Arne Brataas

Finite-element theory of transport in ferromagnet-normal metal systems

We formulate a theory of spin dependent transport in an electronic circuit involving ferromagnetic elements with noncollinear magnetization which is based on the conservation of spin and charge current. The theory considerably simplifies the calculation of the transport properties of complicated ferromagnet-normal metal systems. We illustrate the theory by considering a novel three-terminal device.

Negative Domain Wall Resistance in Ferromagnets

The electrical resistance of a diffusive ferromagnet with magnetic domain walls is studied theoretically, taking into account the spatial dependence of the magnetization. The semiclassical domain wall resistance is found to be either negative or positive depending on the difference between the spin-dependent scattering life-times. The predictions can be tested experimentally by transport studies in doped ferromagnets.

Ballistic and diffuse transport through a ferromagnetic domain wall

We study transport through ballistic and diffuse ferromagnetic domain walls in a two-band Stoner model with a rotating magnetization direction. For a ballistic domain wall, the change in the conductance due to the domain wall scattering is obtained from an adiabatic approximation valid when the length of the domain wall is much longer than the Fermi wavelength. In diffuse systems, the change in the resistivity is calculated using a diagrammatic technique to the lowest order in the domain-wall scattering and taking into account spin dependent scattering lifetimes and screening of the domain-wall potential.

Conductance modulation by spin precession in noncollinear ferromagnet normal-metal ferromagnet systems

conductance of the system in the presence of a magnetic field can be asymmetric with respect to time reversal. The total conductance changes nonmonotonically with the magnetic field strength for different magnetic configurations. This modulation of the conductance is due to the precession of the spin accumulation in the normal metal. The difference between the conductance of the parallel and antiparallel configurations can be either positive or negative as a function of the applied magnetic field. This effect should be best observable on Al single crystals attached to ferromagnetic electrodes by means of tunnel junctions or metallic contacts.

Large magnetoresistance ratio in ferromagnetic single-electron transistors in the strong tunneling regime

We study transport through a ferromagnetic single-electron transistor. The resistance is represented as a path integral, so that systems where the tunnel resistances are smaller than the quantum resistance can be investigated. Beyond the low order sequential tunneling and co-tunneling regimes, a large magnetoresistance ratio at sufficiently low temperatures is found. In the opposite limit, when the thermal energy is larger than the charging energy, the magnetoresistance ratio is only slightly enhanced.

Scattering theory of perpendicular transport in metallic multilayers (invited)

Electronic transport in metallic multilayers is discussed in the language of the Landauer–Buttiker scattering formalism. The semiclassical conductance through a disordered interface can be unambiguously separated into specular and diffuse scattering contributions. Analytical results are derived for the perpendicular conductance of multiple disordered interfaces. Predictions for the transport properties of interfaces with dilute but strongly scattering defects should be accessible to verification by experiments. First results of first‐principles calculations of ballistic transport in magnetic multilayers are presented.

Temperature dependence of tunnel conductance in ferromagnetic double barrier junctions

The spin-dependent conductance and magnetoresistance (MR) in ferromagnetic double junctions are studied theoretically, taking into account discrete energy levels and a finite spin relaxation time. The basic results are applied to hybrid tunnel junctions, in which nano-scale grains are embedded in the tunnel barrier. With increasing size quantization and spin relaxation time the MR is found to be enhanced at low temperatures.

Semiclassical scattering theory of parallel transport in metallic magnetic multilayers

Abstract The parallel (CIP) conductance of magnetic multilayers is calculated in the scattering formalism of transport. Geometric and spin-independent (‘parasitic’) CIP conductance channels are identified in the parallel configuration. The dominance of these channels in bulk systems is an artifact of the semiclassical method employed.

Perpendicular transport through rough interfaces in the metallic regime

Abstract Perpendicular transport through an interface in the metallic regime is considered. The semi-classical theory presented is based on the Landauer-Buttiker formalism taking into account an effective mass mismatch at the interface and a non-zero average of random scattering potentials. The transmission probability for a given mode is found in terms of the effective mass and the conduction band bottom to the left and to the right of the interface, the Fermi energy, the self-energy of the electron, and the transverse wave vector of the electron. The diffuse and specular contributions to the interface roughness scattering are shown to be equally important in the weak scattering limit. Predictions for the transport properties of interfaces with a low concentration of strongly scattering defects should be accessible to verification by experiments. The theory is applied to the spin-valve effect in magnetic multilayers.