arXiv subjects

Arne Brataas

Publications and source records attributed to Arne Brataas.

At least 199 records · Page 11Linked to original sources

arXiv2006

Non-collinear Magnetoelectronics

The electron transport properties of hybrid ferromagnetic|normal metal structures such as multilayers and spin valves depend on the relative orientation of the magnetization direction of the ferromagnetic elements. Whereas the contrast in the resistance for parallel and antiparallel magnetizations, the so-called Giant Magnetoresistance, is relatively well understood for quite some time, a coherent picture for non-collinear magnetoelectronic circuits and devices has evolved only recently. We review here such a theory for electron charge and spin transport with general magnetization directions that is based on the semiclassical concept of a vector spin accumulation. In conjunction with first-principles calculations of scattering matrices many phenomena, e.g. the current-induced spin-transfer torque, can be understood and predicted quantitatively for different material combinations.

cond-mat.mes-hall↗

arXiv2005

Magnetomechanical Torques in Small Magnetic Cantilevers

We study the dnamics of small magnetic cantilevers, either made from Si covered by a magnetic film or entirely ferromagnetic ones. The magnetomechanical torques are found to cause line splittings in ferromagnetic resonance spectra and magnetization reversal facilitated by mechanical degrees of freedom. We show that the magnetomechanical torques can extend the limits of detecting and exciting motion at the nanoscale. A "nanomotor" described here effectively transforms rf magnetic fields into mechanical oscillations. We furthermore propose to integrate mechanical oscillators into magnetoelectronic devices that make use of current-induced spin-transfer torques. This opens new possibilities for electric transducers of nanomechanical motion.

cond-mat.mes-hall↗

arXiv2005

Current-induced macrospin vs spin-wave excitations in spin valves

The mode dependence of current-induced magnetic excitations in spin valves is studied theoretically. The torque exerted on the magnetization by transverse spin currents as well as the Gilbert damping constant are found to depend strongly on the wave length of the excitation (spin wave). Analytic expressions are presented for the critical currents that excite a selected spin wave. The onset of macrospin (zero wavelength) vs finite wavelength instabilities depends on the device parameters and the current direction, in agreement with recent experimental findings.

cond-mat.mes-hall↗

arXiv2005

Spontaneous-Symmetry-Breaking Mechanism of Adiabatic Pumping

We consider heterostructures consisting of regions with a continuous symmetry in contact with regions wherein the symmetry is spontaneously broken. The low-frequency dynamics of the corresponding order parameter are shown to induce nonequilibrium transport, a ``pumping,'' out of the symmetry-broken regions, which is governed by the generator of the broken-symmetry operator. This pumping damps Goldstone-mode excitations and transfers them beyond traditional (static) proximity length scales. Our general conclusions are discussed for specific examples of order parameters in helimagnets, charge/spin-density waves, superconductors, and ferromagnets. We carry out a detailed calculation of such pumping for spiral magnetic orders in helimagnets possessing a duality in the representation of its symmetry-broken states.

cond-mat.mes-hall↗

arXiv2005

Spin Hall effect, Hall effect and spin precession in diffusive normal metals

We study transport in normal metals in an external magnetic field. This system exhibits an interplay between a transverse spin imbalance (spin Hall effect) caused by the spin-orbit interaction, a Hall effect via the Lorentz force, and spin precession due to the Zeeman effect. Diffusion equations for spin and charge flow are derived. The spin and charge accumulations are computed numerically in experimentally relevant thin film geometries. The out-of-plane spin Hall potential is suppressed when the Larmor frequency is larger than the spin-flip scattering rate. The in-plane spin Hall potential vanishes at zero magnetic field and attains its maximum at a finite magnetic field before spin precession starts to dominate. Spin-injection via ferromagnetic contacts creates a transverse charge Hall effect that decays in a finite magnetic field due to spin precession.

cond-mat.mes-hall↗

arXiv2005

Spin accumulation and decay in magnetic Schottky barriers

The theory of charge and spin transport in forward-biased Schottky barriers reveals characteristic and experimentally relevant features. The conductance mismatch is found to enhance the current induced spin-imbalance in the semiconductor. The GaAs|MnAs interface resistance is obtained from an analysis of the magnetic field dependent Kerr rotation experiments by Stephens et al. and compared with first-principles calculations for intrinsic interfaces. With increasing current bias, the interface transparency grows towards the theoretical values, reflecting increasingly efficient Schottky barrier screening.

cond-mat.mes-hall↗

arXiv2005

Spin Transport in Diffusive Superconductors

We employ the Keldysh formalism in the quasiclassical approximation to study transport in a diffusive superconductor. The resulting 4x4 transport equations describe the flow of charge and energy as well as the corresponding flow of spin and spin energy. Spin-flip scattering due to magnetic impurities is included. We find that the spin-flip length is renormalized in the superconducting case and propose an experimental system to measure the spin-accumulation in a superconductor.

cond-mat.supr-con↗

arXiv2005

Spin transport and magnetoresistance in F/S/F spin valves

We consider spin transport and spin relaxation in superconductors using the quasiclassical theory of superconductivity. We include spin relaxation due to spin-orbit interaction as well as magnetic impurities, and show that the energy dependence of the spin-flip rate is different for these two mechanisms. In ferromagnet-superconductor-ferromagnet systems made of Co and Al, interface resistances can be small compared to bulk resistances. This simplifies the description of transport in Co/Al/Co spin valves, for which we numerically calculate the temperature and Al length dependence of the magnetoresistance.

cond-mat.supr-con↗

arXiv2005

Magnetization noise in magnetoelectronic nanostructures

By scattering theory we show that spin current noise in normal electric conductors in contact with nanoscale ferromagnets increases the magnetization noise by means of a fluctuating spin-transfer torque. Johnson-Nyquist noise in the spin current is related to the increased Gilbert damping due to spin pumping, in accordance with the fluctuation-dissipation theorem. Spin current shot noise in the presence of an applied bias is the dominant contribution to the magnetization noise at low temperatures.

cond-mat.mes-hall↗

arXiv2004

Nano-mechanical magnetization reversal

The dynamics of the ferromagnetic order parameter in thin magnetic films is strongly affected by the magnetomechanical coupling at certain resonance frequencies. By solving the equation of motion of the coupled mechanical and magnetic degrees of freedom we show that the magnetic-field induced magnetization switching can be strongly accelerated by the lattice and illustrate the possibility of magnetization reversal by mechanical actuation.

cond-mat.other↗

arXiv2004

Spin Hall effects in diffusive normal metals

We consider spin and charge flow in normal metals. We employ the Keldysh formalism to find transport equations in the presence of spin-orbit interaction, interaction with magnetic impurities, and non-magnetic impurity scattering. Using the quasiclassical approximation, we derive diffusion equations which include contributions from skew scattering, side-jump scattering and the anomalous spin-orbit induced velocity. We compute the magnitude of various spin Hall effects in experimental relevant geometries and discuss when the different scattering mechanisms are important.

cond-mat.mes-hall↗

arXiv2004

Spin and Charge Pumping by Ferromagnetic-Superconductor Order Parameters

We study transport in ferromagnetic-superconductor/normal-metal systems. It is shown that charge and spin currents are pumped from ferromagnetic superconductors into adjacent normal metals by adiabatic changes in the order parameters induced by external electromagnetic fields. Spin and charge pumping identify the symmetry of the superconducting order parameter, e.g., singlet pairing or triplet pairing with opposite or equal spin pairing. Consequences for ferromagnetic-resonance experiments are discussed.

cond-mat.mes-hall↗

arXiv2004

Reducing the critical switching current in nanoscale spin valves

The current induced magnetization reversal in nanoscale spin valves is a potential alternative to magnetic field switching in magnetic memory devices. We show that the critical switching current can be decreased by an order of magnitude by strategically distributing the resistances in the magnetically active region of the spin valve. In addition, we simulate full switching curves and predict a new precessional state.

cond-mat.mes-hall↗

arXiv2003

Magnetoelectronic Spin Echo

We predict a spin echo in electron transport through layered ferromagnetic-normal-ferromagnetic metal structures: whereas a spin current polarized perpendicular to the magnetization direction decays when traversing a single homogeneous ferromagnet on the scale of the ferromagnetic spin-coherence length, it reappears by adding a second identical but antiparallel ferromagnet. This re-entrant transverse spin current resembles the spin-echo effect in the magnetization of nuclei under pulsed excitations. We propose an experimental setup to measure the magnetoelectronic spin echo.

cond-mat.mes-hall↗

arXiv2003

Spin detection in quantum dots by electric currents

We develop a theoretical description of transport through quantum dots connected to reservoirs via spin-polarized ballistic contacts. Rate equations account for spin accumulation inside the dot, electron-electron interactions, and stochastic fluctuations. It is shown that both the ac response (admittance) and the frequency-dependent shot noise are governed by spin-flip scattering, which can be used to detect spin polarization and spin-flip processes in the dot.

cond-mat.mes-hall↗

arXiv2003

Dynamic ferromagnetic proximity effect in photoexcited semiconductors

The spin dynamics of photoexcited carriers in semiconductors in contact with a ferromagnet is treated theoretically and compared with time-dependent Faraday rotation experiments. The long time response of the system is found to be governed by the first tens of picoseconds in which the excited plasma interacts strongly with the intrinsic interface between semiconductor and ferromagnet in spite of the existence of a Schottky barrier in equilibrium.

cond-mat.mes-hall↗

arXiv2003

Non-monotonic angular magnetoresistance in asymmetric spin valves

The electric resistance of ferromagnet/normal-metal/ferromagnet perpendicular spin valves depends on the relative angle between the magnetization directions. In contrast to common wisdom, this angular magnetoresistance is found to be not necessarily a monotone function of the angle. The parameter dependence of the global resistance minimum at finite angles is studied and the conditions for experimental observation are specified.

cond-mat.mes-hall↗

arXiv2003

Magnetovibrational coupling in small cantilevers

A nano-magnetomechanical system consisting of a cantilever and a thin magnetic film is predicted to display magnetovibrational modes, which should enable applications for sensors and actuators. The {}``polaritonic{}'' modes can be detected by line splittings in ferromagnetic resonance spectra.

cond-mat.mes-hall↗