R.N. Costa Filho

Valley polarization due to trigonal warping on tunneling electrons in graphene

The effect of trigonal warping on the transmission of electrons tunneling through potential barriers in graphene is investigated. We present calculations of the transmission coefficient for single and double barriers as a function of energy, incidence angle and barrier heights. The results show remarkable valley-dependent directional effects for barriers oriented parallel to the armchair or parallel to the zigzag direction. These results indicate that electrostatic gates can be used as valley filters in graphene-based devices.

Microscopic theory of dipole-exchange spin waves in ferromagnetic films: Linear and nonlinear processes

The linear and nonlinear processes in ferromagnetic films at low temperatures T<< Tc are studied in a microscopic theory. Both the long-range magnetic dipole-dipole and the Heisenberg exchange interactions to nearest and next-nearest neighbors are included. The results obtained for the linearized spin-wave spectrum are compared with previous macroscopic theories. For ultrathin films (or for large wave vectors) the microscopic theory provides important corrections. The nonlinear dynamics of the spin waves are studied through a finite-temperature perturbation theory based on Feynman diagrams. We obtain explicit results for the energy shift and damping (or reciprocal lifetime) of the dipole-exchange spin waves due to all possible three-magnon and four-magnon processes involving combinations of the surface and quantized bulk spin waves at low temperatures. To investigate different dipole interaction strengths (relative to the exchange) numerical results are presented using parameters for Fe, EuO, and GdCl3.

Spin-wave interactions in ultrathin ferromagnetic films: the dipole-exchange regime

Abstract A perturbation formalism is employed to study nonlinear processes for the interactions between the discrete spin waves in ferromagnetic films, taking account of both dipole–dipole and exchange coupling. By contrast with previous macroscopic theories of spin-wave interactions the dipolar terms are treated within a Hamiltonian method to obtain results applicable to ultrathin films. The roles of three-magnon and four-magnon processes in the spin-wave energy shift and damping are evaluated and numerical applications to EuO are discussed.

Critical Behavior Ising Model S = 1, 3/2 and 2 on Directed Networks

On directed} Barabasi-Albert and Small-World networks the Ising model with spin S=1, 3/2 and 2 is now studied through Monte Carlo simulations. In this model, the order-disorder phase transition of the order parameter is well defined on Small-World networks for Ising model with spin S=1. We calculate the value of the critical temperature T_c for several values of rewiring probability p of the directed Small-World network. This model on directed Small-World networks we obtained a second-order phase transition for p=0.2 and first-order phase transition for p=0.8. The critical exponentes beta/nu, gamma/nu and 1/\nu were calculated for p=0.2. On directed Barabasi-Albert we show that no there is phase transition for Ising model with spin S=1, 3/2 and 2.

Coupled magnetostatic modes of ferromagnetic / antiferromagnetic cylindrical bilayers

A theory is developed for the bulk and surface magnetostatic modes in bilayer systems with cylindrical geometries, where an antiferromagnetic layer is grown in contact with a ferromagnetic layer. Specifically the dispersion relations and localization properties of the modes are derived for ferromagnetic / antiferromagnetic bilayers as long concentric tubes around a nonmagnetic core. The results differ depending on which type of material is in the outer layer and also contrast with the planar bilayer geometries studied previously. Numerical examples are given for Permalloy and Gadolinium Aluminate as the materials.

Transport and thermodynamic properties of 2DEG in the presence of tilted magnetic field

We study the effects of a tilted magnetic field acting on a non‐interacting two‐dimensional electron gas (2DEG). We show that the existence of an angle between the external magnetic field and the normal direction to the 2DEG plane introduces severe changes on the electronic properties of the electron gas, with consequences on the thermodynamic and transport properties. Here, we consider that the electrons of the gas feel the presence of a tilted magnetic field and are confined by a non‐symmetric potential. Then, taking into account the Zeeman and Rashba spin‐orbit interactions, we obtain the Fermi energy, magnetization and the Hall conductivity for magnetic fields with different intensities and tilt angles. The influence of a weak periodic potential on these quantities is also analyzed. Our results show that some specific physical properties can be given to the 2DEG (in a reversible way) with a convenient choice of the intensity and tilt angle.

Microscopic theory of spin-wave interactions and relaxation in ferromagnetic films (abstract)

A diagrammatic perturbation method is employed to study the nonlinear dynamics of spin waves in thin ferromagnetic films, including the effects of the short-range exchange coupling and the longer-range magnetic dipole-dipole coupling between the spins. It is well known1 that, in the linear approximation, the spin-wave spectrum of a finite-thickness film may consist of several discrete branches describing the localized surface modes and the quantized (or standing) bulk modes. When higher-order effects are taken into account, we obtain a description of interactions between these modes where the dominant relaxation terms are due to three- and four-magnon scattering processes, by analogy with infinite ferromagnets.2 However, in the film geometry, the interaction terms may involve spin waves from either the same or different discrete branches, and so the temperature and wave-vector dependence of the spin wave energy shift and damping are modified. Our results represent a generalization of previous calculations...