Showing papers by "Martin Gmitra published in 2006"

Current-driven destabilization of both collinear configurations in asymmetric spin valves.

Abstract: Spin-transfer torque in spin valves usually destabilizes one of the collinear configurations (either parallel or antiparallel) and stabilizes the second one. Apart from this, balance of the spin-transfer and damping torques can lead to steady precessional modes. In this Letter we show that in some asymmetric nanopillars, spin current can destabilize both parallel and antiparallel configurations. As a result, stationary precessional modes can occur at zero magnetic field. The corresponding phase diagram as well as frequencies of the precessional modes have been calculated in the framework of macrospin model. The relevant spin-transfer torque has been calculated in terms of the macroscopic model based on spin diffusion equations.

Macroscopic description of spin transfer torque

Abstract: A macroscopic description of the current-induced torque due to spin transfer has been developed for layered systems consisting of ferromagnetic films, separated by nonmagnetic layers. The description is based on the classical spin diffusion equations for the distribution functions used in the theory of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR), and the relevant boundary conditions for the longitudinal and transverse components of the spin current and spin accumulation. The torque is expressed as a function of the usual parameters derived from CPP-GMR experiments and two additional parameters involved in the transverse boundary conditions. The model describes qualitatively the normal and inverse switching phenomena studied in recent experiments. We also discuss a structure for which the spin torque disappears at a noncollinear magnetic configuration.

Influence of interface spin-flip scattering on spin accumulation and spin currents in magnetic multilayers with collinear magnetizations

Abstract: The macroscopic description of spin-dependent electronic transport in magnetic-layered structures is extended by including the effects due to spin-flip scattering at the interfaces between magnetic and nonmagnetic layers Such processes lead to spin-memory losses at the interfaces and therefore play a significant role in the giant magnetoresistance and current-induced magnetic switching phenomena Interface spin-flip scattering significantly modifies the distribution of spin currents and spin accumulation in the vicinity of the interface Two semi-infinite systems in direct contact, as well as magnetic/nonmagnetic (F∕N) sandwiches and superlattices, are considered for both parallel and antiparallel magnetic configurations

A self-adjusted Monte Carlo simulation as a model for financial markets with central regulation

Abstract: Properties of the self-adjusted Monte Carlo algorithm applied to 2d Ising ferromagnet are studied numerically. The endogenous feedback form expressed in terms of the instant running averages is suggested in order to generate a biased random walk of the temperature that converges to criticality without an external tuning. The robustness of a stationary regime with respect to partial accessibility of the information is demonstrated. Several statistical and scaling aspects have been identified which allow to establish an alternative spin lattice model of the financial market. It turns out that our model alike model suggested by Bornholdt [Int. J. Mod. Phys. C 12 (2001) 667], may be described by Levy-type stationary distribution of feedback variations with unique exponent α 1 ∼ 3.3 . However, the differences reflected by Hurst exponents suggest that resemblances between the studied models seem to be non-trivial.

Current-induced dynamics in asymmetric spin valves

Abstract: Spin transfer in an asymmetric nanopillar spin valve is studied numerically in the diffusive transport regime. The system considered includes a pinned synthetic antiferromagnetic structure, which is separated from the sensing layer by a thin nonmagnetic film. Landau-Lifshitz-Gilbert equation, with the spin-transfer torque taken into account, is used to study current-induced magnetic dynamics in the macrospin model. It is shown that steady dynamical states can occur in the absence of magnetic field. The advantage of systems including a synthetic antiferromagnetic structure, compared to simple spin valves, is also discussed.

The co-evolutionary dynamics of directed network of spin market agents

Abstract: The spin market model [S. Bornholdt, Int. J. Mod. Phys. C 12 (2001) 667] is generalized by employing co-evolutionary principles, where strategies of the interacting and competitive traders are represented by local and global couplings between the nodes of dynamic directed stochastic network. The co-evolutionary principles are applied in the frame of Bak–Sneppen self-organized dynamics [P. Bak, K. Sneppen, Phys. Rev. Lett. 71 (1993) 4083] that includes the processes of selection and extinction actuated by the local (node) fitness. The local fitness is related to orientation of spin agent with respect to the instant magnetization. The stationary regime is formed due to the interplay of self-organization and adaptivity effects. The fat tailed distributions of log-price returns are identified numerically. The non-trivial model consequence is the evidence of the long time market memory indicated by the power-law range of the autocorrelation function of volatility with exponent smaller than one. The simulations yield network topology with broad-scale node degree distribution characterized by the range of exponents 1.3 γ in 3 coinciding with social networks.

Current‐induced spin dynamics in spin‐valve structures

Abstract: The Landau-Lifshitz-Gilbert equation for the magnetization dynamics, extended by including the torque due to spin-transfer from conduction electrons, is used to study current-induced magnetic switching and stationary precessional modes. The spin torque is described in terms of a macroscopic model based on the classical spin diffusion equations and relevant boundary conditions. The approach allows to investigate current-driven switching, in particular the switching time and the frequency of the associated magnetoresistance signal in the precessional regime.

Current induced switching due to spin-transfer in spin valves: macroscopic model

Abstract: We develop a macroscopic description of current-induced torque due to spin transfer in layered systems consisting of ferromagnetic films separated by a nonmagnetic layer. The description is based on the classical spin diffusion equations for the distribution functions inside the films used in the theory of CPP-GMR, and macroscopic boundary conditions for the longitudinal and transverse components of the spin current. Due to strong exchange field in ferromagnetic films, we assume that the perpendicular component of spin current is totally absorbed within the narrow interface region giving rise to the torque. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)