Józef Barnaś

Bio: Józef Barnaś is an academic researcher from Adam Mickiewicz University in Poznań. The author has contributed to research in topics: Spin polarization & Quantum dot. The author has an hindex of 39, co-authored 303 publications receiving 6170 citations. Previous affiliations of Józef Barnaś include Polish Academy of Sciences & Katholieke Universiteit Leuven.

Theory of giant magnetoresistance effects in magnetic layered structures with antiferromagnetic coupling.

Abstract: We present a simple theoretical description of recently measured giant magnetoresistance effects in Fe/Cr layered structures. The resistivity is calculated by solving the Boltzmann transport equation with spin-dependent scattering at the interfaces. The magnitude of the effect depends on the ratio of the layer thickness to the mean free path and on the asymmetry in scattering for spin-up and spin-down electrons. Good agreement with experiment is found for both sandwich structures and superlattices.

Kondo effect in quantum dots coupled to ferromagnetic leads.

Abstract: We study the Kondo effect in a quantum dot coupled to ferromagnetic leads and analyze its properties as a function of the spin polarization of the leads. Based on a scaling approach, we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation of motion technique, we study nonlinear transport through the dot. For parallel alignment, the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.

Thermoelectric effects in transport through quantum dots attached to ferromagnetic leads with noncollinear magnetic moments

Abstract: Charge transport accompanied by heat transfer through a single-level quantum dot coupled to ferromagnetic leads with noncollinear magnetic moments is studied theoretically in the linear and nonlinear transport regimes. Calculations performed in the framework of nonequilibrium Green's function formalism and the equation of motion method reveal a significant influence of Coulomb blockade on thermal transport processes. The thermopower $S$ and thermal efficiency described by the figure of merit $ZT$ depend on magnetic configuration of the system. Two physically different situations are considered; one appears when spin accumulation is excluded and the second one when spin accumulation is relevant. In the latter case we also calculate the corresponding spin thermopower. Apart from this, magnetothermopower is introduced and discussed.

Magnetoresistance Oscillations due to Charging Effects in Double Ferromagnetic Tunnel Junctions

Abstract: Electron tunneling in a double junction consisting of two ferromagnetic electrodes, with a small ferromagnetic metallic grain in between, is analyzed theoretically in the Coulomb blockade regime. A new phenomenon, that of oscillations in tunneling magnetoresistance due to discrete charging effects, is predicted. The corresponding oscillation period depends on the charging energy, and the oscillations disappear when both junctions have the same spin asymmetry. The interplay of the oscillations and a nonoscillatory voltage dependence of the magnetoresistance is also analyzed.

Large enhancement of thermoelectric effects in a double quantum dot system due to interference and Coulomb correlation phenomena

Abstract: Thermoelectric effects in a double quantum dot system coupled to external magnetic/nonmagnetic leads are investigated theoretically. The basic thermoelectric transport characteristics, like thermopower, electronic contribution to heat conductance, and the corresponding figure of merit, have been calculated in terms of the linear response theory and Green function formalism in the Hartree-Fock approximation for Coulomb interactions. An enhancement of the thermal efficiency (figure of merit $ZT$) due to Coulomb blockade has been found. The magnitude of $ZT$ is further considerably enhanced by quantum interference effects. Both the Coulomb correlations and interference effects lead to strong violation of the Wiedemann-Franz law. The influence of spin-dependent transport and spin bias on the thermoelectric effects (especially on Seebeck and spin Seebeck effects) is also analyzed.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Molecular spintronics using single-molecule magnets

Abstract: A revolution in electronics is in view, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. A fundamental link between these two fields can be established using molecular magnetic materials and, in particular, single-molecule magnets. Here, we review the first progress in the resulting field, molecular spintronics, which will enable the manipulation of spin and charges in electronic devices containing one or more molecules. We discuss the advantages over more conventional materials, and the potential applications in information storage and processing. We also outline current challenges in the field, and propose convenient schemes to overcome them.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Spin Hall effects

Abstract: In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.