Papers by renato guimaraes
Materials Chemistry and Physics, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Journal of Fish Diseases, 2017
Zooarchaeological collections from shell mounds in Rio de Janeiro (2,470-4,632 cal BP) contain a ... more Zooarchaeological collections from shell mounds in Rio de Janeiro (2,470-4,632 cal BP) contain a high prevalence of swollen fish bones belonging to the Atlantic spadefish (Chaetodipterus faber), crevalle jack (Caranx hippos) and fat snook (Centropomus parallelus). Given the lack of knowledge of the bone degenerative process in senile fishes, this study analysed hyperostotic bone in zooarchaeological and modern specimens to obtain high-resolution morphology and microstructure reconstruction. We used microCT as well as X-ray diffraction to characterize the crystallographic changes associated with fish senility. Our results showed that trabecular microstructures in hyperostotic bones were consistent with estimated values of the per cent bone volume-to-total volume ratio (BV/TV) and were greater than 60% in cortical bone. Hyperostotic bones indicated a high radiograph density, and X-ray diffractograms showed a decrease in hydroxyapatite [Ca 10 (PO 4) 6 (OH) 2 ] and calcite (CaCO 3) neocrystallization. These crystalline and density changes revealed an advanced stage of fish senile and indicate the vulnerability of ageing fish populations.
AIP Advances, 2012
We report on the magnetic properties of a special configuration of a FeRh thin film. An anomalous... more We report on the magnetic properties of a special configuration of a FeRh thin film. An anomalous behavior on the magnetisation vs. temperature was observed when low magnetic fields are applied in the plane of a thin layer of FeRh deposited on ordered Fe3Pt. The anomalous effect resembles a metamagnetic transition and occur only in the field-cooled-cooling magnetisation curve at temperatures near 120 K in samples without any heat treatment.
Journal of Applied Physics, 1986
Profiles of Bi, Xe, Sn, Kr, Ga, Fe, K, Ar, P, Na, and F implanted into the AZ111 photoresist are ... more Profiles of Bi, Xe, Sn, Kr, Ga, Fe, K, Ar, P, Na, and F implanted into the AZ111 photoresist are compared with recent theoretical predictions. With the exception of the noble gases and F, the experimental results are well fitted by the Biersack–Haggmark [Nucl. Instrum. Methods 174, 257 (1980)] Monte Carlo calculations. For the noble gases we obtain ranges up to a factor of 2 shorter than the above predictions. Fluor changes the profile as function of energy, being nearly Gaussian at 30 keV and distributing according to the calculated ionization at 70 keV.
Journal of Alloys and Compounds, Sep 1, 2012
Lanthanum oxyfluoride (LaOF) powders doped with Eu 3+ and co-doped with Eu 3+ and Yb 3+ were prep... more Lanthanum oxyfluoride (LaOF) powders doped with Eu 3+ and co-doped with Eu 3+ and Yb 3+ were prepared by combustion synthesis. Surface morphology and structure of the powders were investigated by scanning electronic microscopy, energy dispersive X-ray and X-ray powder diffraction. Luminescence experiments were performed in singly doped (Eu 3+) samples illuminated with ultraviolet light (k = 255 nm) and co-doped (Eu 3+ :Yb 3+) samples exposed to near-infrared diode laser (k = 975 nm). The characteristic red luminescence of Eu 3+ corresponding to 5 D 0 ? 7 F J transitions was observed in both cases. The Judd-Ofelt intensity parameters X 2 and X 4 were calculated from experimental data and luminescence quantum efficiencies were estimated. The near-infrared to visible up-conversion luminescence achieved in co-doped samples is due to energy transfer (ET) from pairs of Yb 3+ to Eu 3+. The ET rate was estimated by use of a rate equation model adjusted to the dynamics of up-conversion luminescence.
Physical Review B, 2000
We report specific-heat measurements for the homometallic Fe oxyborate Fe 3 O 2 BO 3. This materi... more We report specific-heat measurements for the homometallic Fe oxyborate Fe 3 O 2 BO 3. This material has a charge ordering crossover at T co Ϸ220 K as shown by the transport measurements. Below T co we find a large linear temperature-dependent contribution to the specific heat that we identify as due to the excitations of a Wigner glass phase. At lower temperatures we observe two large anomalies associated with magnetic transitions. The lowest temperature specific heat is dominated by two-dimensional antiferromagnetic magnons.
Journal of Molecular Structure: THEOCHEM, 2001
Geometrical and Fe±Fe interaction effects on the charge states of the transition metal in the hom... more Geometrical and Fe±Fe interaction effects on the charge states of the transition metal in the homometallic ludwigite Fe 3 O 2 BO 3 have been investigated by using the extended Hu Èckel method. It was found that the high positive charge state of Fe(2), a result not generally found in bimetallic ludwigites, is due to a d±d splitting of the kind s±s p of the t 2g crystalline band. The splitting creates an empty band above the Fermi level and comes from the strong Fe(2)±Fe(3) interaction. The proximity of the empty band to the Fermi level (,0.2 eV above it) suggests the possibility of a high spin con®guration in the compound, whose density would be located in the region around the 3±2±3 crystalline sites.
Physical Review B, 2001
In this paper we report magnetic and specific heat results for the oxyborates Fe 2 OBO 3 and Mn 2... more In this paper we report magnetic and specific heat results for the oxyborates Fe 2 OBO 3 and Mn 2 OBO 3. The former presents L-type ferrimagnetism while the latter presents weak ferromagnetism in distinct ranges of temperature. The Curie constant for Mn 2 OBO 3 is normal while that for Fe 2 OBO 3 is unusually high, as a consequence of short-range ferromagnetic order. The specific heat of both systems at low temperatures is dominated by the magnetic excitations of the ordered magnetic phases. In order to subtract the lattice contribution to the specific heat in these systems, we measured the specific heat of the nearly isostructural, nonmagnetic compound MgScOBO 3. The remaining specific heat contains a magnetic part and an additional component associated with charge delocalization.
Physical Review B, 1994
We present x-ray, susceptibility, and magnetization (up to 18 T) measurements in the compound Mg ... more We present x-ray, susceptibility, and magnetization (up to 18 T) measurements in the compound Mg TiOBO3, which forms in the structure known as warwickite. This structure is characterized by the presence of ribbons along the c axis where the metallic ions are located. This system is magnetic since the Ti ions are in the valence state Ti + with the electronic configuration 3d'. Its susceptibility has a temperature dependence, at low temperatures, given by g(T)~T, with the exponent tr=0.83. The low-temperature behavior of the magnetization, as a function of field, is described by m~H " with y=1a. These results, together with the x-ray measurements, allow us to identify this material as belonging to a family of inorganic, one-dimensional, disordered magnetic systems.
Physical Review B, 2003
We present an investigation of the magnetic properties of the pyroborates Mn 2 B 2 O 5 and MnMgB ... more We present an investigation of the magnetic properties of the pyroborates Mn 2 B 2 O 5 and MnMgB 2 O 5 using magnetization and electron-paramagnetic-resonance measurements. The pyroborates have in common with another well studied family of borates, the warwickites, substructures in the form of ribbons where the metals are located. The homometallic Mn 2 B 2 O 5 system is shown to be a conventional three-dimensional antiferromagnet with Neel temperature T N Ϸ24 K. Its magnetization isotherms present steps indicating spin-flop-like transitions involving distinct pairs of metal sites. The heterometallic compound MnMgB 2 O 5 shows a susceptibility with a power-law temperature dependence (T)ϰT Ϫ␣ , down to Tϭ1.8 K and a magnetization with a power-law field dependence M (H)ϰH (1Ϫ␣) at low temperatures. This behavior is characteristic of a random singlet phase, in this case, in a system with spin Sϭ5/2. We discuss our magnetic results on the pyroborates in connection with their structure and compare them with those obtained in the warwickites.
We present an extensive study of the structural, magnetic and thermodynamic properties of the two... more We present an extensive study of the structural, magnetic and thermodynamic properties of the two hetero- metallic oxy-borates: Co2FeO2BO3 and Ni2FeO2BO3. This has been carried out through X-ray difiraction at RT and 150 K, d.c. and a.c. magnetic susceptibilities, and speciflc heat experiments in single crystals. The magnetic properties of these iron ludwigites are discussed in comparison with those of the other two known homo-metallic ludwigites: Fe3O2BO3 and Co3O2BO3. In the homo-metallic iron ludwigite, Fe3O2BO3, the coexistence of Fe 3+ magnetic ordering and Fe 2+ paramagnetism (1,2) has been observed. Also, a subtle structural change, which dimerises pairs of iron ions along structural sub-units in the form of 3-leg-ladders (3LL) occurs in this compound (3,4). These observations suggested an investigation of the structure and magnetic properties of the only other known homo-metallic ludwigite, Co3O2BO3. Surprisingly, a careful study showed that the homo-metallic cobalt ludwigit...
Physical Review B, 2005
We study the transport properties of the charge-density-wave system Fe 3 O 2 BO 3. ac conductivit... more We study the transport properties of the charge-density-wave system Fe 3 O 2 BO 3. ac conductivity measurements for different frequencies are presented for temperatures above and below the structural transition. dc conductivity, as a function of temperature and pressure, yields the variation of the transition temperature with external pressure. Below this transition the conductivity is thermally activated in a wide range of temperature and the gap obtained is strongly pressure dependent. The ac conductivity at sufficiently low temperatures below the transition is ascribed to the excitation of local defects associated with domain walls and which are characteristic of the one-dimensional nature of the Fe 3 O 2 BO 3 system.
Physica B: Condensed Matter, 2000
Ludwigites are low-dimensional magnetic systems with chemical formula 2MO) MBO. For the ludwigite... more Ludwigites are low-dimensional magnetic systems with chemical formula 2MO) MBO. For the ludwigites in which M"Fe and M"Mg, Fe, Ni and Cu we report the following behavior: (1) The ordering temperatures of the Fe> subsystem in each compound is higher than that of the respective M subsystem ; (2) In particular, the 2FeO) FeBO ludwigite presents a reentrant weak ferromagnetism in the range 40}70 K and the coexistence of paramagnetic and antiferromagnetic spins between 70 and 112 K 2000 Elsevier Science B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials, 2001
Features of the magnetic and charge ordering of the Fe O BO ludwigite have been studied using MoK... more Features of the magnetic and charge ordering of the Fe O BO ludwigite have been studied using MoK ssbauer spectroscopy in the range of 4 K(¹(120 K. From the temperature dependence of the magnetic hyper"ne "elds (B), associated to four di!erent iron sites, three magnetic transitions, at 112, 73 and 40 K, can be distinguished in agreement with previous magnetization measurements. In the temperature region where a weak ferromagnetism appears, the B of some Fe sites show two di!erent values related with di!erent canting angles.
The European Physical Journal B, 1999
The system Cu2FeO2BO3 is an oxyborate belonging to the family of the ludwigites. In this paper we... more The system Cu2FeO2BO3 is an oxyborate belonging to the family of the ludwigites. In this paper we present AC susceptibility, magnetization measurements and Mössbauer spectroscopy on this material which allows for a complete characterization of its complex magnetic behavior. We find an hierarchy of interactions which clearly defines three regimes with decreasing temperature. These are associated with, the freezing of the Fe moments, the antiferromagnetic ordering of the Cu sub-lattice and finally the coupling between both systems.
Chemical Physics Letters, 2008
Red light (two bands peaked at λ ∼680 and ∼697nm), visible by naked eye, is emitted by crystallin... more Red light (two bands peaked at λ ∼680 and ∼697nm), visible by naked eye, is emitted by crystalline ceramic powders of NdAlO3 prepared by direct combustion synthesis when the sample is irradiated by unfocused ultraviolet pulsed laser light (λ=337nm). This is the first observation of luminescence at the visible from NdAlO3 structure. The red emission can be explained by assuming
Physical Review B, 1998
We present an investigation of the magnetic properties of the ludwigite Ni 2 FeO 2 BO 3. This mat... more We present an investigation of the magnetic properties of the ludwigite Ni 2 FeO 2 BO 3. This material is an oxyborate that presents in its crystalline structure subunits in the form of walls where the transition metal ions are located. Our Mössbauer, ac susceptibility, and magnetization measurements show that Fe and Ni ions form two independent subsystems in this material down to T N ϭ15 K. We explain this behavior in terms of a hierarchy of interactions. ͓S0163-1829͑98͒01325-3͔
Physical Review B, 2010
operates a facility that consists of 460 acres of fairly shallow lagoons (~ 5'deep) for biologica... more operates a facility that consists of 460 acres of fairly shallow lagoons (~ 5'deep) for biological wastewater treatment that meets targets for primary and secondary treatments (solids, biological oxygen demand (BOD), and pathogen removal). Significant natural algal growth occurs in these lagoons, which improves BOD removal through oxygenation and also facilitates N removal through volatilization as ammonia under high pH conditions created by algal growth. Phosphorus, however, is non-volatile and stays in the water and likely cycles in and out of algal cells as they grow and die in the lagoons. In the near future, the regulatory limits on phosphorus released from the Logan wastewater treatment facility are likely to become significantly lower to counter potential downstream eutrophication. One way to potentially lower phosphorus levels in the wastewater effluent is through management of iv algal growth in the lagoons. As mentioned above, algae growth naturally occurs in the treatment lagoons and if the algal biomass is harvested when growth yields are highest, the phosphorus contained in the cells could be removed to obtain phosphorus-free water. The algal biomass could then be used for production of biofuels. This research focuses on laboratory and pilot assessments to show the ability of algae indigenous to the Logan lagoons to uptake phosphorus and produce biomass that can be used for biofuel production. (71 pages) v ACKNOWLEDGMENTS I would like to thank the Logan City Environmental Department for funding this project along with the Department of Biological and Irrigation Engineering. I also would like to thank everyone involved in the project for their knowledge and support. I would especially like to thank my committee members, Dr. Sridhar Viamajala, Dr. Ronald Sims, and Mr. Issa Hamud, for their support and assistance. Special thanks to my family for their support, especially my wife, Beckie, for her love and support throughout this process.
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Papers by renato guimaraes