Navegando por Autor "Almeida, N. S."
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Artigo Characterization of the magnetic phases of holmium nanofilms via magnetic neutron scattering(Elsevier, 2019) Mello, V. D.; Santiago, F. A. L.; Anselmo, Dory Hélio Aires de Lima; Vasconcelos, Manoel Silva de; Almeida, N. S.Magnetic phases of holmium films are associated with the intensity of the magnetic neutron scattering. It is shown that each magnetic phase of the system (fan, helifan, spin-slip, helix and ferromagnetic) exhibits a characteristic profile of scattering which can be used as a fingerprint to identify it. In this paper, we present theoretical results obtained for holmium films 24 monolayers thick at a fixed temperature and in the presence of a dc magnetic field applied along the basal plane. A self-consistent local field algorithm was used to obtain the equilibrium configurations of the magnetic moments of the film and, with these results, the spin-spin correlation functions which determine the shape of the neutron scattering intensity were calculatedArtigo Influence of nonlinearities on the pass band structure of magnetic superlattices(Elsevier, 1989) Carriço, Artur da Silva; Almeida, N. S.We consider superlattices constructed with alternate layers of antiferromagnetic and nonmagnetic materials to study the effect produced by the first nonlinear correction of the susceptibility of the antiferromagnetic material, on the electromagnetic pass bands of these artificial structures. We focus specially on the modifications produced on the pass band edges, as compared to the behavior exhibited under very low magnetic fields. Applications are made for FeF2/ZnF2 superlattices with different thicknesses.Artigo Magnetic behavior of thin rare-earth films(American Physical Society, 1999-03-01) Carriço, Artur da Silva; Almeida, N. S.; Mello, V. D.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 q=0 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 con- figuration, magnetization, and the lowest spin-wave modes of 30-monolayer-thick films.Artigo Magnetocaloric effect of thin Terbium films(Elsevier, 2017-12) Anselmo, Dory Hélio Aires de Lima; Mello, V. D.; Vasconcelos, Manoel Silva de; Almeida, N. S.We report a theoretical study of the magnetocaloric effect of Terbium (Tb) thin films due to finite size and surface effects in the helimagnetic phase, corresponding to a temperature range from TC=219 K to TN=231 K, for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field (ΔH=30 kOe, ΔH=50 kOe and ΔH = 70 kOe) we report a significative change in adiabatic temperature, ΔT/ΔH, near the Néel temperature, of the order ten times higher than that observed for Tb bulk. On the other hand, for small values of the magnetic field, large thickness effects are found. For external field strength around few kOe, we have found that the thermal caloric efficiency increases remarkably for ultrathin films. For an ultrathin film with 6 monolayers, we have found ΔT/ΔH = 43 K/T while for thicker films, with 20 monolayers, ΔT/ΔH = 22 K/T. Our results suggest that thin films of Tb are a promising material for magnetocaloric effect devices for applications at intermediate temperaturesArtigo Nonlinear response of antiferromagnetic films to radiation at oblique incidence(American Physical Society, 1993-10-01) Carriço, Artur da Silva; Almeida, N. S.The response of an antiferromagnetic film to incident radiation at oblique incidence is studied in the nonlinear regime. For given values of the frequency and incidence angle, the reflectivity of a film of arbitrary thickness is obtained as an implicit function of the incident field amplitude. The compound effect of the incidence angle and field strength on the reflected radiation is studied by means of the analysis of the reflection coefficient calculated for different values of these external variables. We use the physical parameters of the uniaxial Heisenberg antiferromagnet FeF2 to obtain numerical results. It is found that the threshold for nonlinear optical behavior is strongly dependent on the incidence angle.Artigo Polaritons in modified antiferromagnets(American Physical Society, 1992-11-01) Carriço, Artur da Silva; Fonseca, T. L.; Almeida, N. S.A theoretical study of mixed modes of uniaxial Heisenberg antiferromagnets by a dc field is presented. The static field is applied in an arbitrary direction relative to the easy axis and special attention is given to the magnetostatic limit where previous calculations show intriguing behavior of surface modes. Retarded polaritons are also considered and the spectrum that should be observed through the attenuated-total-reflection technique is calculated. Numerical results are presented for MnF2 and FeF2 semi-infinite specimens.Artigo Propagation of magnetostatic modes on aperiodic rare-earth multilayers(Elsevier, 2018) Anselmo, Dory Hélio Aires de Lima; Santiago, F. A. L.; Almeida, N. S.; Vasconcelos, Manoel Silva de; Cottam, M. G.; Mello, V. D.The localization behavior and scaling properties of magnetostatic modes propagating on magnetic aperiodic superlattices made up by rare-earth materials is analyzed. Both surface and bulk modes are obtained, considering two different aperiodic rules of stacking: Thue-Morse and Fibonacci. The magnetization is considered to be perpendicular to the superlattice’s interfaces. Our model consider as magnetic materials both dysprosium and holmium, at different temperatures. A scaling behavior is observed for the allowed bulk bands, through specific ordering parameters, meaning that the allowed bands have a fractal (or multifractal) structureArtigo Soft mode of antiferromagnetic multilayers near the surface spin-flop transition(Physical review B, 2005) Dantas, Ana L.; Vieira, Selma R.; Almeida, N. S.; Carriço, Artur da SilvaWe investigate the long wavelength soft mode of antiferromagnetic (AF) multilayers near the field induced instability of the AF order. We show that in the vicinity of the surface spin flop field sHSSFd the frequency of the lowest mode is V2 /g2=asHSSF−Hd+bsHSSF−Hd2, where a and b are functions of the magnetic parameters of the multilayer and H is the external field strength. For antiferromagnetic films the first order term is zero (due to the absence of demagnetizing effects) and b=1.