P. B. Tavares

Publications (40)53.6 Total impact

• Article: Magnetically-induced lattice distortions and ferroelectricity in magnetoelectric GdMnO(3).

  J Agostinho Moreira, A Almeida, M R Chaves, J Kreisel, J Oliveira, F Carpinteiro, P B Tavares
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  ABSTRACT: In this work we investigate the magnetic field dependence of A(g) octahedra rotation (tilt) and B(2g) symmetric stretching modes frequencies at different temperatures. Our field-dependent Raman investigation at 10 K is interpreted by an ionic displacive nature of the magnetically-induced ferroelectric phase transition. The increasing frequency of the A(g) tilt mode with magnetic field gives evidence for the increase of the MnO(6) tilt angle, which in turn leads to a weakening of the ferromagnetic exchange interaction, yielding the necessary conditions for the onset of ferroelectricity on the basis of the inverse Dzyaloshinskii-Moriya interaction. The frequency change of the B(2g) symmetric stretching mode at the ferroelectric phase transition enables one to estimate the shift of oxygen ion positions and the corresponding spontaneous polarization of 480 μC m(-2) in magnitude, which is of the same order as earlier reported values in single crystals.
  Journal of Physics Condensed Matter 10/2012; 24(43):436002. · 2.55 Impact Factor
• Article: Competing exchanges and spin-phonon coupling in Eu1-xRxMnO3 (R=Y,Lu)

  D. A. Mota, Y. Romaguera Barcelay, P. B. Tavares, M. R. Chaves, A. Almeida, J. Oliveira, W. S. Ferreira, J. Agostinho Moreira
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  ABSTRACT: This work is focused on the phase diagrams and physical properties of Y-doped and Lu-doped EuMnO3. The differences in the corresponding phase boundaries in the (x,T) phase diagram could be overcome by considering a scaling of the Y3+ and Lu3+ concentrations to the tolerance factor. This outcome evidences that the tolerance factor is in fact a more reliable representative of the lattice deformation induced by doping. The normalization of the phase boundaries using the tolerance factor corroborates previous theoretical outcomes regarding the key role of competitive FM and AFM exchanges in determining the phase diagrams of manganite perovskites. Though, significant differences in the nature and number of phases at low temperatures and concentrations could not be explained by just considering the normalization to the tolerance factor. The vertical phase boundary observed just for Lu-doped EuMnO3, close to 10%Lu, is understood considering a low temperature Peierls-type spin-phonon coupling, which stabilizes the AFM4 phase in Lu-doped EuMnO3.
  08/2012;
• Article: Structure and physical properties of Eu0.8Y0.2MnO3 ceramics

  J. Agostinho Moreira, A. Almeida, W. S. Ferreira, M. R. Chaves, J. B. Oliveira, J. M. Machado da Silva, M. A. Sá, S. M. F. Vilela, P. B. Tavares
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  ABSTRACT: This work is addressed to study the crystal structure and morphology, as well as the thermodynamic, dielectric and magnetic properties of Eu0.8Y0.2MnO3 ceramics, synthesized by urea sol-gel combustion method. The experimental results were systematically compared with data available for the corresponding single crystals. Though the effect of the anisotropy on both dielectric and magnetic properties is missing, they enabled us to investigate the main physical mechanisms associated with their magnetoelectric properties, in particular the one which drives the ferroelectric phase. The phase sequence and critical temperatures are in good agreement with the corresponding values reported for single crystals. Similarly, structural results evidence strong distortions of the crystal lattice, enhancing the ferromagnetic interactions over the antiferomagnetic ones. A significant contribution of the magnetic fluctuations above TN was also evidenced from the magnetization studies. KeywordsSol-gel processing-Ceramics-Rare-earth manganites-Magnetoelectricity PACS classification codes81.20.Ev-75.80.+q-75.40.Cx-77.22. Gm
  Journal of Electroceramics 04/2012; 25(2):203-211. · 1.19 Impact Factor
• Source

  Available from: Joao Pedro Araujo

  Article: Perturbed angular correlations investigations on YMnO3 multiferroic manganite

  T. M. Mendonça, A. M. L. Lopes, J. N. Gonçalves, J. G. Correia, P. B. Tavares, V. S. Amaral, C. Darie, J. P. Araujo
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  ABSTRACT: The Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO3 multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure. KeywordsPAC–Manganites–Phase separation
  Hyperfine Interactions 04/2012; 197(1):83-88. · 0.21 Impact Factor
• Article: Structural and insulator-to-metal phase transition at 50 GPa in GdMnO_ {3}

  J. Oliveira, J. Agostinho Moreira, A. Almeida, V. H. Rodrigues, M. M. R. Costa, P. B. Tavares, P. Bouvier, M. Guennou, J. Kreisel
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  ABSTRACT: We present a study of the effect of very high pressure on the orthorhombic perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to 53 GPa. The experimental results yield a structural and insulator-to-metal phase transition close to 50 GPa, from an orthorhombic to a metrically cubic structure. The phase transition is of first order with a pressure hysteresis of about 6 GPa. The observed behavior under very high pressure might well be a general feature in rare-earth manganites.
  Phys. Rev. B. 02/2012; 85(5).
• Source

  Available from: María Paz Fernández-García

  Article: Superparamagnetic MFe2O4 (M = Fe, Co, Mn) nanoparticles: tuning the particle size and magnetic properties through a novel one-step coprecipitation route

  C. Pereira, A. M. Pereira, C. Fernandes, M. Rocha, R. Mendes, M. P. Fernández-García, A. Guedes, P. B. Tavares, Jean-Marc Grenèche, J. P. Araújo, C. Freire
  Chemistry of Materials. 01/2012; 24:1496−1504.
• Source

  Available from: Gonçalo Oliveira

  Article: Phase control studies in Gd5Si2Ge2 giant magnetocaloric compound

  J. H. Belo, A. M. Pereira, J. Ventura, G. N. P. Oliveira, J. P. Araujo, P. B. Tavares, L. Fernandes, P. A. Algarabel, C. Magen, L. Morellon, M. R. Ibarra
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  ABSTRACT: A systematic set of annealings on arc-melted synthesized Gd5Si2Ge2 sample was performed. Through powder X-ray diffraction (XRD) and magnetometry measurements we monitored the effect of varying the annealing time with constant temperature (T = 1473 K) on the formation of the monoclinic (M) crystallographic phase fraction, which is the one responsible for the giant magnetocaloric effect (GMCE) in this compound. The conversion of the orthorhombic O(I) crystallographic phase into M was achieved, resulting in a significant increase of the M mass fraction. Such conversion led to a change in the magnetic transition nature, evolving from a second to a first order transition for the as-cast and annealed samples, respectively. An optimal annealing time range for the M phase conversion was identified to be within 80-120 min at T = 1473 K followed by a rapid quenching to liquid N-2. Furthermore, an increase up to similar to 50% of the magnetocaloric effect was obtained for the sample annealed during 120 min. (C) 2012 Elsevier B.V. All rights reserved.
  Journal of Alloys and Compounds 01/2012; 529:89-95. · 2.29 Impact Factor
• Article: Magentically-Induced Lattice Distortions and Ferroelectricity in Magnetoelectric GdMnO3

  J. Agostinho Moreira, A Almeida, J Oliveira, M. R. Chaves, J. Kreisel, F. Carpinteiro, P. B. Tavares
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  ABSTRACT: In this work we investigate the magnetic field dependence of Ag octahedra rotation (tilt) and B2g symmetric stretching modes frequency at different temperatures. Our field-dependent Raman investigation at 10K is interpreted by an ionic displacive nature of the magnetically induced ferroelectric phase transition. The frequency change of the Ag tilt is in agreement with the stabilization of the Mn-Gd spin arrangement, yielding the necessary conditions for the onset of ferroelectricity on the basis of the inverse Dzyaloshinskii-Moriya interaction. The role of the Jahn-Teller cooperative interaction is also evidenced by the change of the B2g mode frequency at the ferroelectric phase transition. This frequency change allows estimating the shift of the oxygen position at the ferroelectric phase transition and the corresponding spontaneous polarization of 480 {\mu}C/m2, which agrees with earlier reported values in single crystals. Our study also confirms the existence of a large magnetic hysteresis at the lowest temperatures, which is a manifestation of magnetrostiction.
  11/2011;
• Article: Structural and insulator-to-metal phase transition at 50 GPa in GdMnO3

  J. Agostinho Moreira, A Almeida, J Oliveira, V H Rodrigues, M. M. R. Costa, P. B. Tavares, P. Bouvier, M. Guennou, J. Kreisel
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  ABSTRACT: We present a study of the effect of very high pressure on the orthorhombic perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to 53.2 GPa. The experimental results yield a structural and insulator-to-metal phase transition close to 50 GPa, from an orthorhombic to a metrically cubic structure. The phase transition is of first order with a pressure hysteresis of about 6 GPa. The observed behavior under very high pressure might well be a general feature in rare-earth manganites.
  11/2011;
• Source

  Available from: J.O. Carneiro

  Article: Photocatalytic activity and UV-protection of TiO2 nanocoatings on poly(lactic acid) fibres deposited by pulsed magnetron sputtering.

  J O Carneiro, V Teixeira, J H O Nascimento, J Neves, P B Tavares
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  ABSTRACT: The application of nanocoatings in the textile finishing is increasingly being explored because they open a whole new vista of value-addition possibilities in the textile sector. In the present work, low temperature pulsed DC magnetron sputtering method was used to create functional TiO2 nanocoatings on poly(lactic acid) textile fibres surfaces. In this study, the principal objectives in the application of TiO2 nanocoatings to textile materials are to impart UV protection functions and self-cleaning properties to the textile substrates. The TiO2 films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, UV-visible spectroscopy and contact angle analysis. The Photocatalytic activity of the films was tested by measuring the photodegradation rates of rhodamine-B dye aqueous solution under UV light irradiation. The ultraviolet protection function was tested according to the Australian/New Zealand standards. It was observed that the TiO2 nanocoatings on poly(lactic acid) fibres showed an excellent ultraviolet protection (> 40) function and the photocatalytic efficiency was maintained even after a strong washing treatment.
  Journal of Nanoscience and Nanotechnology 10/2011; 11(10):8979-85. · 1.56 Impact Factor
• Article: Phase diagram of the orthorhombic, lightly lutetium doped EuMnO_ {3} magnetoelectric system

  J. Oliveira, J. Agostinho Moreira, A. Almeida, M. R. Chaves, J. M. M. da Silva, J. B. Oliveira, M. A. Sá, P. B. Tavares, R. Ranjith, W. Prellier
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  ABSTRACT: This paper reports on structural, magnetic, dielectric, thermodynamic, and magnetodielectric properties of Eu1-xLuxMnO3, with 0 ⩽ x ⩽ 0.2, towards the (x, T) phase diagram. The phase diagram reflects the effect of lattice distortions induced by the isovalent substitution of Eu3+ by smaller Lu3+ ions, which gradually unbalances the antiferromagnetic against the ferromagnetic exchange interactions, enabling the emergence of both ferroelectricity and magnetoelectric coupling. For x < 0.1, the paramagnetic phase is followed by a presumably incommensurate collinear antiferromagnetic phase AFM-1, and then a weak ferromagnetic phase seems to be established, with a canted A-type antiferromagnetic order. For 0.1 ⩽ x ⩽ 0.2, the AFM-1 phase is followed by an antiferromagnetic phase AFM-2 with modulated spiral spin arrangement, compatible with ferroelectricity. The disappearance of hysteresis cycles P(E) at low temperatures, clearly indicates the existence of an antiferromagnetic phase AFM-3, whose spin structure is not compatible with both the ferroelectric and ferromagnetic components. The magnetic behavior of EuMnO3 and Eu0.9Lu0.1MnO3 suggests the existence of a phase line separating the AFM-1 phase from the AFM-2 and AFM-3 phases, which is observed for x = 0.1. Magnetodielectric coupling was evidenced for both x = 0.1 and 0.2 compositions. Ferroelectric polarization and magnetodielectric coupling coefficient are larger for the latter composition.
  Phys. Rev. B. 09/2011; 84(9).
• Article: Oxygen ordering in the high-T_ {c} superconductor HgBa_ {2} CaCu_ {2} O_ {6+ δ} as revealed by perturbed angular correlation

  T. M. Mendonça, J. G. Correia, H. Haas, P. Odier, P. B. Tavares, M. R. da Silva, A. M. L. Lopes, A. M. Pereira, J. N. Gonçalves, J. S. Amaral, C. Darie, J. P. Araujo
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  ABSTRACT: Lattice sites and collective ordering of oxygen atoms in HgBa2CaCu2O6+δ were studied using the perturbed angular correlation (PAC) technique at ISOLDE/CERN. The electric field gradients (EFG) at 199mHg nuclei have been measured as functions of oxygen doping on the Hg planes, above and below Tc. In comparison with the results obtained for oxygen and fluorine doping in Hg-1201, the analysis shows a different oxygen ordering exhibited by Hg-1212. Moreover, for all studied cases, the experimental results show that at a local scale there is non uniform oxygen distribution. A series of ab initio EFG calculations allowed to infer that at low concentrations, regions without oxygen coexist with regions where O2δ dumbbell molecules are located at the center of the Hg mesh. On the other side, at high concentrations, O2δ dumbbell molecules coexist with single Oδ atoms occupying the center of the Hg mesh. The present results suggest that oxygen sits on the Hg planes in the form of a molecule and not as a single atom.
  Phys. Rev. B. 09/2011; 84(9).
• Article: Designing novel hybrid materials by one-pot co-condensation: from hydrophobic mesoporous silica nanoparticles to superamphiphobic cotton textiles.

  C Pereira, C Alves, A Monteiro, C Magén, A M Pereira, A Ibarra, M R Ibarra, P B Tavares, J P Araújo, G Blanco, J M Pintado, A P Carvalho, J Pires, M F R Pereira, C Freire
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  ABSTRACT: This work reports the synthesis and characterization of mesoporous silica nanoparticles (MSNs) functionalized with tridecafluorooctyltriethoxysilane (F13) and their in situ incorporation onto cotton textiles. The hybrid MSNs and the functional textiles were prepared by a one-pot co-condensation methodology between tetraethylorthosilicate (TEOS) and F13, with hexadecyltrimethylammonium chloride (CTAC) as the template and triethanolamine as the base. The influence of the F13 to TEOS molar ratio (1:10, 1:5 and 1:3) on the nanoparticle morphology, porosity, degree of functionalization, and hydro/oleophobic properties is discussed. The hybrid nanosilicas presented high colloidal stability and were spherical and monodispersed with average particle size of ∼45 nm. They also showed high surface areas, large pore volumes, and a wormhole-type mesoporous structure. The increase in the organosilane proportion during the co-condensation process led to a more radially branched wormhole-like mesoporosity, a decrease in the surface area, pore volume, and amount of surface silanol groups, and an enrichment of the surface with fluorocarbon moieties. These changes imparted hydrophobic and oleophobic properties to the materials, especially to that containing the highest F13 loading. Cotton textiles were coated with the F13-MSNs through an efficient and less time-consuming route. The combination between surface roughness and mesoporosity imparted by the MSNs, and the low surface energy provided by the organosilane resulted in superhydrophobic functional textiles. Moreover, the textile with the highest loading of fluorocarbon groups was superamphiphobic.
  ACS Applied Materials & Interfaces 06/2011; 3(7):2289-99. · 4.53 Impact Factor
• Article: Oxygen ordering in the high-T-c superconductor HgBa2CaCu2O6+delta as revealed by perturbed angular correlation

  T M Mendonca, J G Correia, H Haas, P Odier, P B Tavares, M R Silva, A M L Lopes, A M Pereira, J N Goncalves, J S Amaral, C Darie, J P Araujo
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  ABSTRACT: Lattice sites and collective ordering of oxygen atoms in HgBa2CaCu2O6+delta were studied using the perturbed angular correlation (PAC) technique at ISOLDE/CERN. The electric field gradients (EFG) at Hg-199m nuclei have been measured as functions of oxygen doping on the Hg planes, above and below T-c. In comparison with the results obtained for oxygen and fluorine doping in Hg-1201, the analysis shows a different oxygen ordering exhibited by Hg-1212. Moreover, for all studied cases, the experimental results show that at a local scale there is non uniform oxygen distribution. A series of ab initio EFG calculations allowed to infer that at low concentrations, regions without oxygen coexist with regions where O-2 delta dumbbell molecules are located at the center of the Hg mesh. On the other side, at high concentrations, O-2 delta dumbbell molecules coexist with single O-delta atoms occupying the center of the Hg mesh. The present results suggest that oxygen sits on the Hg planes in the form of a molecule and not as a single atom.
  Physical Review B 01/2011; 84(9). · 3.69 Impact Factor
• Source

  Available from: J. Agostinho Moreira

  Article: THz and infrared studies of multiferroic hexagonal Y1− x Eu x MnO3 (x = 0–0.2) ceramics

  V. Goian, S. Kamba, C. Kadlec, D. Nuzhnyy, P. Kužel, J. Agostinho Moreira, A. Almeida, P.B. Tavares
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  ABSTRACT: We report an investigation of hexagonal Y1− x Eu x MnO3 ceramics with x = 0, 0.1, and 0.2 using infrared (IR) and terahertz (THz) spectroscopies in the temperature range between 5 and 900 K. The temperature dependence of the THz permittivity reveals a kink near the antiferromagnetic (AFM) phase transition temperature T N ≈ 70 K, giving evidence of a strong spin–lattice coupling. Below T N, two absorption peaks were revealed in the THz spectra close to 43 and 73 cm−1. While the first peak corresponds to a sharp AFM resonance exhibiting softening on heating towards T N, the second one may be attributed to an impurity mode or a multiphonon absorption peak. High-temperature THz spectra measured up to 900 K reveal only small gradual increase of the permittivity in agreement with a weak phonon softening observed in the IR reflectance spectra upon heating. This corresponds to an improper ferroelectric character of the phase transition proposed from first principle calculations by Fennie and Rabe (Phys. Rev. B 72: 100103(R), 2005).
  Phase Transitions 10/2010; 83(Nos. 10–11):931-941. · 1.01 Impact Factor
• Article: Dielectric and Magnetic Properties of ReMnO3 (Re = Eu, Gd) Ceramics

  W. S. Ferreira, J. Agostinho Moreira, A. Almeida, J. P. Araújo, P. B. Tavares, T. M. Mendonça, P. Simeão Carvalho, S. Mendonça
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  ABSTRACT: EuMnO 3 and GdMnO 3 ceramics have been synthesised by urea sol-gel combustion method. The dielectric and magnetic properties of high quality ceramics of both compounds were performed in the 5 K–300 K temperature range. The magnetic phase transitions in both compounds are marked by anomalies in the temperature behaviour of both dielectric constant and induced magnetization. The experimental results are discussed and compared with those ones obtained in single crystals.
  Ferroelectrics. 09/2010; 368(2008):107-113.
• Article: Polar properties and phase sequence in Eu0.8Y0.2MnO3

  J Agostinho Moreira, A Almeida, W S Ferreira, M R Chaves, J P Araújo, A M Pereira, S M F Vilela, P B Tavares
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  ABSTRACT: In this work, we have studied in detail the temperature dependence of the electric polarization of Eu0.8Y0.2MnO3 aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperature magnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in 30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinear spiral-antiferromagnetic phase, stable in that temperature range. Contrary to results published in previous works, our experimental data clearly show that the weak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsically ferroelectric. The misinterpretation, regarding the polar character of the lower temperature magnetic phases, stems from the existence of an induced polarization occurring below 30 K. The mechanisms associated with polar and magnetic properties, and their correlation with both spin and lattice structures are also discussed.
  Journal of Physics Condensed Matter 03/2010; 22(12):125901. · 2.55 Impact Factor
• Article: Polar properties and phase sequence in Eu(0.8)Y(0.2)MnO(3).

  J Agostinho Moreira, A Almeida, W S Ferreira, M R Chaves, J P Araújo, A M Pereira, S M F Vilela, P B Tavares
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  ABSTRACT: In this work, we have studied in detail the temperature dependence of the electric polarization of Eu(0.8)Y(0.2)MnO(3) aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperature magnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in 30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinear spiral-antiferromagnetic phase, stable in that temperature range. Contrary to results published in previous works, our experimental data clearly show that the weak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsically ferroelectric. The misinterpretation, regarding the polar character of the lower temperature magnetic phases, stems from the existence of an induced polarization occurring below 30 K. The mechanisms associated with polar and magnetic properties, and their correlation with both spin and lattice structures are also discussed.
  Journal of Physics Condensed Matter 03/2010; 22(12):125901. · 2.55 Impact Factor
• Article: Effect of the external fields on the polar and dielectric properties of Eu0.8Y0.2MnO3

  J. Agostinho Moreira, A. Almeida, W. S. Ferreira, M. R. Chaves, S. M. F. Vilela, P. B. Tavares, B. Kundys, R. Ranjith, W. Prellier
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  ABSTRACT: Eu _0.8 Y _0.2 MnO ₃ has been widely studied due to its very distinctive phase diagram, where it is still poorly understood the actual ferroelectric character of the low temperature magnetic phases. In order to figure out what is the origin of the microscopic mechanisms that drive its behavior, we carried out a detailed study of the displacement currents for both different starting conditions and polarizing electric fields, and of the field dependent magnetodielectric effect in Eu _0.8 Y _0.2 MnO ₃ ceramics. The experimental results provide clear evidence for the existence of two dipolar systems, one stemming from an electric field-induced process, likely associated with the isovalent substitution of Eu ³⁺ by the smaller off-center Y ³⁺ ions at A-lattice sites, which is independent of any cooperative phenomena occurring in the system. The other dipolar system, strongly dependent on the existence of the first one, drives the polar behavior of the nonmodulated magnetic phase AFM-2, stable in the temperature range of 23–30 K, and is associated with the ferroelectric character of this phase. The magnetic field dependence of the complex dielectric constant clearly shows that the magnetodielectric effect is strongly dependent on the phase it is being considered, and provides further evidence for the ferroelectric character of the AFM-2 phase referred to above.
  Journal of Applied Physics 02/2010; · 2.17 Impact Factor
• Article: Superparamagnetic γ-Fe2O3@SiO2 nanoparticles: a novel support for the immobilization of [VO(acac)2].

  C. Pereira, A. M. Pereira, P. Quaresma, P. B. Tavares, E. Pereira, J. P. Araújo, C. Freire
  Dalton Transactions 01/2010; · 3.84 Impact Factor