Navegando por Autor "Dantas, Ana L."

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Confinement of magnetic vortex and domain walls in dipolar coupled concentric nanocylinders
(IEEE, 2015-05) Carriço, Artur da Silva; Dantas, Ana L.; Rebouças, G. O. G.; Souza Júnior, I. D. Q.; Souza, C. M.; Oliveira, Leonardo L.; Nunes, Marcos S.
We report a theoretical study of the magnetic phases of core–shell nanocylinders, consisting of a Py cylindrical core, dipolar coupled to a coaxial Fe cylindrical shell. A few nanometers thick nonmagnetic cylindrical layer separates the core from the shell, and controls the magnitude of the core–shell dipolar interaction. New magnetic phases emerge from the dipolar interaction, and may consist of either the combination of the intrinsic magnetic phases or new phases that are not seen in isolated cylinders and shells. We discuss typical examples. The magnetic phases of a 21 nm-height nanocylinder composed of a 57 nm-diameter Py core coupled to a 12 nm-thick Fe shell may be set to be a Py vortex with the same chirality of the Fe shell circular state, or a Py uniform domain coupled to a pair of domain walls of the Fe shell onion state. A magnetic vortex may be stabilized in a 6 nm-height, 42 nm-diameter Py cylinder coupled to a 6 nm-thick Fe shell.
Controlling the core-to-core distance of vortex pairs in exchange-biased iron elliptical nanoelements
(AIP Publishing LLC, 2012-02-10) Carriço, Artur da Silva; Oliveira, Felipe F.; Moura, Thiago R. S.; Dantas, Ana L.; Rebouças, G. O. G.
We report a theoretical study of vortex pairs in exchange-biased elliptical iron nanoelements. We show that the remanent state may be tailored to fit vortex pairs with opposite chiralities separated by a diamond-like domain. Flat nanoelements with lateral dimensions ranging from 115 nm × 425 nm to 195 nm × 425 nm have the core-to-core distance tunable by the interface field strength.
Controlling the vortex core of thin Permalloy nano-cylinders dipolar coupled to Co polarizers
(AIP Publishing LLC, 2014-01-13) Carriço, Artur da Silva; Souza, C. M.; Dantas, Ana L.; Queiroz Júnior, I. S.
We report a theoretical study of the vortex profile of in-plane magnetized PyTM nano-cylinders subjected to the stray field of perpendicular anisotropy Co nano-cylinders. We consider 6 nm thick PyTM cylinders dipolar coupled to 60 nm thick Co cylinders, at distances from 1.5 nm to 30 nm, with diameters (D) ranging from 45 nm to 105 nm. We find considerable reduction of critical diameter for stable PyTM magnetic vortices and spiral-vortex phases, as well as vortex core diameters twice as large as the bulk value.
Depinning field of a periodic domain wall array in vicinal nanowires
(Journal of Applied Physics, 2009) Dantas, Ana L.; Rebouças, G. O. G.; Carriço, Artur da Silva
We report a theoretical investigation of the magnetic states and depinning field of a periodic array of head-to-head domain walls of flat Fe rectangular nanowires, exchange coupled with a vicinal two-sublattice uniaxial antiferromagnetic substrate. We show that for strong interface exchange energy, domain walls are pinned at interface steps perpendicular to the antiferromagnetic easy axis, separating terraces with opposite interface exchange field. The array sequence, which alternates head-to-head and tail-to-tail domain walls, may form a structure with alternate chirality or with the same chirality. The domain wall dipolar field affects the chirality sequence, which is tunable by the geometrical constraints and the strength of the interface exchange field. The depinning field of 10 nm thick, 1 μm long wires, with widths of 100 and 200 nm, is of the order of the interface field strength, and the depinning process involves domain wall motion and the transversal displacement of a periodic array of vortices.
Design of Magnetic Polymeric Particles as a Stimulus-Responsive System for Gastric Antimicrobial Therapy
(Springer Nature Switzerland AG, 2016-12-13) Carriço, Artur da Silva; Silva-Freitas, Erica L.; Pontes, Thales R. F.; Araújo-Neto, Rafael P.; Damasceno, Ítalo H. M.; Silva, Kátia L.; Carvalho, Juliana F.; Medeiros, Aldo C.; Silva, Rodolfo B.; Silva, Amanda K. A.; Morales, Marco A.; Egito, Eryvaldo S. T.; Dantas, Ana L.
The treatment of peptic ulcers induced by H. pylori remains challenging due to the deep mucous layer location of bacteria preventing antimicrobial drug access. The present work aimed to design and evaluate in vitro dual responsive (both pH and magnetic field-sensitive) polymeric magnetic particles loaded with amoxicillin as a smart drug carrier for deep mucous layer penetration and in situ drug release. Magnetite particles were produced by the co-precipitation method and subsequently coated with the Eudragit®S100 and amoxicillin by using the spray-drying technique. The physicochemical characterization of the obtained particles was carried out by optical and scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, nitrogen adsorption/desorption isotherms, and vibrating sample magnetometry. Additionally, drug release tests and antibacterial activity tests were evaluated in vitro. Microparticles presented 17.2 ± 0.4 μm in size and their final composition was 4.3 ± 1.5% of amoxicillin, 87.0 ± 2.3% of Eudragit, and 9.0 ± 0.3% of magnetite. They were both pH and magnetic field responsive while presenting antimicrobial activity. On one side, magnetic field responsiveness of particles is expected to prompt them to reach bacterium niche in deep mucous layer by means of magnetic forces. On the other side, pH responsiveness is expected to enable drug release in the neutral pH of the deep mucous layer, preventing undesired delivery in the acidic gastric lumen. Smart microparticles were designed presenting both pH and magnetic field responsiveness as well as antimicrobial activity. These may be promising assets for peptic ulcer treatment.
Dipolar field effects on the critical current for spin transfer switch of iron and permalloy nanoelements
(AIP Publishing LLC, 2014-02-10) Carriço, Artur da Silva; Oliveira, L. L.; Dantas, J. T. S.; Dantas, Ana L.; Souza, R. M.
We report a theoretical study of dipolar effects on the switching current density of soft ferromagnetic elliptical nanoelements. Relevant changes on the critical current value are found according to the orientation of the magnetization and the spin polarization with the major axis. We show that the critical current density may be reduced by as much as 92% for thin nanoelements magnetized along the minor axis direction, using in-plane spin polarization parallel to the magnetization.
Effects of composition on the stability of the magnetic order of ferrimagnetic multilayers
(APS PHYSICS, 2002) Dantas, Ana L.; Vieira, Selma R.; Carriço, Artur da Silva
We study the field induced instability of the antiferromagnetic phase of magnetic multilayers consisting of a stacking of uniaxial ferromagnetic films, F1 and F2, with thicknesses t1 and t2, coupled through a nonmagnetic ~NM! spacer. The external field is along the uniaxial axis, in the plane of the films, and the composition is varied by choosing arbitrary values of the relative thickness b5t1 /t2. We calculate the critical value of the external field H*, and examine the nature of the phase transition. We find that H* does not change with the multilayer thickness in the case of multilayers with an even number of magnetic layers, (@F1 /NM/F2 /NM#n , n arbitrary!. The nature of the phase transition and the critical field depend on the value of b, and the minimum value of H* corresponds to b51. For a multilayer with an odd number of magnetic layers we find that the thickness dependence of the critical field is significantly affected by the value of b.
Ferromagnetic resonance of compensated ferromagnetic/antiferromagnetic bilayers
(AIP Publishing LLC, 2012-10-02) Carriço, Artur da Silva; Dantas, Ana L.; Oliveira, L. L.; Silva, M. L.
We report a theoretical study of the ferromagnetic resonance (FMR) frequency Ω(H) of Fe/FeF2(110) and Fe/MnF2(110) uniaxial anisotropy compensated bilayers. We show that under external field perpendicular to the anisotropy axis, the uniform mode of the Fe-film becomes soft at an external field strength (H∗=HFA−H⊥int) smaller than the Fe anisotropy field. For strong interface exchange coupling, there is a gap in the FMR spectrum. In this case, Ω(H) is a monotonically increasing function of the external field strength and for any value of the external field strength Ω(H)>Ω(0). The value of the effective interface field downshifts (H⊥int) and the value of Ω(0) may be used to estimate the interface exchange energy.
Heat capacity of compensated F/AF bilayers
(ELSEVIER, 2005) Silva, M. L.; Dantas, Ana L.; Carriço, Artur da Silva
We report a theoretical study of the magnetic heat capacity, cV(T), of exchange coupled compensated ferromagnet/antiferromagnetic bilayers. We show that cV(T) may display a large peak at the temperature TR where the interface magnetic frustration induces a 90° reorientation of the F layer. We study Fe/FeF2 bilayers and show that TR and the magnitude of the cV(T) peak depend on the strength of the interface exchange. We suggest how the sign of the interface exchange energy parameter can be investigated using features produced by external fields.
Impact of core-shell dipolar interaction on magnetic phases of spherical core-shell nanoparticles
(American Physical Society, 2015-08-26) Carriço, Artur da Silva; Medeiros Filho, F. C.; Oliveira, L. L.; Pedrosa, S. S.; Rebouças, G. O. G.; Dantas, Ana L.
We show that confinement in small volumes affects the interplay of exchange and dipolar interactions and the magnetic phases of hard and soft spherical core-shell nanoparticles. Large variations in the magnetization of thin shells may occur due to the core dipolar field gradient within the shell. The reversal field is tunable by the trends imposed by the dipolar and core-shell interface exchange energies. We show, for instance, that the reversal field of a CoFe2O4 (30 nm)@ MnFe 2O4 (6 nm) particle ranges from 15.5 kOe for antiferromagnetic coupling down to 2.5 kOe for ferromagnetic coupling.
Interface roughness effects on coercivity and exchange bias
(Journal of Applied Physics, 2005) Dantas, Ana L.; Rebouças, G. O. G.; Silva, André S. W. T.; Carriço, Artur da Silva
We report model calculations of the hysteresis loops of exchange-coupled ferromagnet/antiferromagnet bilayers with monolayer-scale roughness and show that the loops are affected by the combined effect of the interface field strength, the degree of magnetic roughness and magnetostatic effects. The magnetization reversal may occur via domain-wall nucleation at the edges of monoatomic interface steps or coherent magnetization rotation. A magnetic phase diagram is constructed for a 10-nm-thick Fe film, subjected to nanometer-scale interface roughness.
Magnetic excitations of interface pinned domains
(ELSEVIER, 2001) Carriço, Artur da Silva; Dantas, Ana L.
We study the uniform excitations of the domains of a thin magnetic film on a two sublattice antiferromagnetic substrate.An interface step defect stabilizes a domain wall which holds two large ferromagnetic domains with opposite magnetization on each side of the defect line.The orientation of the magnetization in the domains, in response to an external field, parallel to the film surface, differs from the single domain situation and leads to new dispersion relations for the uniform precession modes of the ferromagnetic film.We apply the model to the case of a Neel wall of a uniaxial "ferromagnetic film and discuss the implications of the results in measurements of the interface effective exchange field.
Magnetic hysteresis of interface-biased flat iron dots
(Physical review B, 2009-03-04) Rebouças, G. O. G.; Silva, A. S. W. T.; Dantas, Ana L.; Camley, R. E.; Carriço, Artur da Silva
We report a theoretical study of the coercivity and bias of iron dots exchange coupled with an antiferromagnetic substrate. We show that flat dots, with height close to the iron exchange length, and lateral dimensions of a few exchange lengths, exhibit large enhancement of coercivity and exchange bias. For small interface field strength the magnetization reversal is nearly a coherent rotation with symmetrical loops. Interface pinning leads to large reduction in coercivity and asymmetrical loops, if the interface field strength is comparable to the value of the iron exchange field. We discuss the impact of geometrical confinement and interface pinning on the magnetization reversal mechanisms. We show that small area dots with height larger than the exchange length display stronger interface effects.
Magnetic thermal hysteresis in Fem/Dyn /Fem and Gdm/Dyn /Gdm trilayers
(Physical review B, 2007) Dantas, Ana L.; Camley, R. E.; Carriço, Artur da Silva
We have performed a theoretical investigation of thermal hysteresis in the magnetization of Fem/Dyn /Fem and Gdm/Dyn /Gdm trilayers, where the interface coupling is antiferromagnetic and ferromagnetic, respectively. Our results show a large number of stable states can exist in these structures. The thermal hysteresis occurs because the transitions between these states occur at different temperatures for the heating and cooling pro- cesses. The thermal hysteresis can be quite large, spanning about 90 K. Furthermore the temperature span of the thermal hysteresis can be substantially reduced by applying a modest external magnetic field.
Magnetocaloric effect of thin Dy films
(ELSEVIER, 2006) Mello, V.D.; Dantas, Ana L.; Carriço, Artur da Silva
We report a theoretical study of the magnetocaloric effect (MCE) of Dy thin films, and show that the confinement of spins in the direction perpendicular to the surfaces leads to a considerable enhancement of the MCE. The adiabatic temperature change, 1T , for external field strength change, 1H, of the order of a few kOe, is much larger than what is found in bulk Dy, reaching 1T 1H = 6 K/T for ultra-thin films.
Monodisperse sodium oleate coated magnetite high susceptibility nanoparticles for hyperthermia applications
(Elsevier B.V., 2014-08-13) Carriço, Artur da Silva; Araújo Neto, R. P.; Silva Freitas, E. L.; Carvalho, J. F.; Pontes, T. R. F.; Silva, K. L.; Damasceno, I. H. M.; Egito, E. S. T.; Dantas, Ana L.; Morales, Marco A.
We report a simple and low cost methodology to synthesize sodium oleate coated magnetite nanoparticles for hyperthermia applications. The system consists of oleate coated magnetite nanoparticles with large susceptibility (1065 emu/gT), induced by the dipolar inter-particle interaction, with a magnetic core diameter in the 6 nm–12 nm size range. In aqueous medium, the nanoparticles agglomerate to form a monodisperse system, exhibiting a mean hydrodynamic diameter of 60.6 nm±4.1 nm, with a low average polydispersity index of 0.128±0.003, as required for intravenous applications. The system exhibits promising efficiency for magnetic hyperthermia, with a specific absorption rate of 14 W/g at a low field amplitude of 15.9 kA/m and frequency of 62 kHz. In a 50 mg/mL density in 1 mL, the temperature rises to 42.5 °C in 1.9 min.
Nucleation of vortex pairs in exchange biased nanoelements
(American Institute of Physics, 2011-03-24) Carriço, Artur da Silva; Rebouças, G. O. G.; Dantas, Ana L.; Silva, A. S. M.
We report a theoretical investigation of interface effects in the magnetic order of interface biased iron and Permalloy™ elliptical nano-elements. Contrary to intuition, there is a partial pinning of the interface layer, favoring double vortex states along the hysteresis loop. Interface biasing affects the relative chirality and the distance of the vortices. Unbiased nanoelements may nucleate vortex pairs with the same chirality separated by an antivortex. For interface biased nanoelements the vortex pair forms with opposite chirality separated by a magnetic domain.
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 Silva
We 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.
Synthesis of magnetite nanoparticles by high energy ball milling
(Elsevier B.V., 2013) Carriço, Artur da Silva; Carvalho, J. F.; Medeiros, S. N.; Morales, M. A.; Dantas, Ana L.
We report on the preparation of magnetite nanoparticles, with size ranging from 12 nm to 20 nm, by high energy ball milling. The synthesis is made using stoichiometric amounts of distilled water and metallic iron powder. The milled powder samples were analyzed by Mossbauer spectroscopy (MS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Our results indicate that the milling time is a key parameter of the synthesis. By increasing the milling time one achieves high purity magnetite samples. Also, the particle size decreases with the milling time. The sample milled during 10 h contained a fraction of 56 nm metallic Fe particles and 20 nm magnetite particles. By increasing the milling time to 96 h we have obtained a sample that is mainly composed of 12 nm magnetite particles. MS performed at room temperature showed a spectrum consisting of two sextets with hyperfine parameters related to iron ions occupying octahedral (A) and tetrahedral (B) sites. We have used a self-consistent method to investigate the impact of the dipolar interaction to drive the system to a magnetically blocked regime.
Synthesis of stoichiometric Ca2Fe2O5 nanoparticles by high-energy ball milling and thermal annealing
(Elsevier, 2016-05-01) Amorim, Bruno Ferreira; Morales, Marco Antonio Morales; Bohn, Felipe; Carriço, Artur Silva; Medeiros, Suzana Nóbrega de; Dantas, Ana L.
We report the synthesis of Ca2Fe2O5 nanoparticles by high-energy ball milling and thermal annealing from α-Fe2O3 and CaCO3. Magnetization measurements, Mössbauer and X-ray spectra reveal that annealing at high temperatures leads to better quality samples. Our results indicate nanoparticles produced by 10 h high-energy ball milling and thermal annealing for 2 h at 1100 °C achieve improved stoichiometry and the full weak ferromagnetic signal of Ca2Fe2O5. Samples annealed at lower temperatures show departure from stoichiometry, with a higher occupancy of Fe3+ in octahedral sites, and a reduced magnetization. Thermal relaxation for temperatures in the 700–1100 °C range is well represented by a Néel model, assuming a random orientation of the weak ferromagnetic moment of the Ca2Fe2O5 nanoparticles

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Navegando por Autor "Dantas, Ana L."