J. Barnas

Exchange interaction of magnetic impurities in graphene

The indirect exchange interaction between magnetic impurities localized in a graphene plane is considered theoretically, with the influence of intrinsic spin-orbit interaction taken into account. Such an interaction gives rise to an energy gap at the Fermi level, which makes the usual RKKY model not applicable. The results show that the effective indirect exchange interaction is described by a range function which decays exponentially with the distance between magnetic moments. The interaction is also shown to depend on whether the two localized moments belong to the same sublattice or are located in two different sublattices.

Spin Hall effect in a system of Dirac fermions in the honeycomb lattice with intrinsic and Rashba spin-orbit interaction

We consider spin Hall effect in a system of massless Dirac fermions in a graphene lattice. Two types of spin-orbit interaction, pertinent to the graphene lattice, are taken into account - the intrinsic and Rashba terms. Assuming perfect crystal lattice, we calculate the topological contribution to spin Hall conductivity. When both interactions are present, their interplay is shown to lead to some peculiarities in the dependence of spin Hall conductivity on the Fermi level.

Electron tunnelling in a double ferromagnetic junction with a magnetic dot as a spacer

Electron tunnelling in a double-barrier junction with ferromagnetic external electrodes and a magnetic quantum dot as a spacer is analysed theoretically in the framework of the nonequilibrium Green function technique. The considerations are restricted to spin-conserving tunnelling processes through a quantum dot with a single spin-split discrete level. The Coulomb correlations on the dot are taken into account in terms of the Hubbard Hamiltonian. Transport characteristics, including tunnelling magnetoresistance due to rotation of the magnetic moments of external electrodes, are calculated selfconsistently.

Tunable Conductance of Magnetic Nanowires with Structured Domain Walls

We show that in a magnetic nanowire with double magnetic domain walls, quantum interference results in spin-split quasistationary states localized mainly between the domain walls. Spin-flip-assisted transmission through the domain structure increases strongly when these size-quantized states are tuned on resonance with the Fermi energy, e.g., upon varying the distance between the domain walls which results in resonance-type peaks of the wire conductance. This novel phenomenon is shown to be utilizable to manipulate the spin density in the domain vicinity. The domain wall parameters are readily controllable, and the predicted effect is hence exploitable in spintronic devices.

Surface/interface roughness effects on magneto-electrical properties of thin films

Abstract We investigate roughness effects on electrical conductivity and giant magnetoresistance (GMR) in thin films with self-affine roughness, characterized by the r.m.s. roughness amplitude w , the in-plane correlation length ξ , and the roughness exponent H . It is shown that dynamic roughening (evolution of the roughness parameters) plays an important role in electrical conductivity of growing films, and consequently also in the GMR. In both cases our theoretical predictions are in qualitative agreement with experimental observations.

Underscreened Kondo effect in S=1 magnetic quantum dots: Exchange, anisotropy, and temperature effects

We present a theoretical analysis of the effects of uniaxial magnetic anisotropy and contact-induced exchange field on the underscreened Kondo effect in S=1 magnetic quantum dots coupled to ferromagnetic leads. First, by using the second-order perturbation theory we show that the coupling to spin-polarized electrode results in an effective exchange field

Shot noise and tunnel magnetoresistance in multilevel quantum dots : Effects of cotunneling

B_{\rm eff}

Backhopping effect in magnetic tunnel junctions: Comparison between theory and experiment

and an effective magnetic anisotropy

Spin Waves in a Bilayer with Biquadratic Interlayer Coupling

D_{\rm eff}

Magnetization Dynamics in a Magnetic Tunnel Junction Due to Spin Transfer Torque in the Presence of Interlayer Exchange Coupling

. Second, we confirm these findings by using the numerical renormalization group method, which is employed to study the dependence of the quantum dot spectral functions, as well as quantum dot spin, on various parameters of the system. We show that the underscreened Kondo effect is generally suppressed due to the presence of effective exchange field and can be restored by tuning the anisotropy constant, when