Effective attraction induced by repulsive interaction in a spin-transfer system

In magnetic systems with dominating easy-plane anisotropy the magnetization can be described by an effective one dimensional equation for the in-plane angle. Re-deriving this equation in the presence of spin-transfer torques, we obtain a description that allows for a more intuitive understanding of spintronic devices' operation and can serve as a tool for finding new dynamic regimes. A surprising prediction is obtained for a planar ``spin-flip transistor'': an unstable equilibrium point can be stabilized by a current induced torque that further repels the system from that point. Stabilization by repulsion happens due to the presence of dissipative environment and requires a Gilbert damping constant that is large enough to ensure overdamped dynamics at zero current

Analytic treatment of the precessional (ballistic) contribution to the conventional magnetic switching

We consider a switching of the magnetic moment with an easy axis anisotropy from an "up" to a "down" direction under the influence of an external magnetic field. The driving field is applied parallel to the easy axis and is continuously swept from a positive to a negative value. In addition, a small constant perpendicular bias field is present. It is shown that while the driving field switches the moment in a conventional way, the perpendicular field creates an admixture of the precessional (ballistic) switching that speeds up the switching process. Precessional contribution produces a non-monotonic dependence of the switching time on the field sweep time with a minimum at a particular sweep time value. We derive an analytic expressions for the optimal point, and for the entire dependence of the switching time on the field sweep time. Our approximation is valid in a wide parameter range and can be used to engineer and optimize of the magnetic memory devices.Comment: 13 pages, 7 figure

Anomalous stabilization in a spin-transfer system at high spin polarization

Switching diagrams of nanoscale ferromagnets driven by a spin-transfer torque are studied in the macrospin approximation. We consider a disk-shaped free layer with in-plane easy axis and external magnetic field directed in-plane at 90 degrees to that axis. It is shown that this configuration is sensitive to the angular dependence of the spin-transfer efficiency factor and can be used to experimentally distinguish between different forms of $g(\theta)$, in particular between the original Slonczewski form and the constant $g$ approximation. The difference in switching diagrams is especially pronounced at large spin polarizations, with the Slonczewski case exhibiting an anomalous region.Comment: 3 pages, 4 figure

Local injection of pure spin current generates electric current vortices

We show that local injection of pure spin current into an electrically disconnected ferromagnetic - normal-metal sandwich induces electric currents, that run along closed loops inside the device, and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.Comment: Journal version. Improved notation, suggestions for experimental observation adde

Growth Optimal Portfolio : Analysis and construction on a discrete multi-period market

This thesis provides an analysis of Growth Optimal Portfolio (GOP) in discrete time. Growth Optimal Portfolio is a portfolio optimization method that aims to maximize expected long-term growth. One of the main properties of GOP is that, as time horizon increases, it outperforms all other trading strategies almost surely. Therefore, when compared with the other common methods of portfolio construction, GOP performs well in the long-term but might provide riskier allocations in the short-term. The first half of the thesis considers GOP from a theoretical perspective. Connections to the other concepts (numeraire portfolio, arbitrage freedom) are examined and derivations of optimal properties are given. Several examples where GOP has explicit solutions are provided and sufficiency and necessity conditions for growth optimality are derived. Yet, the main focus of this thesis is on the practical aspects of GOP construction. The iterative algorithm for finding GOP weights in the case of independently log-normally distributed growth rates of underlying assets is proposed. Following that, the algorithm is extended to the case with non-diagonal covariance structure and the case with the presence of a risk-free asset on the market. Finally, it is shown how GOP can be implemented as a trading strategy on the market when underlying assets are modelled by ARMA or VAR models. The simulations with assets from the real market are provided for the time period 2014-2019. Overall, a practical step-by-step procedure for constructing GOP strategies with data from the real market is developed. Given the simplicity of the procedure and appealing properties of GOP, it can be used in practice as well as other common models such as Markowitz or Black-Litterman model for constructing portfolios

Ballistic (precessional) contribution to the conventional magnetic switching

We consider a magnetic moment with an easy axis anisotropy energy, switched by an external field applied along this axis. Additional small, time-independent bias field is applied perpendicular to the axis. It is found that the magnet's switching time is a non-monotonic function of the rate at which the field is swept from "up" to "down". Switching time exhibits a minimum at a particular optimal sweep time. This unusual behavior is explained by the admixture of a ballistic (precessional) rotation of the moment caused by the perpendicular bias field in the presence of a variable switching field. We derive analytic expressions for the optimal switching time, and for the entire dependence of the switching time on the field sweep time. The existence of the optimal field sweep time has important implications for the optimization of magnetic memory devices