Navegando por Autor "Cottam, Michael G."
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Artigo Magnetization in quasiperiodic magnetic multilayers with biquadratic exchange and uniaxial anisotropy(Elsevier Science B.V., 2002-02) Bezerra, Claudionor Gomes; Cottam, Michael G.A theoretical study is made of the magnetization versus applied field curves of ferromagnetic/nonmagnetic multilayers constructed according to a Fibonacci quasiperiodic sequence. The ferromagnetic films are assumed to have uniaxial anisotropy and are coupled by both bilinear and biquadratic effective exchange. The effects of quasiperiodicity in the magnetic phases are illustrated numerically for Fe/Cr systemsArtigo Multifractal spectra of spin waves in Fibonacci magnetic superlattices with biquadratic exchange coupling(Elsevier B.V., 2002-06-01) Bezerra, Claudionor Gomes; Cottam, Michael G.The multifractal properties are investigated for the bulk spin wave bands of quasiperiodic magnetic superlattices arranged according to the Fibonacci sequence. The superlattices are considered to be composed of layers of two simple cubic ferromagnetic materials. The layers are characterized by a bulk bilinear exchange coupling and they interact through an additional biquadratic exchange coupling at the interfaces. The function f(α), which describes the spectrum of singularities, is calculated for different values of the dimensionless in-plane wave vector kxa and ratio R=Jbq/JblI between the biquadratic and interfacial bilinear exchange couplings. Our results show the effects of the biquadratic exchange coupling on the bulk band structure, which becomes more disordered as the ratio R increasesArtigo Surface and bulk spin waves in magnetic superlattices with biquadratic exchange coupling(American Institute of Physics Publishing, 2002-05-13) Bezerra, Claudionor Gomes; Cottam, Michael G.A study is made of the bulk and surface spin wave modes propagating in magnetic superlattices that are composed of two simple cubic ferromagnetic materials which interact by bilinear and biquadratic exchange couplings at the interfaces. 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 obtained show that from a certain value of the ratio R between the biquadratic and interfacial bilinear exchange couplings the surface acoustic modes are suppressed and only the optical modes are present