We study theoretically the transmission spectra in one-dimensional photonic quasicrystals, made up of SiO 2 (A) and TiO 2 (B) materials, organized following the Octonacci sequence, where the nth-stage of the multilayer S n is given by the rule S n = S n−1 S n−2 S n−1 , for n ≥ 3 and with S 1 = A and S 2 = B. The expression for transmittance was obtained by employing a theoretical calculation based in the transfer-matrix method. To normally incident waves, we observe that, for a same generation, the transmission spectra for TE and TM waves are equal, at least qualitatively, and they present a scaling property where a self-similar behavior is obtained, as an evidence that these spectra are fractals. The spectra show regions where the omnidirectional band gaps emerges for specific generations of Octonacci photonic structure, except to TM waves. For TE waves, we note that all of them have the almost same width, for different generations. We also report the localization of modes as a consequence of the quasiperiodicity of the heterostructure.

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We report a theoretical investigation of thermal hysteresis of fourfold anisotropy ferromagnetic ͑FM͒ film exchange coupled to a compensated antiferromagnetic substrate. Thermal hysteresis occurs if the temperature interval includes the reorientation transition temperature, below which the frustration of the interface exchange coupling leads to a 90°rotation of the magnetization of the ferromagnetic layer. The temperature width of the thermal hysteresis is tunable by external magnetic fields of modest magnitude, with values of 43 K for an external field of 110 Oe and of 14 K for a field of 210 Oe, for a Fe͑12 nm͒ / MnF 2 ͑110͒ bilayer. For a Fe͑3 nm͒ / FeF 2 ͑110͒ bilayer the width of the thermal hysteresis is 23 K at 110 Oe and 13 K at 300 Oe. We discuss how the thickness of the iron film affects the field tuning of the thermal hysteresis width, and also how the thermal loops may be used to identify the nature of the interface exchange energy.

We consider a thin rare-earth film with the c axis perpendicular to the surface, to study its nonuniform magnetic structure induced by the lower coordination near the surfaces. The influence of a dc magnetic field, applied perpendicular to the c axis, on the equilibrium configurations is also investigated. The spin-wave frequencies, in the qϭ0 limit, are calculated as a function of the strength of the external dc field. We use the bulk parameters of dysprosium and holmium to obtain the external field dependence of the equilibrium configuration, magnetization, and the lowest spin-wave modes of 30-monolayer-thick films.