Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

Can spin-dependent scattering mimic spin-dependent tunneling in heterogeneous ferromagnets? The discussion is triggered by the surprising similarity in magnetoresistance properties of granular ferromagnets with metallic, insulating or point-contact spacers.     

Spin-dependent transport in organic-ferromagnets

Based on the modified Su-Schrieffer-Heeger model and the non-equilibrium Green's function current formula, the spin polarization of the ferromagnet-electrode connected organic ferromagnet is theoretically studied. The spin polarization can be suppressed by atomic dimerization and be driven by an applied electric field. We investigate the spin polarization from the viewpoint of energy competitions in different interactions under the electric field. In addition, the ferromagnetic electrodes significantly enhance the spin polarization.     

Spin-dependent valence-band density of states of 3d ferromagnets

Spin-resolved X-ray photoelectron spectroscopy (SRXPS) and high resolution X-ray photoelectron spectroscopy (HRXPS) studies of the valence bands of ferromagnetic Fe, Co, Co₆₆Fe₄Ni₁B₁₄Si₁₅ and Ni are reported. The SRXPS and HRXPS spectra are compared with theoretical densities of states (DOS) that are corrected for photoelectric cross section variations within the valence band. Agreement between theory and experiment is very good for ferromagnetic Fe and Co₆₆Fe₄Ni₁B₁₄Si₁₅. For Co metal, experiment agrees poorly with theory incorporating a 1.5 eV exchange splitting. Agreement is improved if a reduced Co exchange splitting of 1.2 eV is adopted theoretically. The reduced exchange splitting is attributed to valence electron correlation in Co metal. Ferromagnetic Ni shows poor agreement between experiment and theory. The SRXPS Ni spectra demonstrate that most of the disagreement concerns the ↑-spin channel.     

Magneto-transport properties of dilute granular ferromagnets

We present magnetoresistance (MR) measurements performed on quench-condensed granular Ni thin films which are on the verge of electric continuity. In these systems, the electric conductivity is believed to be governed by the resistance between a very small number of grains. The films exhibit sharp resistance jumps as a function of magnetic field. We interpret these findings as being the result of magneto-mechanical distortions that occur in single grains which act as bottlenecks in the dilute percolation network. The observed features provide a unique measure of magnetostriction effects in nano-grain structures as well as being able to shed light on some of the properties of regular granular magnetic films.     

Spin-dependent transport in molecular tunnel junctions

We present measurements of magnetic tunnel junctions made using a self-assembled-monolayer molecular barrier. Ni-octanethiol-Ni samples were fabricated in a nanopore geometry. The devices exhibit significant changes in resistance as the angle between the magnetic moments in the two electrodes is varied, demonstrating that low-energy electrons can traverse the molecular barrier while remaining spin polarized. An analysis of the voltage and temperature dependence of the data suggests that the spin-polarized transport signals can be degraded by localized states in the molecular barriers.     

Electron transport in nanogranular ferromagnets

We study electronic transport properties of ferromagnetic nanoparticle arrays and nanodomain materials near the Curie temperature in the limit of weak coupling between the grains. We calculate the conductivity in the Ohmic and non-Ohmic regimes and estimate the magnetoresistance jump in the resistivity at the transition temperature. The results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays and a certain class of manganites, where localization effects within the clusters can be neglected.     

Spin-dependent transport through magnetic nanojunctions

Coherent electronic transport through a molecular device is studied using non-equilibrium Green's function (NEGF) formalism. Such device is made of atomic nanowire which is connected to ferromagnetic electrodes. The molecule itself is described with the help of Hubbard model (Coulomb interactions are treated by means of the Hartree-Fock approximation), while the coupling to the electrodes is modeled through the use of a broad-band theory. It was shown that magnetoresistance varies periodically with increasing length of the atomic wire (in the linear response regime) and oscillates with increasing bias voltage (in the nonlinear response regime). Since the TMR effect for analyzed structures is predicted to be large (tens of percent), these junctions seem to be suitable for application as magnetoresistive elements in future electronic circuits.     

Spin-dependent electron transport in manganite bicrystal junctions

Magnetic bicrystal films and junctions of magnetic La_0.67Sr_0.33MnO₃ (LSMO) and La_0.67Ca_0.33MnO₃ (LCMO) films epitaxially grown on NdGaO₃ substrates with the (110) planes of their two parts misoriented (tilted) at angles of 12°, 22°, 28°, and 38° are investigated. For comparison, bicrystal boundaries with a 90° misorientation of the axes of the NdGaO₃ (110) planes were fabricated. The directions of the axes and the magnetic anisotropy constants of the films on both sides of the boundary are determined by two independent techniques of magnetic resonance spectroscopy. The magnetic misorientation of the axes in the substrate plane has been found to be much smaller than the crystallographic misorientation for tilted bicrystal boundaries, while the crystallographic and magnetic misorientation angles coincide for boundaries with rotation of the axes. An increase in the magnetoresistance and characteristic resistance of bicrystal junctions with increasing misorientation angle was observed experimentally. The magnetoresistance of bicrystal junctions has been calculated by taking into account the uniaxial anisotropy, which has allowed the contributions from the tunneling and anisotropic magnetoresistances to be separated. The largest tunneling magnetoresistance was observed on LCMO bicrystal junctions, in which the characteristic resistance of the boundary is higher than that in LSMO boundaries.     

Spin-dependent quasiparticle transport in aluminum single-electron transistors

We investigate the effect of Zeeman splitting on quasiparticle transport in normal-superconducting-normal (NSN) aluminum single-electron transistors (SETs). In the above-gap transport, the interplay of Coulomb blockade and Zeeman splitting leads to spin-dependence of the sequential tunneling. This creates regimes where either one or both spin species can tunnel onto or off the island. At lower biases, spin-dependence of the single quasiparticle state is studied, and operation of the device as a bipolar spin filter is suggested.     

Peculiarities of the electronic transport in Co2CrAl and Co2CrGa half-metallic ferromagnets

We present results on the transport properties of the half-metallic ferromagnetic Heusler alloys Co₂CrAl and Co₂CrGa in the temperature range from 4 to 900 K. The peculiarities of the resistivity and the absolute differential thermoelectric power are considered within a two-current model of conductivity, taking into account the energy gap at the Fermi level in the electronic spectrum of alloys for electrons with spin opposite to the direction of the magnetization vector.     

Visualizing spin-dependent electronic collisions In ferromagnets

This work demonstrates experimentally and theoretically that the coincident two-electron emission from a ferromagnetic surface, upon the impact of a polarized electron, carries detailed information on the spin-dependent electronic collisions in ferromagnets. The analysis of the calculated and the measured two-electron spectra reveals the potential of the electron-pair emission technique for the study of (a) surface magnetism and (b) spin-dependent electron scattering dynamics in ferromagnets.     

Spin-dependent transport in trilayer junctions of doped manganites

Trilayer junctions of doped manganites provide a model system for the study of spin-polarized transport across a half metallic interface. Novel phenomena observed in these systems include a large low-field magnetoresistance (MR), with nearly an order of magnitude change in junction resistance in 100 Oe at 14 K, and a strong temperature and bias dependence of the junction MR. They also enabled the first demonstration of a spin-angular momentum transfer-induced reversal of a magnetic moment. Systems containing both manganite and transition metal ferromagnetic electrodes demonstrate a complex interface structure between the perovskite oxide such as SrTiO₃ and the transition metal electrode such as Co or Fe. It reveals the important effect this interface has on the magnetotransport property of the junction.     

Giant magnetoresistance of granular microwires: Spin-dependent scattering in integranular spacers

The anomalous behavior of magnetoresistance has been revealed in a number of granular microwires. In contrast to the giant magnetoresistance of granular alloys, which is associated with the spin-dependent scattering in the bulk of grains and at their surface, is linear in the square of the magnetization, and decreases with an increase in temperature, the magnetoresistance, for example, in Co₁₀Cu₉₀ microwires is negative, increases with an increase in temperature below the Curie temperature, and does not reach saturation in the field dependence in the high-field range. A simple mechanism of negative giant magnetoresistance due to scattering of spin-polarized charge carriers by impurity magnetic moments localized in the nonmagnetic intergranular spacers has been proposed taking into account that a considerable part of magnetic ions in microwires exhibiting this behavior is dissolved in the intergranular spacers. It has been shown that the corresponding contribution to magnetoresistance can reach 10–20%.     

Spin-dependent transport in a nanopillar non-local spin valve

We investigate the injection of a pure spin current into a non-magnetic Cu wire in a lateral spin valve. We detect the spin accumulation occurring at the interfaces between the magnetic nanopillars and the non-magnetic wire in the non-local geometry. We confirm that the accumulated spins diffuse equally in the Cu wire irrespective of the presence of a charge current. The inversion of the injector and detector magnetic nanopillars does not affect the spin signal, in agreement with analytical predictions for this system.     

Spin-dependent transport in a magnetic two-dimensional electron gas

Magneto-transport and magneto-optical probes are used to interrogate spin-dependent transport in magnetic heterostructures wherein a two dimensional electron gas (2DEG) is exchange-coupled to local moments. At low temperatures, the significant s–d exchange-enhanced spin splitting in these “magnetic” 2DEGs is responsible for the observation of unusual transport properties such as a complete spin polarization of the gas at large Landau level filling factors and a pronounced, non-monotonic background magneto-resistance. Magneto-transport measurements of gated samples performed in a parallel field geometry are used to systematically study the variation of the magneto-resistance with sheet concentration, yielding new insights into the dependence of spin transport on the Fermi energy of the majority spin carriers.     

Spin-dependent thermoelectric transport through double quantum dots

We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green’s function in the linear response regime.It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling,the relative magnetic configurations,and the spin polarization of leads.In particular,the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantum dots and the leads for the spin-down electrons are small.Moreover,the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures.In the presence of the magnetic field,the spin accumulation in the leads strongly suppresses the thermoelectric efficiency,and a pure spin thermopower can be obtained.     

Spin-dependent tunneling in Co/insulating granular systems: an AC approach

The spin-dependent tunneling of cobalt clusters embedded in Al₂O₃ or SiO_x has been analyzed as a function of the frequency at room temperature. Two sets of samples, with one or several layers of clusters, have been produced by alternate physical deposition of the metal and the insulator. The impedance versus frequency curves were measured with and without an external magnetic field. The results suggest that when the distance between successive cluster layers is small, some correlations between the cluster positions are present.     

Spin-dependent negative differential conductance in transport through single-molecule magnets

Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance (NDC) emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.     

Phenomenological theory of spin transport in molecular-field ferromagnets

The effects of electron-lattice collisions on the nonuniform transverse susceptibility of an itinerant ferromagnet are found from the quantum-statistical equations of motion in the molecularfield approximation. The effects of pure orbital collisions and the corresponding “diffusion” correction are found to be negligible, in agreement with the conclusions of P. Fulde and A. Luther. The effect of magnetic (spin-orbital) collisions is included in the Gilbert damping and does not contribute to the “diffusion” correction. The form of the susceptibility and of the corresponding macroscopic equation of motion, and also the contradictions between this and a number of previous studies are discussed in detail.     

Spin-dependent transport through an interacting quantum dot system

Using an equation of motion technique, we report on a theoretical analysis of transport characteristics of a spin-valve system formed by a quantum dot coupled to ferromagnetic leads, whose magnetic moments are oriented at an angle θ with respect to each other, and a mesoscopic ring by the Anderson Hamiltonian. We analyse the density of states of this system, and our results reveal that the density of states show some noticeable characteristics depending on the relative angle θ of magnetic moment M, and the spin-polarised strength P in ferromagnetic leads, and also the magnetic flux Φ and the number of lattice sites N_R in the mesoscopic ring. These effects might have some potential applications in spintronics.