Alvaro S. Núñez

Changing Exchange Bias in Spin Valves with an Electric Current

An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. In this paper we demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density (~10^12 A/m^2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.

Thermally-Assisted Current-Driven Domain Wall Motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall r(dw) as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero-temperature r(dw) initially varies linearly with current, even in the absence of nonadiabatic spin torques.

Influence of a uniform current on collective magnetization dynamics in a ferromagnetic metal

We discuss the influence of a uniform current j on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energye(q ) has a current-induced contribution proportional to qiJ W , where J W is the spin current, and predict that collective dynamics will be more strongly damped at finite j. We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for j*10 9 Ac m 22 . We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.

Theory of spin-charge-coupled transport in a two-dimensional electron gas with Rashba spin-orbit interactions

We use microscopic linear response theory to derive a set of equations that provide a complete description of coupled spin and charge diffusive transport in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO) interaction. These equations capture a number of interrelated effects including spin accumulation and diffusion, Dyakonov-Perel spin relaxation, magnetoelectric, and spin-galvanic effects. They can be used under very general circumstances to model transport experiments in 2DEG systems that involve either electrical or optical spin injection. We comment on the relationship between these equations and the exact spin and charge density operator equations of motion. As an example of the application of our equations, we consider a simple electrical spin injection experiment and show that a voltage will develop between two ferromagnetic contacts if a spin-polarized current is injected into a 2DEG, that depends on the relative magnetization orientation of the contacts. This voltage is present even when the separation between the contacts is larger than the spin diffusion length.

Ab initio giant magnetoresistance and current-induced torques in Cr/Au/Cr multilayers

We report on an ab-initio study of giant magnetoresistance (GMR) and current-induced-torques (CITs) in Cr/Au multilayers that is based on non-equilibrium Green’s functions and spin density functional theory. We find substantial GMR due primarily to a spin-dependent resonance centered at the Cr/Au interface and predict that the CITs are strong enough to switch the antiferromagnetic order parameter at current-densities � 100 times smaller than typical ferromagnetic metal circuit switching densities. Magnetic metals are often well described using the effective mean-field description provided by the KohnSham equations of spin-density functional theory. In this description, the Kohn-Sham quasiparticles experience exchange-correlation potentials with a spin dependence that is comparable in strength to band widths and other characteristic electronic energy scales. The spindependent part of the Kohn-Sham quasiparticle potential acts like an effective magnetic field that is locally aligned with the electron spin-density. Because of these strong spin-dependent potentials, the resistance of a ferromagnetic metal circuit will change substantially when the magnetization orientation in any part of the circuit is altered, an effect known as giant magnetoresistance (GMR). Conversely, transport currents can destabalize magnetization configurations that are metastable in the absence of a current and change collective magnetization dynamics. In the case of circuits containing ferromagnetic elements the influence of transport currents on the magnetization can be understood as following from the conservation of total spin angular momentum; the torques that reorient quasiparticle spins as they traverse a non-collinear magnetic circuit are accompanied by current-induced reaction torques (CITs) that act on the magnetic condensate. This type of phenomenon is not by any means limited to magnetic systems. For example, there have been recent studies of the interaction between transport and charge density waves (CDW), which find that the CDW order parameter configuration influences transport and conversely that transport can alter the CDW. 5

Interlayer transport in bilayer quantum Hall systems.

Bilayer quantum Hall systems have a broken symmetry ground state at a filling factor which can be viewed either as an excitonic superfluid or as a pseudospin ferromagnet. We present a theory of interlayer transport in quantum Hall bilayers that highlights remarkable similarities and critical differences between transport in Josephson junction and ferromagnetic metal spin-transfer devices. Our theory is able to explain the size of the large but finite low-bias interlayer conductance and the voltage width of this collective transport anomaly.

2D Bands and Electron-Phonon Interactions in Polyacene Plastic Transistors

We present a simple tight-binding model for the two-dimensional energy bands of polyacene field-effect transistors and for the coupling of these bands to lattice vibrations of their host molecular crystal. We argue that the strongest electron-phonon interactions in these systems originate from the dependence of intermolecule hopping amplitudes on collective molecular motion, and introduce a generalized Su-Schrieffer-Heeger model and is parameter-free once the band mass has been specified. We compute alpha(2)F(omega) as a function of two-dimensional hole density, and are able to explain the onset of superconductivity near 2D carrier density n(2D) approximately 10(14) cm(-2), observed in recent experiments.

Thermally assisted current-driven skyrmion motion

We study the behavior of skyrmions in thin films under the action of stochastic torques arising from thermal fluctuations. We find that the Brownian motion of skyrmions is described by a stochastic Thieles equation and its corresponding Fokker-Planck equation. The resulting Fokker-Planck equation is recognized as the one for a high-friction Brownian particle which has been studied extensively in different physical contexts. It is shown that thermal fluctuations favor the skyrmion motion allowing a finite mobility even in presence of pinning traps. We calculate explicitly the mobility tensor of skyrmions in linear response to an electric current finding that it increases with temperature.

Electron–Phonon Interactions in Polyacene Organic Transistors

We present a simple model for the electron-phonon interactions between the energy subbands in polyacene field-effect transistors and the vibrations of the crystal. We introduce a generalized SuSchrieffer-Heeger model, arguing that the strongest electron-phonon interactions in these systems originate from the dependence of inter-molecule hopping amplitudes on collective molecular motion. We compute the electron-phonon spectral function α 2 F(ω) as a function of two-dimensional hole density and the coupling strength constant. Our results are in agreement with the sharp onset of superconductivity near half-filling discovered in recent experiments by Schon et al. [1] and predict an increase of T c with pressure. We further speculate on the implications that the observation of the quantum Hall effect in these systems has on the effective band mass in the low carrier density regime.