the surface states and the quasi-2D bulk states to the transport from localization and interaction effects. We present two-terminal resistance measurement with FM and NM contacts on the surface of epitaxial and exfoliated Bi2Te3 films, and find change of resistance with reversal of the FM magnetization direction. We also measure magnetic hysteresis properties of sputtered Bi2Te3-Fe heterostructure and obtain enhancement of coercive field of Fe in the heterostructure, which could be due to strong spin-orbit coupling proximity effect arising from the Bi2Te3 film.Electrical and Computer Engineerin

Room Temperature Ferroelectricity, Ferromagnetism, and Anomalous Hall Effect in Half-metallic Monolayer CrTe

Two-dimensional materials hosting ferroelectricity and ferromagnetism are crucial for low-power and high-speed information processing technologies. However, intrinsic 2D multiferroics in the monolayer limit are rare. Here, we demonstrate that monolayer CrTe, obtained by cleaving the [002] surface, is dynamically stable multiferroic at temperatures beyond room temperature. We show that it orders ferromagnetically with significant in-plane magnetocrystalline anisotropy, and it is a half-metal featuring a large half-metal gap. Remarkably, the broken inversion symmetry and buckled geometry of monolayer CrTe make it a ferroelectric with a large spontaneous out-of-plane polarization and significant magnetoelectric coupling. In addition, we demonstrate polarization or electric field-induced tunability of the anomalous Hall effect, accompanied by substantial bandstructure modulation. Our findings establish monolayer CrTe as a room-temperature multiferroic with great potential for applications in spintronics and ferroelectric devices.Comment: 9 double column pages, and 5 figures; A significantly updated version predicting monolayer CrTe to be an intrinsic multiferroic at room temperatur