Multifractal spectra of spin waves in Fibonacci magnetic superlattices with biquadratic exchange coupling

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 increases.