Spin-dependent transport for a two-dimensional electron gas with magnetic barriers

The spin-dependent conductance and magnetoresistance ratio (MRR) for a semiconductor heterostructures consisting of two magnetic barriers with different height and space have been investigated by the transfer-matrix method. It is shown that the splitting of the conductance for parallel and antiparallel magnetization configurations results in tremendous spin-dependent MRR, and the maximal MRRs reach 5300\% and 3800\% for the magnetic barrier spaces W=81.3 and 243.9~nm, respectively. The obtained spin-filtering transport property of nanostructures with magnetic barriers may be useful to magnetic-barrier-based spintronics.     

The magnetoresistance effect in a nanostructure with the periodic magnetic barriers

The magnetoresistance (MR) effect is theoretically investigated in a periodic magnetically modulated nanostructure, which can be realized experimentally by depositing periodic parallel ferromagnetic strips on the top of a heterostructure. We find that there exists a significant conductance difference for electrons through the parallel (P) and antiparallel (AP) magnetization configurations, which results in a considerable magnetoresistance effect. We also find that the magnetoresistance effect depends not only on the temperature but also on the number of the periodic magnetic barriers.     

Magnetoresistance effect in a hybrid ferromagnetic/semiconductor nanostructure

We propose a Magnetoresistance device in a magnetically modulated two-dimensional electron gas, which can be realized experimentally by the deposition of two parallel ferromagnetic strips on the top and bottom of a semiconductor heterostructure. It is shown that there exists a significant transmission difference for electrons through the parallel and antiparallel magnetization configurations of such a device, which leads to a considerable magnetoresistance effect. It is also shown that the magnetoresistance ratio of the device depends greatly on the magnetic strength difference in the two delta barriers of the system.     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

Switching magnetization of a nanoscale ferromagnetic particle using nonlocal spin injection

We have performed nonlocal spin injection into a nanoscale ferromagnetic particle configured in a lateral spin-valve structure to switch its magnetization only by spin current. The nonlocal spin injection aligns the magnetization of the particle parallel to the magnetization of the spin injector. The spin current responsible for switching is estimated from the experiment to be about 200 microA, which is reasonable compared with the values obtained for conventional pillar structures. Interestingly, the switching always occurs from antiparallel to parallel in the particle-injector magnetic configurations, where no opposite switching is observed. Possible reasons for this discrepancy are discussed.     

Spin-Dependent Transport and Densities of States in Non-collinear Magnetic Barriers

The spin-dependent transport properties in the non-collinear pattern of series of δ-magnetic barriers are studied by using scattering theory and Green＇s function methods. The Green＇s function is obtained by using distorted wave approach and the scattering matrix is related by Fisher-Lee relationship. In addition to reproducing the results of Papp＇s and Xu＇s in parallel and antiparallel configurations, we also obtain further results, where arbitrary orientations of the magnetic barriers and arbitrary number of barriers are included. The main finding of our results is that the signs of polarizations can be switched around some ＂geometric unpolarized windows＂. The well-known antiparallel configuration has no such characteristics. Furthermore, we discuss spin-related partial densities of states in both polarized and unpolarized structures.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Temperature-dependent asymmetry of the nonlocal spin-injection resistance: evidence for spin nonconserving interface scattering

We report nonlocal spin injection and detection experiments on mesoscopic Co-Al2O3-Cu spin valves. We have observed a temperature-dependent asymmetry in the nonlocal resistance between parallel and antiparallel configurations of the magnetic injector and detector. This strongly supports the existence of a nonequilibrium resistance that depends on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal providing evidence for increasing interface spin scattering with temperature.     

Shot noise in magnetic field modulated graphene superlattice

We investigate the shot noise properties in a monolayer graphene superlattice modulated by N parallel ferromagnets deposited on a dielectric layer. It is found that for the antiparallel magnetization configuration or when magnetic field is zero the new Dirac-like point appears in graphene superlattice. The transport is almost forbidden at this new Dirac-like point, and the Fano factor reaches its maximum value 1/3. In the parallel magnetization configuration as the number of magnetic barriers increases, the shot noise increases. In this case, the transmission can be blocked by the magnetic–electric barrier and the Fano factor approaches 1, which is dramatically distinguishable from that in antiparallel alignment. The results may be helpful to control the electron transport in graphene-based electronic devices.     

Magnetization-orientation dependence of the superconducting transition temperature in the ferromagnet-superconductor-ferromagnet system: CuNi/Nb/CuNi

The superconducting critical temperature (T(c)) of ferromagnet-superconductor-ferromagnet systems has been predicted to exhibit a dependence on the magnetization orientation of the ferromagnetic layers such that T(AP)(c)>T(P)(c) for parallel (P) and antiparallel (AP) configurations of the two ferromagnetic layers. We have grown CuNi/Nb/CuNi films via magnetron sputtering and confirmed the theoretical prediction by measuring the resistance of the system as a function of temperature and magnetic field. We find an approximately 25% resistance drop occurs near T(c) in Cu0.47Ni0.53(5 nm)/Nb(18)/CuNi(5) when the two CuNi layers change their magnetization directions from parallel to antiparallel, whereas there is no corresponding resistance change in the normal state.     

Giant magnetoresistance effect in a magnetic barrier nanostructure

The giant magnetoresistance (MR) effect is theoretically studied in a magnetically modulated two-dimensional electron gas. We find that the significant transmission difference for electron tunneling through parallel and antiparallel magnetization configurations results in a considerable MR effect. We also find that the MR ratio strongly depends on the magnetic strength and the distance between the left edges of two ferromagnetic strips as well as the temperature.     

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene.In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance(TMR) effect.The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.     

Optical properties of a magneto-donor in a quantum dot

The temperature-dependent magnetoresistance effect is investigated in a magnetically modulated two-dimensional (2D) electron gas (2DEG) which can be realized by depositing two parallel ferromagnets on top of a 2DEG electron gas. In the resonant tunnelling regime the transmission for the parallel and antiparallel magnetization configurations shows a quite distinct dependence on the longitudinal wave vector of the incident electrons. This leads to a very large magneto resistance ratio with a strong temperature dependence.     

Spin-polarized transport properties of a pyridinium-based molecular spintronics device

By applying a first-principles approach based on non-equilibrium Green's functions combined with density functional theory, the transport properties of a pyridinium-based “radical-π-radical” molecular spintronics device are investigated. The obvious negative differential resistance (NDR) and spin current polarization (SCP) effect, and abnormal magnetoresistance (MR) are obtained. Orbital reconstruction is responsible for novel transport properties such as that the MR increases with bias and then decreases and that the NDR being present for both parallel and antiparallel magnetization configurations, which may have future applications in the field of molecular spintronics.     

Giant magnetoresistance effect in nanostructures consisting of magnetic–electric barriers

The GMR effect in magnetic–electric barrier nanostructure, which can be realized experimentally by depositing two parallel metallic ferromagnetic strips with an applied voltage on the top of heterostructure, is investigated theoretically. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly dependent upon the applied voltage to metallic ferromagnetic strips in nanosystems, thus may leading to voltage-tunable GMR devices.     

Magnetotransport in a zigzag monolayer MoS2 nanoribbon with ferromagnetic electrodes

In this research, using the three-band tight-binding model combined with a non-equilibrium Green's function technique, spin-dependent electron transport was investigated in the quantum structure of zigzag monolayer molybdenum disulfide with ferromagnetic electrodes. It was shown that in parallel configurations, the conductance exhibited a quantized oscillating phenomenon, while in the antiparallel configurations with increasing magnetization, the conductance showed a zero platform in a large-energy region. We observed a giant magnetoresistance effect. Moreover, the length of the central part of the structure had a certain influence on the magnetoresistance ratio. It was found that as the length of the middle region increased, the magnetoresistance ratio decreased gradually. The results not only extended our understanding of novel electronic structures of monolayer MoS₂ but also provided the possibility for the technological applications of spintronics device.     

Transport Through a Precessing Spin Coupled to Noncollinearly Polarized Ferromagnetic Leads

The quantum electronic transport through a precessing magnetic spin coupled to noncollinearly polarized ferromagnetic leads （F-MS-F） has been studied in this paper. The nonequilibrium Green function approach is used to calculate local density of states （LDOS） and current in the presence of external bias. The characters of LDOS and the electronic current are obtained. The tunneling current is investigated for different precessing angle and different configurations of the magnetization of the leads. The investigation revea/s that when the precessing angle takes θ 〈 π /2 and negative bias is applied, the resonant tunneling current appears, otherwise, it appears when positive bias is applied. When the leads are totally polarized and the precessing angel takes O, the tunneling current changes with the configuration of two leads; and it becomes zero when the two leads are antiparallel.     

Phase diagrams for the precession states of the nanoparticle magnetization in a rotating magnetic field

Using the analytical and numerical solutions of the Landau–Lifshitz equation, we calculate the phase diagrams for the precession states of the nanoparticle magnetization in a rotating magnetic field. We show that there are three different scenarios for the magnetization switching. The bias magnetic field applied antiparallel to the nanoparticle magnetization strongly decreases the switching amplitudes and frequencies of the rotating field.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Exchange bias in FeNi/FeMn/Gd–Co trilayers: The role of the magnetic prehistory

FeNi/FeMn/Gd_xCo_100-x multilayered films were prepared by magnetron sputtering. The Gd–Co layer had different temperature dependences of the spontaneous magnetization due to the different Gd content. The magnetic properties of the films were determined from hysteresis loops measured in the temperature range 5–473 K. In order to determine the existence of a long-range interaction and a mutual influence of two exchange bias systems through the formation of bulk continuous magnetic structure in the antiferromagnetic layer special cooling procedure with FeNi and Gd–Co magnetizations saturated in a direction parallel or antiparallel to each other was used. The observed difference in the exchange bias between the two cooling configurations was discussed.