Spin Polarized transport

Top PDF Spin Polarized transport:

Spin polarized transport through single molecule magnet Mn6 complexes

Spin polarized transport through single molecule magnet Mn6 complexes

height and the width of such resonances depend on the elec- tronic coupling between the particular molecular level and the electrodes and it is ultimately determined by the symmetry of the given level. In this case, however, the Fermi level of the electrodes lies well within the HOMO–LUMO gap, meaning that the transport is tunnelling in nature. As a consequence the transmission at the Fermi level, which determines the low-bias conductance, depends on the tail of the transmission reso- nances closer to E F . We then nd that T(E F ) for the alpha Fig. 2 Projected density of states (PDOS) and zero-bias transmission spectra, T ( E ), of the Au(111) – Mn 6 – Au(111) junction with the molecule in the S ¼ 12 spin state. Red
Show more

7 Read more

Spin Polarized Transport and Spin Seebeck Effect in Triple Quantum Dots with Spin Dependent Interdot Couplings

Spin Polarized Transport and Spin Seebeck Effect in Triple Quantum Dots with Spin Dependent Interdot Couplings

We study the spin-dependent electronic and thermoelectric transport through a structure composed of triple quantum dots (TQDs) coupled to two metallic leads in the presence of spin-dependent interdot couplings, which is reliable by applying a static magnetic field on the tunnel junctions between different dots. When the TQDs are serially connected, a 100% spin-polarized conductance and thermopower emerge even for very small spin-polarization of the interdot coupling as the dots are weakly coupled to each other. Whereas if the TQDs are connected in a ring shape, the Fano antiresonance will result in sharp peaks in the conductance and thermopower. In the presence of spin-dependent interdot couplings, the peaks of the spin-up and spin-down thermopowers will shift to opposite directions in the dot level regime, resulting large either 100% spin-polarized or pure spin thermopowers. The latter generally arises at low temperatures and is robust against the level detuning, the dot-lead coupling, and the system equilibrium temperature.
Show more

8 Read more

A Two-Band Tight-Binding Model of Spin-Polarized Transport in Magnetic Tunnel Junctions

A Two-Band Tight-Binding Model of Spin-Polarized Transport in Magnetic Tunnel Junctions

For the last twenty years, the intensive investigation of giant magnetoresistance (GMR) in magnetic multilayers and tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJ) has offered a rather comprehensive view of these phenomena. Various methods and models were pro- posed to treat spin-dependent transport in magnetic multi- layers. Semiclassical approaches based on Boltzmann equ- ation account for spin-dependent scattering in the bulk of magnetic layer via spin-dependent mean free path [1, 2]. Quantum theories of spin-dependent transport in magnetic multilayers [3-6] take into account the quantization of elec- tron momentum and the scattering on the potential barriers between successive layers. These theories give the descrip- tion of ballistic as well as diffusive spin-dependent transport in magnetic multilayers. Among the quantum models of spin-dependent transport in the multilayers, the free-electron as well as tight-binding models were used. Model calcula- tions [7, 8] predicted huge increase of TMR in double (and more) barrier structure due to resonant tunneling. The fitting of first principle calculation of band parameters was used to get quantitative description of specific systems.
Show more

10 Read more

Spin polarized localization in a magnetized chain

Spin polarized localization in a magnetized chain

We investigate a simple tight-binding Hamiltonian to understand the stability of spin-polarized transport of states with an arbitrary spin content in the presence of disorder. the general spin state is made to pass through a linear chain of magnetic atoms, and the localization lengths are computed. Depending on the value of spin, the chain of magnetic atoms unravels a hidden transverse dimensionality that can be exploited to engineer energy regimes where only a selected spin state is allowed to retain large localization lengths. We carry out a numerical anmalysis to understand the roles played by the spin projections in different energy regimes of the spectrum. For this purpose, we introduce a new measure, dubbed spin-resolved localization length. We study uncorrelated disorder in the potential profile offered by the magnetic substrate or in the orientations of the magnetic moments concerning a given direction in space. Our results show that the spin filtering effect is robust against weak disorder and hence the proposed system should be a good candidate model for experimental realizations of spin-selective transport devices.
Show more

12 Read more

Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Here we present results which clarify the dependence of spin- polarized transport on the Schottky barrier height. We fabri- cated 5 nm thick ferromagnetic layers (Ni Fe , Co and Fe) directly onto GaAs substrates in an ultrahigh vacuum (UHV) chamber. Conventional measurements were performed both with and without photoexcitation. A circularly polarized laser beam was then used together with an external magnetic field to investigate the spin-dependence of the photocurrent at the FM/GaAs interface at room temperature. By varying the Schottky barrier according to either the various FM materials or the doping density in the SC, the roles of photoexcitation in the SC and the FM are investigated. The photon energy dependence of the spin-dependent photocurrent is also studied and compared with that measured by photoemission experiments. For the FM, permalloy was chosen due to a large polarization difference at its Fermi level [20] and a small magneto-optical background [due to magnetic circular dichroism (MCD)], and compared with Co and Fe samples. Since the epitaxial growth of the FM transition metals on GaAs substrates has been well established [21], GaAs substrates were chosen for the present study [22].
Show more

7 Read more

Dissipation through spin Coulomb drag in electronic spin transport and optical excitations

Dissipation through spin Coulomb drag in electronic spin transport and optical excitations

Spintronics applications are receiving increasing attention in the hope of revolutionizing traditional technology by a pow- erful exploitation of the spin – as well as the charge – degrees of freedom. An intense research effort is underway to im- prove our understanding of spin dynamics, especially related to nanocircuits and their components, such as quantum wells and wires. In this context the theory of spin Coulomb drag (SCD) was recently developed [1–5]. This theory analyzes the role of Coulomb interactions between different spin popula- tions in spin-polarized transport. Coulomb interactions trans- fer momentum between different spin populations, so that the total momentum of each spin population is not preserved. This provides an intrinsic source of friction for spin currents, a measure of which is given by the spin-transresistivity [1]. SCD is generally small in metals, due to a typical Fermi tem- perature of the order of 10 5
Show more

7 Read more

Spin pumping and inelastic electron tunneling spectroscopy in topological insulators

Spin pumping and inelastic electron tunneling spectroscopy in topological insulators

The calculated populations of the various spin states are plotted as a function of bias in Fig. 3(a) for both an (11,6) and a (7,4) ribbon. An S = 1 spin in a uniaxial anisotropy field and in thermal equilibrium with an electron bath presents an equal probability to occupy the |+1 and the |−1 states, i.e., for V = 0 one has P +1 = P −1 = 1/2. As soon as the bias is increased at and above | D | /e, excitations to the | 0 state become possible due to spin-flip backscattering. In this case, however, the current is intense, so that in between two scattering events the impurity spin does not have the time to relax back to the degenerate ground state. This means that now a spin-up electron (the majority specie in the upper edge right-going channel) can also induce the transition |0 → |+1. The consequence is that the electronic current flowing at the upper edge, in virtue of its spin polarization and its intensity, produces a net flow of population between the two degenerate ground states, i.e., for V > +| D | /e one has P +1 > P −1 . In other words the impurity spin is driven by the current away from its uniaxial anisotropy axis. This can be fully appreciated by looking at Fig. 3(b), where we show the average magnetization S z =
Show more

5 Read more

Spin dynamics in highly frustrated pyrochlore magnets

Spin dynamics in highly frustrated pyrochlore magnets

of the non-Kramers Tb 3 + ions along the local 111 axes [46,47]. In spite of effective antiferromagnetic interactions leading to a Curie-Weiss temperature of −13 K [48], which should drive the system into long-range order [32,49], prior works pointed out a disordered fluctuating ground state down to 20 mK [50,51]. Various subsequent studies have suggested complex spin dynamics, where different time and temperature scales coexist, as revealed by muons [52–54], magnetization [55,56] and neutron scattering experiments [57–66]. Recently, power law spin correlations have also been reported [67], bearing some resemblance with the pinch point pattern observed in the aforementioned spin ices. The peculiar form of these elastic correlations has been confirmed by recent triple axis measurements carried out at 4F (LLB), shown in Fig. 2a [68].
Show more

7 Read more

Spin Polarized Quasiparticle Transport in Cuprate Superconductors

Spin Polarized Quasiparticle Transport in Cuprate Superconductors

There are many people who have helped me throughout the years in my graduate studies. First and foremost, I would like to thank my thesis advisor, Prof. Nai-Chang Yeh, whose guidance was instrumental in my research experiments and preparation of this thesis. Without her encouragement and patience during the rough periods in my life, I would not have been able to complete this work. I admire her dedication and work ethic, as well as her ability as a physicist to navigate through the maze in which we often find ourselves while doing research. Second, I would like to thank Dr. Richard Vasquez for supplying our group with seemingly infinite number of samples (and all those sample transport jars), as well as for giving me the opportunity (along with my advisor) to learn new techniques, to meet new people, and to expand my horizon working at JPL. I would also like to thank the members of my committee, Prof. Tombrello, Prof. Cross, and Prof. Zmuidzinas, for their time out of busy schedules.
Show more

174 Read more

Preparation of magnetic tips for spin polarized STM on Fe1+yTe

Preparation of magnetic tips for spin polarized STM on Fe1+yTe

In many unconventional superconductors, the super- conducting phase is reached from a magnetically ordered state by some external tuning parameter, such as dop- ing, pressure or chemical substitution. Superconductiv- ity emerges in close vicinity to a magnetically ordered phase[1]. This suggests an intimate relation between magnetism and superconductivity in these materials. Of- ten, the phase diagrams exhibit even regimes of coexis- tence between the two, however the important question about whether the two coexist or compete at the micro- scopic level remains unresolved. One difficulty in probing their relation at the atomic scale is that most methods employed to characterize magnetic order, such as neu- tron scattering, probe a macroscopic sample volume, ren- dering statements about local phase separation difficult. A method which has been very successful to character- ize both superconductivity and magnetism locally on an atomic scale is Scanning Tunneling Microscopy (STM). It has provided important information both about local variations in the superconducting properties and charge ordering in strongly correlated electron materials[2–4] and, using magnetic tips in spin-polarized STM, it has also been shown to allow for characterization of mag- netism at the atomic scale in nanostructures[5, 6]. Ap- plication of spin-polarized STM to strongly correlated materials has recently been demonstrated in the non- superconducting parent compound of the iron chalco- genide superconductors[7], providing real space images of the magnetic structure Fe 1+δ Te. Preparing and calibrat-
Show more

6 Read more

Polarized parton densities and spin dependent structure function of the nucleon

Polarized parton densities and spin dependent structure function of the nucleon

Abstract. We study the polarized parton distribution functions (PPDFs) from recent ex- perimental data at the next-to-leading order (NLO) approximation in the fixed-flavor number scheme. In this analysis, we can compare our results with experimental data such as very recent COMPASS data. It would be very worth to study the PPDFs parametriza- tion form when we want to use this new COMPASS data in QCD analysis on polarized proton structure and parton distribution functions.

6 Read more

Neutron spin rotation measurements

Neutron spin rotation measurements

Abstract. The neutron spin rotation (NSR) collaboration used parity-violating spin rotation of transversely polarized neutrons transmitted through a 0.5 m liquid helium target to constrain weak coupling constants between nucleons. While consistent with theoretical expectation, the upper limit set by this measurement on the rotation angle is limited by statistical uncertainties. The NSR collaboration is preparing a new measurement to improve this statistically-limited result by about an order of magnitude. In addition to using the new high-flux NG-C beam at the NIST Center for Neutron Research, the apparatus was upgraded to take advantage of the larger-area and more divergent NG-C beam. Significant improvements are also being made to the cryogenic design. Details of these improvements and readiness of the upgraded apparatus are presented. We also comment on how recent theoretical work combining effective field theory techniques with the 1 / N c
Show more

5 Read more

Ultracold collisions in metastable helium

Ultracold collisions in metastable helium

Currently we are investigating elastic and ionizing collisions in spin-polarized metastable helium in the absence of an exciting laser, where the spin-dipole interaction can induce relaxation from the initial spin-polarized state to states from which Penning and associative ionization is highly probable.

7 Read more

Colossal Proximity Effect in a Superconducting Triplet Spin Valve Based on the Half-Metallic Ferromagnet CrO2

Colossal Proximity Effect in a Superconducting Triplet Spin Valve Based on the Half-Metallic Ferromagnet CrO2

Combining superconductors ( S ) and ferromagnets ( F ) offers the opportunity to create a new class of superconducting spintronic devices. In particular, the S=F interface can be specifically engineered to convert singlet Cooper pairs to spin-polarized triplet Cooper pairs. The efficiency of this process can be studied using a so-called triplet spin valve (TSV), which is composed of two F layers and a S layer. When the magnetizations in the two F layers are not collinear, singlet pairs are drained from the S layer, and triplet generation is signaled by a decrease of the critical temperature T c . Here, we build highly efficient TSVs
Show more

6 Read more

Progress towards spin polarized scanning tunneling microscopy

Progress towards spin polarized scanning tunneling microscopy

Compound Crystal structure, symmetry Lattice constants at room temperature, A Curie temperature, K The number of magnetic sublattices Magnetic moments for different sublattices, pa Mdssb[r]

11 Read more

Circular polarization in n type resonant tunneling diodes with Si delta doping in the quantum well

Circular polarization in n type resonant tunneling diodes with Si delta doping in the quantum well

can also accumulate next to the barrier forming a two­ dimensional hole-gas (2DRG). Therefore, under light excitation resonant tunneling can also occur between the 20HG states and hole confined states in the QW. For Si 8- doped QWs, the bound state of a shallow donor is lower than the 2D sub-band. The donor assisted resonant tunneling is observed when the energy level of the 20EG in the accumulation layer is aligned with the bound state of a shallow donor in the QW. Under applied voltage and light excitation, optical recombination occurs in different regions of the structure as illustrated in figure I (a). When a magnetic field is applied parallel to current, the confined levels in the QW and the contact layers split into spin-up and spin-down Zeeman states and the optical recombination can occur with well defined selection rules, giving information about the spin polarization of the carriers in the structure.
Show more

5 Read more

Origami based spintronics in graphene

Origami based spintronics in graphene

Driven by the necessity of a bandgap and by the grow- ing interest in graphene-based spintronics, in this letter we propose a simple mechanism that not only produces a gapped electronic structure in graphene but that also spin-polarizes its current. We show that this effect arises quite simply by the combined presence of two key ingre- dients: the SOI and an externally applied magnetic field. While magnetic fields are controllable, the SOI of a mate- rial is normally constant and small in the case of carbon. Therefore, it might seem too ambitious to amplify both ingredients enough for the appearence of a possible gap. Nevertheless, recent discoveries have demonstrated that the SOI is enhanced when graphene is mechanically bent away from its planar geometry [20–23] suggesting that folding might function as a viable mechanism to induce a bandgap. In fact, here we show that folded graphene sheets in the presence of externally applied magnetic fields may display both a bandgap and spin-polarized currents. Not possible with bulk 3-dimensional structures, folding may pave the way to a whole new approach of dealing with spin electronics in 2-dimensional systems, giving rise to the so-called origami spintronics.
Show more

7 Read more

Spin polarized tunneling, magnetoresistance and interfacial effects in ferromagnetic junctions

Spin polarized tunneling, magnetoresistance and interfacial effects in ferromagnetic junctions

Cu ) ® lm thickness in the range 5± 8 A Ê , a negative JMR e€ ect and an unexpected bias voltage dependence was observed, as shown in ® gure 6 ( Moodera et al. 1999 ) , Theoretical calculations by Vedyayev et al. ( 1997 ) and Zhang and Levy ( 1998 ) had predicted oscillations of JMR in FM/NM/I/NM/FM systems as a function of the normal ( NM ) metal thickness, the interface layer behaving like a quantum well leading to the formation of quantum well states ( QWSs ) when a resonance condition was ful® lled. Also, according to calculations by Zhang and Levy, the JMR suppres- sion length in the NM layer could be as much as even 100 A Ê when it was ¯ at whereas, for a rougher FM± NM interface, the coherence was broken, thereby reducing the JMR more rapidly with increasing NM layer thickness. However, in interpreting the experimental results, one has to pay attention to the possibility of interfacial mixing of the atoms ( especially in sputtered samples, e.g. Co/Cu ) , which would yield a spurious decay length. Numerical calculations by our group for the presence of QWSs, based on a model ® rst proposed by Slonczewski ( 1989 ) , qualitatively explained the experimental features, including its bias dependence ( Moodera et al. 1999 ) . Such studies may allow one to engineer a special electrode with strong spin ® ltering, for example by choosing a FM/NM/FM trilayer electrode with a suitable NM layer thickness.
Show more

12 Read more

A Theoretical Study of Spin-orbit Coupling in Polarized Fermionic Condensate

A Theoretical Study of Spin-orbit Coupling in Polarized Fermionic Condensate

the spin-orbit coupled fermionic condensate. In this formalism, one investigated a two component atomic Fermi gas with population imbalance in the presence of Rashba type spin- orbit coupling. As a competition between SOC and population imbalance the finite temperature phase diagram reveals a large varities of new features. This includes the expanding of super fluid state regime and the shrinking of both the phase separation and the normal regimes. It was observed that for large value of SOC parameter the phase separation disappears. It gives way to the super fluid state. One also observes that the transition point moves towards a regime of low temperature, high magnetic field and high polarization as the SOC increases. In Table T1, we have shown the evaluated results of
Show more

7 Read more

Wavevector-dependent spin filtering and spin transport through magnetic barriers in graphene

Wavevector-dependent spin filtering and spin transport through magnetic barriers in graphene

In this paper we shall focus on the effects of Zeeman and exchange spin splitting and postpone the interesting problem of spin-orbit coupling to a future work. We show that the spin-resolved transmissions exhibit a strong de- pendence on the incidence angle, which allows in princi- ple for a selective transmission of spin-up and spin-down electrons. This effect can be qualitatively understood by a simple semiclassical argument. The bending of the elec- tron trajectories under the barrier depends on the energy and thus, in the presence of spin splitting, is different for the two spin projections. The magnitude of the polariza- tion that can be achieved depends on the spin splitting and can be very large in the presence of a large split- ting, as, e.g., that originating from a proximity-induced exchange field. Moreover, in a resonant double barrier configuration, the polarization can be enhanced by in- creasing the distance between the barriers. In this case, in fact, even for relatively small splitting there exists an energy range where the polarization reaches values close to one for large enough distance.
Show more

11 Read more

Show all 10000 documents...