Amanda Arelaro - Academia.edu (original) (raw)
Papers by Amanda Arelaro
In-field M�ssbauer characterization of MFe2O4 (M = Fe, Co, Ni) nanoparticles
Journal of Physics Conference Series, 2010
Nanoparticle samples of Fe3O4, CoFe2O4 and NiFe2O4 with mean sizes in the 5-8 nm range and excell... more Nanoparticle samples of Fe3O4, CoFe2O4 and NiFe2O4 with mean sizes in the 5-8 nm range and excellent crystallinity have been synthesized by thermal decomposition of organic precursors. Mössbauer spectra taken at 4.2 K in applied fields from 0 to 12 T could be fitted with two sextets corresponding to the A and B Fe sites of the spinel structure. The cation distribution was deduced from the relative intensities of both sextets. The Fe oxide spectrum was more similar to γ-Fe2O3 than to Fe3O4, suggesting this sample has undergone partial or total oxidation. A small residual intensity at lines 2 and 5 reveals some spin canting at both Fe sites, however the absence of a third sextet indicates that a "core-shell" spin structure, with disordered spins near the surface, does not occur in these nanoparticles.
Physica B: Condensed Matter, 2008
We investigate the formation of ferrihydrite nanoparticles (NPs) by hydrolysis of the Fe(III) alk... more We investigate the formation of ferrihydrite nanoparticles (NPs) by hydrolysis of the Fe(III) alkoxide Fe(O t Bu) 3 . Controlled amounts of water, up to 3.0 vol%, were added to the precursor solution yielding a series of hydrolyzed samples ranging from P0.0 (the unreacted precursor) to P3.0. X-ray diffraction (XRD) analysis evidenced the formation of high-crystalline ferrihydrite NP in sample P3.0, with grain size estimate of about 3.2 nm. The transition from the molecular precursor to the formation of crystalline magnetic NPs was followed through magnetization measurements M(T) and M(H), as well as Mö ssbauer spectroscopy (MS). M(T) measurements indicate a paramagnetic (PM) behavior for sample P0.0, characteristic of binuclear Fe-O-Fe units, which evolves to a superparamagnetic (SPM) behavior, with an energy barrier for the blocking process estimated for sample P3.0 as E a ¼ 4.9 Â 10 À21 J (E a /k B ¼ 355 K), resulting in a high effective anisotropy constant K eff ¼ 290 kJ/m 3 . Magnetization loops at 5 K progressively change from PM-like to ferromagnetic-like shape upon increasing the hydrolysis process, although hysteresis (H c E500 Oe) only is apparent for P2.0 and higher. MS spectra at room temperature are PM/SPM doublets for all samples, while the MS spectra at T ¼ 4.2 K reveal increasingly well-defined magnetic ordering as hydrolysis of the precursor stepwise progresses until wellcrystallized ferrihydrite particles are formed.
In-field Mössbauer characterization of MFe 2 O 4 (M = Fe, Co, Ni) nanoparticles
Journal of Physics: Conference Series, 2010
... 1Corresponding author. E-mail: hercilio@if.usp.br Abstract. ... Rev. Lett. 77 394-7 [8] Reche... more ... 1Corresponding author. E-mail: hercilio@if.usp.br Abstract. ... Rev. Lett. 77 394-7 [8] Rechenberg HR, Sousa EC, Depeyrot J, Sousa MH, Aquino R, Tourinho FA, Perzynski R (2008) Hyperfine Interact. 184 423-8 [9] Mørup S, Topsøe H (1996) J. Magn. Magn. Mater. ...
Journal of Magnetism and Magnetic Materials, 2008
The influence of different M 2+ cations on the effective magnetic anisotropy of systems composed ... more The influence of different M 2+ cations on the effective magnetic anisotropy of systems composed of MFe 2 O 4 (M ¼ Fe, Co and Mn) nanoparticles was investigated. Samples were prepared by the high-temperature (538 K) solution phase reaction of Fe (acac) 3 , Co (acac) 2 and Mn (acac) 2 with 1,2 octanodiol in the presence of oleic acid and oleylamine. The final particles are coated by an organic layer of oleic acid that prevents agglomeration. Transmission electron microscopy (TEM) images show that particles present near spherical form and a narrow grain size distribution, with mean diameters in the range of 4.5-7.6 nm. Powder samples were analyzed by ac susceptibility and Mo¨ssbauer measurements, and K eff for all samples was evaluated using both techniques, showing a strong dependence on the nature of the divalent cation. r
Journal of Applied Physics, 2005
We have investigated the dynamic magnetic properties of dextran-coated magnetite ͑Fe 3 O 4 ͒ nano... more We have investigated the dynamic magnetic properties of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles in the form of ͑a͒ particles suspended in a carrier liquid and ͑b͒ concentrated powder obtained from lyophilization. The blocking temperature was found to increase from T B =42͑2͒ to 52͑2͒ K ͑@ 0 H =10 mT͒ after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field B hyp ͑T͒ reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy E a = 3.6ϫ 10 −21 J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K 1 =10 kJ/m 3 was obtained by assuming the cubic phase with easy magnetic direction ͓111͔ of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at T V could be observed in our measurements. From the dynamical parameters of ac susceptibility ͑f , T͒ curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be ⍀ = 4.5ϫ 10 −21 J, larger than the single-particle value.
![Research paper thumbnail of Spin disorder and magnetic anisotropy in Fe[sub 3]O[sub 4] nanoparticles](https://attachments.academia-assets.com/47677772/thumbnails/1.jpg)
](Journal of Applied Physics, 2006
We have studied the magnetic behavior of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles with ... more We have studied the magnetic behavior of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles with median particle size ͗d͘ = 8 nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment M S of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy K eff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, T B ͑H , ͒, suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions.
IEEE Transactions on Magnetics, 2000
We have studied the magnetic and power absorption properties of a series of magnetic nanoparticle... more We have studied the magnetic and power absorption properties of a series of magnetic nanoparticles (MNPs) of Fe 3 O 4 with average sizes d ranging from 3 to 26 nm. Heating experiments as a function of particle size revealed a strong increase in the specific power absorption (SPA) values for particles with d = 25-30 nm. On the other side saturation magnetization M S values of these MNPs remain essentially constant for particles with d above 10 nm, suggesting that the absorption mechanism is not determined by M S . The largest SPA value obtained was 130 W/g, corresponding to a bimodal particle distribution with average size values of 17 and 26 nm.
In-field M�ssbauer characterization of MFe2O4 (M = Fe, Co, Ni) nanoparticles
Journal of Physics Conference Series, 2010
Nanoparticle samples of Fe3O4, CoFe2O4 and NiFe2O4 with mean sizes in the 5-8 nm range and excell... more Nanoparticle samples of Fe3O4, CoFe2O4 and NiFe2O4 with mean sizes in the 5-8 nm range and excellent crystallinity have been synthesized by thermal decomposition of organic precursors. Mössbauer spectra taken at 4.2 K in applied fields from 0 to 12 T could be fitted with two sextets corresponding to the A and B Fe sites of the spinel structure. The cation distribution was deduced from the relative intensities of both sextets. The Fe oxide spectrum was more similar to γ-Fe2O3 than to Fe3O4, suggesting this sample has undergone partial or total oxidation. A small residual intensity at lines 2 and 5 reveals some spin canting at both Fe sites, however the absence of a third sextet indicates that a "core-shell" spin structure, with disordered spins near the surface, does not occur in these nanoparticles.
Physica B: Condensed Matter, 2008
We investigate the formation of ferrihydrite nanoparticles (NPs) by hydrolysis of the Fe(III) alk... more We investigate the formation of ferrihydrite nanoparticles (NPs) by hydrolysis of the Fe(III) alkoxide Fe(O t Bu) 3 . Controlled amounts of water, up to 3.0 vol%, were added to the precursor solution yielding a series of hydrolyzed samples ranging from P0.0 (the unreacted precursor) to P3.0. X-ray diffraction (XRD) analysis evidenced the formation of high-crystalline ferrihydrite NP in sample P3.0, with grain size estimate of about 3.2 nm. The transition from the molecular precursor to the formation of crystalline magnetic NPs was followed through magnetization measurements M(T) and M(H), as well as Mö ssbauer spectroscopy (MS). M(T) measurements indicate a paramagnetic (PM) behavior for sample P0.0, characteristic of binuclear Fe-O-Fe units, which evolves to a superparamagnetic (SPM) behavior, with an energy barrier for the blocking process estimated for sample P3.0 as E a ¼ 4.9 Â 10 À21 J (E a /k B ¼ 355 K), resulting in a high effective anisotropy constant K eff ¼ 290 kJ/m 3 . Magnetization loops at 5 K progressively change from PM-like to ferromagnetic-like shape upon increasing the hydrolysis process, although hysteresis (H c E500 Oe) only is apparent for P2.0 and higher. MS spectra at room temperature are PM/SPM doublets for all samples, while the MS spectra at T ¼ 4.2 K reveal increasingly well-defined magnetic ordering as hydrolysis of the precursor stepwise progresses until wellcrystallized ferrihydrite particles are formed.
In-field Mössbauer characterization of MFe 2 O 4 (M = Fe, Co, Ni) nanoparticles
Journal of Physics: Conference Series, 2010
... 1Corresponding author. E-mail: hercilio@if.usp.br Abstract. ... Rev. Lett. 77 394-7 [8] Reche... more ... 1Corresponding author. E-mail: hercilio@if.usp.br Abstract. ... Rev. Lett. 77 394-7 [8] Rechenberg HR, Sousa EC, Depeyrot J, Sousa MH, Aquino R, Tourinho FA, Perzynski R (2008) Hyperfine Interact. 184 423-8 [9] Mørup S, Topsøe H (1996) J. Magn. Magn. Mater. ...
Journal of Magnetism and Magnetic Materials, 2008
The influence of different M 2+ cations on the effective magnetic anisotropy of systems composed ... more The influence of different M 2+ cations on the effective magnetic anisotropy of systems composed of MFe 2 O 4 (M ¼ Fe, Co and Mn) nanoparticles was investigated. Samples were prepared by the high-temperature (538 K) solution phase reaction of Fe (acac) 3 , Co (acac) 2 and Mn (acac) 2 with 1,2 octanodiol in the presence of oleic acid and oleylamine. The final particles are coated by an organic layer of oleic acid that prevents agglomeration. Transmission electron microscopy (TEM) images show that particles present near spherical form and a narrow grain size distribution, with mean diameters in the range of 4.5-7.6 nm. Powder samples were analyzed by ac susceptibility and Mo¨ssbauer measurements, and K eff for all samples was evaluated using both techniques, showing a strong dependence on the nature of the divalent cation. r
Journal of Applied Physics, 2005
We have investigated the dynamic magnetic properties of dextran-coated magnetite ͑Fe 3 O 4 ͒ nano... more We have investigated the dynamic magnetic properties of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles in the form of ͑a͒ particles suspended in a carrier liquid and ͑b͒ concentrated powder obtained from lyophilization. The blocking temperature was found to increase from T B =42͑2͒ to 52͑2͒ K ͑@ 0 H =10 mT͒ after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field B hyp ͑T͒ reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy E a = 3.6ϫ 10 −21 J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K 1 =10 kJ/m 3 was obtained by assuming the cubic phase with easy magnetic direction ͓111͔ of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at T V could be observed in our measurements. From the dynamical parameters of ac susceptibility ͑f , T͒ curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be ⍀ = 4.5ϫ 10 −21 J, larger than the single-particle value.
Journal of Applied Physics, 2006
We have studied the magnetic behavior of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles with ... more We have studied the magnetic behavior of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles with median particle size ͗d͘ = 8 nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment M S of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy K eff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, T B ͑H , ͒, suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions.
IEEE Transactions on Magnetics, 2000
We have studied the magnetic and power absorption properties of a series of magnetic nanoparticle... more We have studied the magnetic and power absorption properties of a series of magnetic nanoparticles (MNPs) of Fe 3 O 4 with average sizes d ranging from 3 to 26 nm. Heating experiments as a function of particle size revealed a strong increase in the specific power absorption (SPA) values for particles with d = 25-30 nm. On the other side saturation magnetization M S values of these MNPs remain essentially constant for particles with d above 10 nm, suggesting that the absorption mechanism is not determined by M S . The largest SPA value obtained was 130 W/g, corresponding to a bimodal particle distribution with average size values of 17 and 26 nm.