C.G. Bezerra

SELF-SIMILAR MAGNETORESISTANCE OF FIBONACCI ULTRATHIN MAGNETIC FILMS

We study numerically the magnetic properties (magnetization and magnetoresistance) of ultrathin magnetic films (Fe/Cr) grown following the Fibonacci sequence. We use a phenomenological model which includes Zeeman, cubic anisotropy, bilinear, and biquadratic exchange energies. Our physical parameters are based on experimental data recently reported, which contain biquadratic exchange coupling with magnitude comparable to the bilinear exchange coupling. When biquadratic exchange coupling is sufficiently large a striking self-similar pattern emerges.

Magnetization in quasiperiodic magnetic multilayers with biquadratic exchange coupling

A theoretical study of the magnetization curves of quasiperiodic magnetic multilayers is presented. We consider structures composed by ferromagnetic films (Fe) with interfilm exchange coupling provided by intervening nonferromagnetic layers (Cr). The theory is based on a realistic phenomenological model, which includes the following contributions to the free magnetic energy: Zeeman, cubic anisotropy, bilinear, and biquadratic exchange energies. The experimental parameters used here are based on experimental data recently reported, which contain sufficiently strong biquadratic exchange coupling.

Anomalous Magnetoresistance in Fibonacci Multilayers

We theoretically investigated magnetoresistance curves in quasiperiodic magnetic multilayers for two different growth directions, namely, [110] and [100]. We considered identical ferromagnetic layers separated by nonmagnetic layers with two different thicknesses chosen based on the Fibonacci sequence. Using parameters for Fe/Cr multilayers, four terms were included in our description of the magnetic energy: Zeeman, cubic anisotropy, bilinear coupling, and biquadratic coupling. The minimum energy was determined by the gradient method and the equilibrium magnetization directions found were used to calculate magnetoresistance curves. By choosing spacers with a thickness such that biquadratic coupling is stronger than bilinear coupling, unusual behaviors for the magnetoresistance were observed: (i) for the [110] case, there is a different behavior for structures based on even and odd Fibonacci generations, and, more interesting, (ii) for the [100] case, we found magnetic field ranges for which the magnetoresistance increases with magnetic field.

The anisotropic Ashkin–Teller model: a renormalization group study

The two-dimensional ferromagnetic anisotropic Ashkin–Teller model is investigated through a real-space renormalization-group approach. The critical frontier, separating five distinct phases, recovers all the known exact results for the square lattice. The correlation length (νT) and crossover (φ) critical exponents are also calculated. With the only exception of the four-state Potts critical point, the entire phase diagram belongs to the Ising universality class.

Transmission fingerprints in quasiperiodic magnetic structures

In this paper we study the spin wave transmission spectra of quasiperiodic magnetic multilayers following Fibonacci, double period and Thue–Morse sequences. We consider multilayers that are composed of two simple cubic ferromagnetic materials surrounded by two semi-infinite slabs of a third ferromagnetic material. A transfer matrix treatment is employed with the calculations carried out for the exchange-dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. The numerical results illustrate the self-similar aspects of the spectra as well as their magnetic transmission fingerprints, which can be used to identify the sequence applied to construct the system.

Magnetoresistance profiles of quasiperiodic Fe/Cr structures

We study theoretically the magnetoresistance properties of magnetic films (Fe/Cr) grown following the Fibonacci sequence. We use a theoretical Hamiltonian which includes Zeeman, cubic and uniaxial anisotropy, bilinear and biquadratic exchange energies. In particular, the presence of the uniaxial anisotropy in the Hamiltonian gives new symmetries to the system which were not discussed in previous works. Our physical parameters are based on experimental data recently reported, which contain biquadratic exchange coupling with magnitude comparable to the bilinear exchange coupling. We show the influence of the biquadratic exchange and anisotropies on the self-similar properties of the magnetoresistance profiles.