Antonio Azevedo | UFPA - Federal University of Pará (original) (raw)
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Papers by Antonio Azevedo
Journal of The Operational Research Society, 2000
Physical Review B, 2006
The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized dire... more The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized direct current is studied. In such a system, spin waves ͑magnons͒ may be critically driven out of equilibrium by an effective spin-injection field that is proportional to the current density. A direct comparison between the predicted critical current and previous experimental results sheds light on the nature of the excited mode. Beyond the threshold, it is assumed that the spin waves are coupled through nonlinear interactions arising from dipolar and surface anisotropy energies. It is shown that the magnon-magnon interactions play two major roles in the dynamics: ͑i͒ They govern and put a limit to the growth in the population of the unstable mode from the thermal level, and ͑ii͒ directly contribute to the renormalization of the magnon energy, which manifests itself through a shift in the precession frequency of the magnetic moments with varying current intensity. Numerical results are presented in remarkable quantitative agreement with recent experiments in nanometric magnetic multilayers, where microwave oscillations generated by direct currents have been observed in the postthreshold regime.
Physical Review Letters, 2000
The technique of Brillouin light scattering is used to observe strong excitation of magnons in an... more The technique of Brillouin light scattering is used to observe strong excitation of magnons in antiferromagnetically coupled trilayers of Fe/Cr/Fe at room temperature. The magnons are driven out of equilibrium by a microwave current applied in the trilayer through point contacts. The magnitude of the scattering intensity is investigated as a function of the magnon wave number and applied magnetic field. Confirming recent theoretical predictions, the observations provide strong evidence of electronic spin injection in the rf driving field.
Physical Review B, 2000
Ferromagnetic resonance has been used to study the room-temperature linewidth and frequency shift... more Ferromagnetic resonance has been used to study the room-temperature linewidth and frequency shift of the q=0 spin-wave mode in thin films of NiFe sputtered on Si(100) substrates. The data on the variation of the linewidth and resonance field with film thickness are completely ...
Physical Review Letters, 1991
A microwave-pumped spin-wave-instability experiment is used to demonstrate that chaos can be cont... more A microwave-pumped spin-wave-instability experiment is used to demonstrate that chaos can be controlled by a small periodic perturbation of an available system parameter, as recently proposed by Ott, Grebogi, and Yorke. The experiment is performed in an yttrium-iron-garnet sphere ...
Physical Review Letters, 2005
A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is tr... more A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is traversed by spin-polarized carriers at high direct-current densities. It is shown that nonlinear effects due to four-magnon interactions arising from dipolar and surface anisotropy energies limit the growth of the driven spin wave and produce shifts in the microwave frequency oscillations. The theory explains quantitatively recent experimental results in nanometric point contacts onto magnetic multilayers showing downward frequency shifts (redshifts) with increasing current, if the external field is on the film plane, and upward shifts (blueshifts), if the field is perpendicular to the film.
Physica B-condensed Matter, 2006
We describe the influence of the temperature of sintering on the structure and the static magneti... more We describe the influence of the temperature of sintering on the structure and the static magnetic properties in Ni 0.5 Zn 0.5 Sm x Fe 2-x O 4 ferrites prepared by combustion synthesis. The increase in Sm content increases the lattice parameter and decreases the mean grain size. The material treated at 600 1C should be single spinel ferrite but with low magnetization saturation. Calcinations carried out at 800 and 1000 1C resulted in materials with cubic spinel phase, but a small amount of SmFeO 3 was also formed. A smooth diminution of the saturation magnetization with an increase in Sm amount was observed. The highest Curie temperatures were obtained for sample treatment at 1000 1C with higher Sm content. r
Coronary Artery Disease, 1998
New strategies to increase coronary patency rate before primary angioplasty are under discussion.... more New strategies to increase coronary patency rate before primary angioplasty are under discussion. We tested the hypothesis that use of a high dose of a standard heparin bolus could achieve an acceptable rate of re-opening occluded infarct-related arteries thus providing an alternative to chemical thrombolysis before admission of the patient to hospital, and a pretreatment for primary angioplasty. Forty-eight patients who presented within 12 h of acute myocardial infarction with ST segment elevation were assigned randomly to groups to receive aspirin (200 mg orally) and high-dose standard heparin 300 U/kg as an intravenous bolus (n = 25), or aspirin and placebo bolus (n = 23). Thereafter, all patients underwent coronary arteriography to assess their suitability for primary angioplasty. The high-dose heparin group had greater patency rate (Thrombolysis in Myocardial Infarction grade 2 or 3 flow in the infarct-related artery) than the placebo group (52% compared with 13%, P = 0.006). Hemorrhages related to the puncture site that required blood transfusion occurred in two of 25 and in one of 23 patients in the high-dose heparin and placebo groups, respectively. Our study suggests that high-dose standard heparin does have a thrombolytic action when administered as an intravenous bolus.
Coronary Artery Disease, 1998
New strategies to increase coronary patency rate before primary angioplasty are under discussion.... more New strategies to increase coronary patency rate before primary angioplasty are under discussion. We tested the hypothesis that use of a high dose of a standard heparin bolus could achieve an acceptable rate of re-opening occluded infarct-related arteries thus providing an alternative to chemical thrombolysis before admission of the patient to hospital, and a pretreatment for primary angioplasty. Forty-eight patients who presented within 12 h of acute myocardial infarction with ST segment elevation were assigned randomly to groups to receive aspirin (200 mg orally) and high-dose standard heparin 300 U/kg as an intravenous bolus (n = 25), or aspirin and placebo bolus (n = 23). Thereafter, all patients underwent coronary arteriography to assess their suitability for primary angioplasty. The high-dose heparin group had greater patency rate (Thrombolysis in Myocardial Infarction grade 2 or 3 flow in the infarct-related artery) than the placebo group (52% compared with 13%, P = 0.006). Hemorrhages related to the puncture site that required blood transfusion occurred in two of 25 and in one of 23 patients in the high-dose heparin and placebo groups, respectively. Our study suggests that high-dose standard heparin does have a thrombolytic action when administered as an intravenous bolus.
Journal of The Operational Research Society, 2000
Physical Review B, 2006
The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized dire... more The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized direct current is studied. In such a system, spin waves ͑magnons͒ may be critically driven out of equilibrium by an effective spin-injection field that is proportional to the current density. A direct comparison between the predicted critical current and previous experimental results sheds light on the nature of the excited mode. Beyond the threshold, it is assumed that the spin waves are coupled through nonlinear interactions arising from dipolar and surface anisotropy energies. It is shown that the magnon-magnon interactions play two major roles in the dynamics: ͑i͒ They govern and put a limit to the growth in the population of the unstable mode from the thermal level, and ͑ii͒ directly contribute to the renormalization of the magnon energy, which manifests itself through a shift in the precession frequency of the magnetic moments with varying current intensity. Numerical results are presented in remarkable quantitative agreement with recent experiments in nanometric magnetic multilayers, where microwave oscillations generated by direct currents have been observed in the postthreshold regime.
Physical Review Letters, 2000
The technique of Brillouin light scattering is used to observe strong excitation of magnons in an... more The technique of Brillouin light scattering is used to observe strong excitation of magnons in antiferromagnetically coupled trilayers of Fe/Cr/Fe at room temperature. The magnons are driven out of equilibrium by a microwave current applied in the trilayer through point contacts. The magnitude of the scattering intensity is investigated as a function of the magnon wave number and applied magnetic field. Confirming recent theoretical predictions, the observations provide strong evidence of electronic spin injection in the rf driving field.
Physical Review B, 2000
Ferromagnetic resonance has been used to study the room-temperature linewidth and frequency shift... more Ferromagnetic resonance has been used to study the room-temperature linewidth and frequency shift of the q=0 spin-wave mode in thin films of NiFe sputtered on Si(100) substrates. The data on the variation of the linewidth and resonance field with film thickness are completely ...
Physical Review Letters, 1991
A microwave-pumped spin-wave-instability experiment is used to demonstrate that chaos can be cont... more A microwave-pumped spin-wave-instability experiment is used to demonstrate that chaos can be controlled by a small periodic perturbation of an available system parameter, as recently proposed by Ott, Grebogi, and Yorke. The experiment is performed in an yttrium-iron-garnet sphere ...
Physical Review Letters, 2005
A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is tr... more A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is traversed by spin-polarized carriers at high direct-current densities. It is shown that nonlinear effects due to four-magnon interactions arising from dipolar and surface anisotropy energies limit the growth of the driven spin wave and produce shifts in the microwave frequency oscillations. The theory explains quantitatively recent experimental results in nanometric point contacts onto magnetic multilayers showing downward frequency shifts (redshifts) with increasing current, if the external field is on the film plane, and upward shifts (blueshifts), if the field is perpendicular to the film.
Physica B-condensed Matter, 2006
We describe the influence of the temperature of sintering on the structure and the static magneti... more We describe the influence of the temperature of sintering on the structure and the static magnetic properties in Ni 0.5 Zn 0.5 Sm x Fe 2-x O 4 ferrites prepared by combustion synthesis. The increase in Sm content increases the lattice parameter and decreases the mean grain size. The material treated at 600 1C should be single spinel ferrite but with low magnetization saturation. Calcinations carried out at 800 and 1000 1C resulted in materials with cubic spinel phase, but a small amount of SmFeO 3 was also formed. A smooth diminution of the saturation magnetization with an increase in Sm amount was observed. The highest Curie temperatures were obtained for sample treatment at 1000 1C with higher Sm content. r
Coronary Artery Disease, 1998
New strategies to increase coronary patency rate before primary angioplasty are under discussion.... more New strategies to increase coronary patency rate before primary angioplasty are under discussion. We tested the hypothesis that use of a high dose of a standard heparin bolus could achieve an acceptable rate of re-opening occluded infarct-related arteries thus providing an alternative to chemical thrombolysis before admission of the patient to hospital, and a pretreatment for primary angioplasty. Forty-eight patients who presented within 12 h of acute myocardial infarction with ST segment elevation were assigned randomly to groups to receive aspirin (200 mg orally) and high-dose standard heparin 300 U/kg as an intravenous bolus (n = 25), or aspirin and placebo bolus (n = 23). Thereafter, all patients underwent coronary arteriography to assess their suitability for primary angioplasty. The high-dose heparin group had greater patency rate (Thrombolysis in Myocardial Infarction grade 2 or 3 flow in the infarct-related artery) than the placebo group (52% compared with 13%, P = 0.006). Hemorrhages related to the puncture site that required blood transfusion occurred in two of 25 and in one of 23 patients in the high-dose heparin and placebo groups, respectively. Our study suggests that high-dose standard heparin does have a thrombolytic action when administered as an intravenous bolus.
Coronary Artery Disease, 1998
New strategies to increase coronary patency rate before primary angioplasty are under discussion.... more New strategies to increase coronary patency rate before primary angioplasty are under discussion. We tested the hypothesis that use of a high dose of a standard heparin bolus could achieve an acceptable rate of re-opening occluded infarct-related arteries thus providing an alternative to chemical thrombolysis before admission of the patient to hospital, and a pretreatment for primary angioplasty. Forty-eight patients who presented within 12 h of acute myocardial infarction with ST segment elevation were assigned randomly to groups to receive aspirin (200 mg orally) and high-dose standard heparin 300 U/kg as an intravenous bolus (n = 25), or aspirin and placebo bolus (n = 23). Thereafter, all patients underwent coronary arteriography to assess their suitability for primary angioplasty. The high-dose heparin group had greater patency rate (Thrombolysis in Myocardial Infarction grade 2 or 3 flow in the infarct-related artery) than the placebo group (52% compared with 13%, P = 0.006). Hemorrhages related to the puncture site that required blood transfusion occurred in two of 25 and in one of 23 patients in the high-dose heparin and placebo groups, respectively. Our study suggests that high-dose standard heparin does have a thrombolytic action when administered as an intravenous bolus.