P. De La Presa - Academia.edu (original) (raw)

Papers by P. De La Presa

PHYSICAL REVIEW B 75, 187401 (2007)

In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transit... more In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transitions of RCo2 [Phys. Rev. B 68, 014409 (2003)], Herrero-Albillos [Phys. Rev. B 73, 134410 (2006)] come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos on the order of the magnetic phase transitions of PrCo2 and NdCo2. In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo2 and NdCo2 undergoes a discontinuous, first-order phase transition at T-C.

Zeitschrift für Naturforschung A, 2000

The quadrupole hyperfine interactions of 11 'Cd/ 111 In probe nuclei in single-crystalline SrTi0 ... more The quadrupole hyperfine interactions of 11 'Cd/ 111 In probe nuclei in single-crystalline SrTi0 3 perovskite samples were investigated using Perturbed Angular Correlation spectroscopy. Three electric field gradients were detected and their fractions and hyperfine parameters were measured in the temperature range from 26 to 700 K. The fraction / 0 having a vanishing quadrupole frequency, as expected for 111 Cd on substitutional sites in a cubic lattice, starts to develop around 300 K and reaches 100% at 700 K. Two well-defined EFG's having closely lying quadrupole frequencies and asymmetry parameters of cj qi = 49.1(3) Mrad/s, 77, = 0.10(2) and u Q2 = 51.8(3) Mrad/s, r] 2 = 0.12(2), at room temperature, were identified in the temperature range from 250 to 530 K, and their (100) orientation in the lattice was determined. They are associated with electronic defects at the probe atoms.

Solid State Communications, 1998

Samples of PbZra.s2Ti0.4s03 (PZT) were fabricated by the Sol-Gel method and characterized by X-ra... more Samples of PbZra.s2Ti0.4s03 (PZT) were fabricated by the Sol-Gel method and characterized by X-ray diffraction analysis. The Rietveld method was used to obtain lattice parameters and grain sizes as a function of the annealing temperature. A powder sample previously annealed at 873 K was irradiated to activate the "'Ta probe. Then, Perturbed Angular Correlation (PAC) spectra were measured at RT and temperatures above 873 K to study the thermal behaviour on samples capsulated in Si02 tubes. The hyperfine parameters observed as a function of temperature show changes in this PZT compound that would correspond to thermal evolution of defects. Furthermore, the reaction of PZT with the substrate was observed at 1173 K, obtaining at this temperature the hyperfine parameters corresponding to SiZrO+ At this temperature no trace of the PZT was left.

Journal of Nanoparticle Research, 2014

The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite ... more The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite sizes 30, 40, and 50 nm, are studied. These crystallite sizes correspond to average particle sizes of 33, 87, and 90 nm, respectively, as determined by TEM. By HRTEM images, it is observed that grain boundary widths decrease as crystallite size increases. Electrical and microstructural properties are correlated based on the theoretical definition of charging energy. Conduction phenomena are investigated as a function of grain boundaries widths, which in turn depend on crystallite size: the calculations suggest that charging energy has a strong dependence on crystallite size. By zero-fieldcooling and susceptibility measurements, it is observed that Verwey transition is crystallite size dependent, with values ranging from 85 to 95 K. In addition, a kink at the out-phase susceptibility curves at 35 K, and a strong change in coercivity is associated to a spin-glass transition, which is independent of crystallite size but frequency dependent. The activation energy associated to this transition is calculated to be around 6-7 meV. Finally, magnetic saturation and coercivity are found to be not significantly affected by crystallite size, with saturation values close to fine powders values. A detailed knowledge on the effects of grain boundary width and crystallite size on conductivity and magnetic properties is relevant for optimization of materials that can be used in magnetoresistive devices.

Physical Review B, 1997

Electronic structure calculations of cubic SrTiO 3 and SrHfO 3 are presented. The full-potential ... more Electronic structure calculations of cubic SrTiO 3 and SrHfO 3 are presented. The full-potential linear augmented-plane-wave method is used and exchange-correlation effects are treated by the local-density approximation. The tendency to ferroelectricity of both compounds is explored and compared by displacing the transition metal atom ͑Ti or Hf͒ towards one of the oxygens ͑001 direction͒. The calculations show that ferroelectricity is favored in SrTiO 3 with respect to SrHfO 3 and that this fact may be correlated with the degree of hybridization between transition metal d-O p bands as has been found for other related systems. Also a detailed discussion of the calculated electric field gradients is presented.

The Journal of Physical Chemistry C, 2012

The specific absorption rate (SAR) of γ-Fe 2 O 3 nanoparticles (NPs) under an alternating magneti... more The specific absorption rate (SAR) of γ-Fe 2 O 3 nanoparticles (NPs) under an alternating magnetic field has been investigated as a function of size, concentration, coating, liquid carrier, and frequency and amplitude of the applied magnetic field. The NPs have been synthesized by coprecipitation method with sizes ranging from 6 to 14 nm with low polydispersity (0.2) and high crystallinity degrees. The small NPs size (6−14 nm) and the value of the maximum applied field (<7.5 kA/m) allow the use of the linear response theory for the analysis of the experimental SARs values. Under this condition, Neel−Brown relaxation times of about 10 −7 s are obtained from SAR field frequency dependence. The NPs have been immobilized in agar to investigate the heating mechanisms, i.e., inversion of the magnetic moments inside the monodomain volume or particle rotation. The results suggest that there is a critical size of around 12 nm for obtaining the most effective heating in viscous media. Furthermore, the surface modification by aminopropylsilane coating does not affect the heating efficiency, making these NPs good candidates for hyperthermia treatment as well as model samples for standardization of hyperthermia apparatus.

The Journal of Physical Chemistry C, 2010

The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferr... more The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferromagnetic nanoparticles of manganese oxide perovskite La 0.56 (SrCa) 0.22 MnO 3 have been studied. The particles were subjected to a size selection process and coated with silica to improve their biocompatibility. The control assays made with HeLa tumor cells showed that cell survival and growth rate were not affected by the particle internalization in cells, or by the electromagnetic field on cells without nanoparticles. However, the application of an alternating electromagnetic field to cells incubated with this silica-coated manganese oxide induced significant cellular damage that finally lead to cell death by an apoptotic mechanism. Cell death is triggered even though the temperature increase in the cell culture during the hyperthermia treatment is lower than 0.5°C.

J. Phys. Chem. C, 2011, 115 (13), pp 5500–5508

FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon thermal annealing, magnet... more FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon thermal annealing, magnetic nanocomposites with combined magnetic properties. Although the pre- and postannealed samples are usually well-characterized, a detailed investigation during annealing is necessary to reveal the role of intermediate processes to produce a desirable composite. We present an alternative method using in situ XANES to investigate the thermal evolution of oleic acid- and oleylamine-coated Pt-rich FePt/Fe3O4 heterodimers. As the temperature increases, a progressive reduction of Fe3O4 to FeO occurs helped by the thermolysis of the surfactants, while above 550 K Fe3Pt starts to be formed. At 840 K an abrupt increase of FeO further drives the phase transformation to stabilize the iron platinum soft phase. Thus, the Fe3O4 reduction acts as catalyst that promotes the Fe and Pt interdiffusion between the Pt-rich FePt and Fe3O4/FeO to form Fe3Pt instead of exchange-coupled FePt/Fe3O4 with hard magnetic properties. In addition, the role of the interface of the heterodimer ends is discussed. The pre- and postannealed samples were also characterized by TEM, XRD, EXAFS, magnetometry, and Mössbauer spectroscopy.

The Journal of Physical Chemistry B, 2009

Gold nanoparticles have been synthesized inside ethosomes, vesicles composed of phospholipid, eth... more Gold nanoparticles have been synthesized inside ethosomes, vesicles composed of phospholipid, ethanol, and water, which could be very efficient not only in delivery probes to the skin but also as diagnostic and therapeutic multimodal agents. High efficiency encapsulation of gold nanoparticles is achieved by a simple strategy: the nanoparticles synthesis occurs simultaneously with the ethosomes formation in the absence of any undesirable reducing agents. A three-dimensional reconstruction of a gold-embedded ethosome generated by cryoelectron tomography reveals that the gold particle is localized inside the lipid bilayer, leaving the ethosome surface and core free for further functionalization. The resulting gold nanoparticles are homogeneous in size and shape and, depending on synthesis temperature, the size ranges from 10 to 20 nm, as revealed by TEM. The ethosome-nanoparticles hybrids' size has been investigated by means of dynamic light scattering and has been found to vary with temperature and gold salt concentration from 700 to 400 nm. Gold nanoparticlesencapsulated ethosomes offer a versatile platform for the enhancement of pharmacological efficacy in transdermal and dermal delivery systems.

Physical Review Letters, 2002

We present an ab initio study of the relaxations introduced in TiO 2 when a Cd impurity substitut... more We present an ab initio study of the relaxations introduced in TiO 2 when a Cd impurity substitutes a Ti atom and an experimental test of this calculation by a perturbed-angular-correlation (PAC) measurement of the orientation of the electric-field gradient (EFG) tensor at the Cd site. The ab initio calculation predicts strong anisotropic relaxations of the nearest oxygen neighbors of the impurity and a change of the orientation of the largest EFG tensor component, V 33 , from the 001 to the 110 direction upon substitution of a Ti atom by a Cd impurity. The last prediction is confirmed by the PAC experiment that shows that V 33 at the Cd site is parallel to either the 110 or the 110 crystal axis.

Physical Review B, 2008

Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanopartic... more Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO 2 and ZrO 2 synthesized in a microwave-plasma process have been investigated by x-ray diffraction ͑XRD͒, transmission electron microscopy ͑TEM͒, and perturbed angular correlation ͑PAC͒ spectroscopy. The PAC technique was used to measure the electric quadrupole interactions ͑QIs͒ of the nuclear probes 181 Ta and 111 Cd in nanocrystalline HfO 2 and ZrO 2 as a function of temperature. For comparison, the QI of 181 Ta in the bulk oxides was determined in the same temperature range 300 K ഛ T ഛ 1550 K. The oxygen-metal ratio of the as-synthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4ഛ x ഛ 1.8. A hydrate surface layer with a hydrogen content of 5 -10 wt %, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by 1 H magic-angle spinning NMR. XRD and TEM show that bare n-ZrO 2 , Al 2 O 3 -coated n-ZrO 2 , and Al 2 O 3 -coated n-HfO 2 are synthesized in the tetragonal or cubic modification with a particle size d Ͻ 5 nm, whereas bare n-HfO 2 is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO 2 is Q A =32͑5͒ kJ/ mol. Coating with Al 2 O 3 stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While TEM micrographs of the native nanoparticles reveal a well-ordered cation sublattice, the observation of a broad QI distribution in the PAC spectra suggests a high degree of disorder of the oxygen sublattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature QI measurements. Hafnia nanoparticles persist in the monoclinic ͑m͒ phase up to T ഛ 1400 K. In coated n-ZrO 2 / Al 2 O 3 , the monoclinic and tetragonal ͑t͒ phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic ͑t or c͒ n-ZrO 2 presents a sharp m ↔ t transition. A "defect" component involving 30%-40% of the probe nuclei appears in the 181 Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ജ 1200 K. The temperature dependence of this component can be reproduced by assuming that Ta impurities in hafnia and zirconia may trap electrons at low temperatures. The observation that the defect component appears only in nanoparticles with diameter d Ͻ 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies ͑V O ͒ and/or V O -hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion is supported by the absence of a size effect for 111 Cd probes in HfO 2 and ZrO 2 . The temperature dependence of the 181 Ta defect fraction is consistent with a Ta + impurity level at E d ϳ 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.

Physical Review B, 2012

A combination of experiments and ab initio quantum-mechanical calculations has been applied to ex... more A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine electronic, structural, and hyperfine interactions in pure and Ta-doped zirconium dioxide in its monoclinic phase (m-ZrO 2 ). From the theoretical point of view, the full-potential linear augmented plane wave plus local orbital (APW + lo) method was applied to treat the electronic structure of the doped system including the atomic relaxations introduced by the impurities in the host in a fully self-consistent way using a supercell approach. Different charge states of the Ta impurity were considered in the study and its effects on the electronic, structural, and hyperfine properties are discussed. Our results suggest that two different charge states coexist in Ta-doped m-ZrO 2 . Further, ab initio calculations predict that depending on the impurity charge state, a sizeable magnetic moment can be induced at the Ta-probe site. This prediction is confirmed by a new analysis of experimental data.

Physical Review B, 2005

The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus 111 Cd on the rar... more The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus 111 Cd on the rare earth ͑R͒ site of the pseudobinary Laves-phase compounds R 1−x Y x Co 2 has been investigated by perturbed angular correlation ͑PAC͒ spectroscopy at 10 K for the rare earth R = Tb and Ho at various Y concentrations x ഛ 0.8 and for R = Gd, Dy, Er at the concentration x = 0.3. Up to four components with different magnetic interaction frequencies M i could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor ͑NN͒ R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n R of NN R atoms and the magnetic hyperfine frequencies:

Physical Review B, 2010

A combination of experiments and ab initio quantum-mechanical calculations has been applied to ex... more A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine hyperfine interactions in Ta-doped hafnium dioxide. Although the properties of monoclinic HfO 2 have been the subject of several earlier studies, some aspects remain open. In particular, time differential perturbed angular correlation spectroscopy studies using 181 Ta as probe atom revealed the coexistence of two hyperfine interactions in this material but an explanation was only given for the more populated one. Until now, no models have been proposed that explain the second interaction, and it has not yet been associated with a specific crystallographic site. In this work, a detailed study of the different charge states for the impurity-probe atom ͑Ta͒ was performed in order to understand the second interaction observed in Ta-doped monoclinic HfO 2 . The combination of experiments and theory suggests that two different charge states coexist in this compound. Further, ab initio calculations predict that, depending on the impurity charge state, a sizeable magnetic moment can be induced at the probe site. This is confirmed by a new analysis of experimental data.

Physical Review B, 2004

Rare-earth-deficient R 1−x Ni 2 Laves phases, which reportedly crystallize in a C15 superstructur... more Rare-earth-deficient R 1−x Ni 2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation ͑PAC͒ measurements of electric quadrupole interactions at the site of the probe nucleus 111 Cd. Although 111 Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction ͑QI͒ with frequencies q Ϸ 265-275 MHz has been found in the paramagnetic phases of R 1−x Ni 2 with R = Pr, Nd, Sm, Gd. This interaction is not observed for the heavy R constituents R = Tb, Dy, Ho, Er. The fraction of probe nuclei subject to the QI in R 1−x Ni 2 , R =Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T Ͼ 300 K and 500 K for R = Sm, Gd and R = Pr, Nd, respectively. At T = 100 K the QI is static within the PAC time window, but at T = 200 K fluctuations with correlation times C Ͻ 10 −6 s, have been detected. These observations can be explained consistently by two assumptions: (i) the mother isotope 111 In of the PAC probe 111 Cd constitutes an attractive potential for vacancies and (ii) the R vacancies in R 1−x Ni 2 are highly mobile at temperatures T Ͻ 300 K, which is incompatible with a static vacancy superstructure. The measurements indicate a decrease of the vacancy-probe binding energy from the light to the heavy R constituents of R 1−x Ni 2 . For R = Pr, Nd, Sm, Gd the binding energy is in the range 0.15-0.40 eV. The activation energy E A for vacancy jumps near the probe derived from the temperature dependence of the nuclear spin relaxation at 200 K ഛ T ഛ 300 K is small. The values observed in different samples cover a range of 0.1 eVഛ E A ഛ 0.23 eV. The trial frequency w 0 of these jumps appears to be correlated to the activation energy: ln w 0 ͑MHz͒Ϸ58E A ͑eV͒. At high temperatures T Ͼ 500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R 1−x Ni 2 . Auxiliary 111 Cd PAC measurements have been carried in Sc 0.95 Ni

Physical Review B, 2007

In their analysis of our perturbed angular correlation ͑PAC͒ study of the magnetic phase transiti... more In their analysis of our perturbed angular correlation ͑PAC͒ study of the magnetic phase transitions of RCo 2 ͓Phys. Rev. B 68, 014409 ͑2003͔͒, Herrero-Albillos et al. ͓Phys. Rev. B 73, 134410 ͑2006͔͒ come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos et al. on the order of the magnetic phase transitions of PrCo 2 and NdCo 2 . In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo 2 and NdCo 2 undergoes a discontinuous, first-order phase transition at T C . In a recent differential scanning calorimetry ͑DSC͒ study of the magnetic phase transitions of the rare earth ͑R͒ Laves phases RCo 2 Herrero-Albillos et al. 1,2 report that the magnetic phase transitions of the light R compounds PrCo 2 and NdCo 2 are second-order transitions ͑SOT͒. This conclusion is in conflict with the results of a perturbed angular correlation ͑PAC͒ investigation 3 of RCo 2 according to which the magnetization of PrCo 2 and NdCo 2 undergoes a first-order transition ͑FOT͒. As documented in Ref. 3, earlier studies of PrCo 2 and NdCo 2 have not been conclusive as to the order of the phase transition of these compounds.

Physica B: Condensed Matter, 2004

AC susceptibility measurements, as a function of temperature and frequency, have been carried out... more AC susceptibility measurements, as a function of temperature and frequency, have been carried out in Y 1Àx Tb x Co 2 (0.1p x p 0.9) compounds to investigate the magnetic phase transition. It is known that YCo 2 exhibits very strongly enhanced Pauli paramagnetism, while TbCo 2 orders magnetically with a Co moment induced by the Tb molecular field. The phase transition of TbCo 2 at 231 K is a second-order phase transition. We found that in Y 1Àx Tb x Co 2 the transition changes from first-to second-order as x increases. The increase in the order temperature with Tb concentration is observed and the type of the phase transition is analyzed in the framework of the Inoue-Shimizu model. r

Journal of Solid State Chemistry, 2009

We present a study on the magnetic properties of naked and silica-coated Fe 3 O 4 nanoparticles w... more We present a study on the magnetic properties of naked and silica-coated Fe 3 O 4 nanoparticles with sizes between 5 and 110 nm. Their efficiency as heating agents was assessed through specific power absorption (SPA) measurements as a function of particle size and shape. The results show a strong dependence of the SPA with the particle size, with a maximum around 30 nm, as expected for a N eel relaxation mechanism in single-domain particles. The SiO 2 shell thickness was found to play an important role in the SPA mechanism by hindering the heat outflow, thus decreasing the heating efficiency. It is concluded that a compromise between good heating efficiency and surface functionality for biomedical purposes can be attained by making the SiO 2 functional coating as thin as possible.

Journal of Solid State Chemistry, 2001

PHYSICAL REVIEW B 75, 187401 (2007)

In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transit... more In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transitions of RCo2 [Phys. Rev. B 68, 014409 (2003)], Herrero-Albillos [Phys. Rev. B 73, 134410 (2006)] come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos on the order of the magnetic phase transitions of PrCo2 and NdCo2. In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo2 and NdCo2 undergoes a discontinuous, first-order phase transition at T-C.

Zeitschrift für Naturforschung A, 2000

The quadrupole hyperfine interactions of 11 'Cd/ 111 In probe nuclei in single-crystalline SrTi0 ... more The quadrupole hyperfine interactions of 11 'Cd/ 111 In probe nuclei in single-crystalline SrTi0 3 perovskite samples were investigated using Perturbed Angular Correlation spectroscopy. Three electric field gradients were detected and their fractions and hyperfine parameters were measured in the temperature range from 26 to 700 K. The fraction / 0 having a vanishing quadrupole frequency, as expected for 111 Cd on substitutional sites in a cubic lattice, starts to develop around 300 K and reaches 100% at 700 K. Two well-defined EFG's having closely lying quadrupole frequencies and asymmetry parameters of cj qi = 49.1(3) Mrad/s, 77, = 0.10(2) and u Q2 = 51.8(3) Mrad/s, r] 2 = 0.12(2), at room temperature, were identified in the temperature range from 250 to 530 K, and their (100) orientation in the lattice was determined. They are associated with electronic defects at the probe atoms.

Solid State Communications, 1998

Samples of PbZra.s2Ti0.4s03 (PZT) were fabricated by the Sol-Gel method and characterized by X-ra... more Samples of PbZra.s2Ti0.4s03 (PZT) were fabricated by the Sol-Gel method and characterized by X-ray diffraction analysis. The Rietveld method was used to obtain lattice parameters and grain sizes as a function of the annealing temperature. A powder sample previously annealed at 873 K was irradiated to activate the "'Ta probe. Then, Perturbed Angular Correlation (PAC) spectra were measured at RT and temperatures above 873 K to study the thermal behaviour on samples capsulated in Si02 tubes. The hyperfine parameters observed as a function of temperature show changes in this PZT compound that would correspond to thermal evolution of defects. Furthermore, the reaction of PZT with the substrate was observed at 1173 K, obtaining at this temperature the hyperfine parameters corresponding to SiZrO+ At this temperature no trace of the PZT was left.

Journal of Nanoparticle Research, 2014

The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite ... more The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite sizes 30, 40, and 50 nm, are studied. These crystallite sizes correspond to average particle sizes of 33, 87, and 90 nm, respectively, as determined by TEM. By HRTEM images, it is observed that grain boundary widths decrease as crystallite size increases. Electrical and microstructural properties are correlated based on the theoretical definition of charging energy. Conduction phenomena are investigated as a function of grain boundaries widths, which in turn depend on crystallite size: the calculations suggest that charging energy has a strong dependence on crystallite size. By zero-fieldcooling and susceptibility measurements, it is observed that Verwey transition is crystallite size dependent, with values ranging from 85 to 95 K. In addition, a kink at the out-phase susceptibility curves at 35 K, and a strong change in coercivity is associated to a spin-glass transition, which is independent of crystallite size but frequency dependent. The activation energy associated to this transition is calculated to be around 6-7 meV. Finally, magnetic saturation and coercivity are found to be not significantly affected by crystallite size, with saturation values close to fine powders values. A detailed knowledge on the effects of grain boundary width and crystallite size on conductivity and magnetic properties is relevant for optimization of materials that can be used in magnetoresistive devices.

Physical Review B, 1997

Electronic structure calculations of cubic SrTiO 3 and SrHfO 3 are presented. The full-potential ... more Electronic structure calculations of cubic SrTiO 3 and SrHfO 3 are presented. The full-potential linear augmented-plane-wave method is used and exchange-correlation effects are treated by the local-density approximation. The tendency to ferroelectricity of both compounds is explored and compared by displacing the transition metal atom ͑Ti or Hf͒ towards one of the oxygens ͑001 direction͒. The calculations show that ferroelectricity is favored in SrTiO 3 with respect to SrHfO 3 and that this fact may be correlated with the degree of hybridization between transition metal d-O p bands as has been found for other related systems. Also a detailed discussion of the calculated electric field gradients is presented.

The Journal of Physical Chemistry C, 2012

The specific absorption rate (SAR) of γ-Fe 2 O 3 nanoparticles (NPs) under an alternating magneti... more The specific absorption rate (SAR) of γ-Fe 2 O 3 nanoparticles (NPs) under an alternating magnetic field has been investigated as a function of size, concentration, coating, liquid carrier, and frequency and amplitude of the applied magnetic field. The NPs have been synthesized by coprecipitation method with sizes ranging from 6 to 14 nm with low polydispersity (0.2) and high crystallinity degrees. The small NPs size (6−14 nm) and the value of the maximum applied field (<7.5 kA/m) allow the use of the linear response theory for the analysis of the experimental SARs values. Under this condition, Neel−Brown relaxation times of about 10 −7 s are obtained from SAR field frequency dependence. The NPs have been immobilized in agar to investigate the heating mechanisms, i.e., inversion of the magnetic moments inside the monodomain volume or particle rotation. The results suggest that there is a critical size of around 12 nm for obtaining the most effective heating in viscous media. Furthermore, the surface modification by aminopropylsilane coating does not affect the heating efficiency, making these NPs good candidates for hyperthermia treatment as well as model samples for standardization of hyperthermia apparatus.

The Journal of Physical Chemistry C, 2010

The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferr... more The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferromagnetic nanoparticles of manganese oxide perovskite La 0.56 (SrCa) 0.22 MnO 3 have been studied. The particles were subjected to a size selection process and coated with silica to improve their biocompatibility. The control assays made with HeLa tumor cells showed that cell survival and growth rate were not affected by the particle internalization in cells, or by the electromagnetic field on cells without nanoparticles. However, the application of an alternating electromagnetic field to cells incubated with this silica-coated manganese oxide induced significant cellular damage that finally lead to cell death by an apoptotic mechanism. Cell death is triggered even though the temperature increase in the cell culture during the hyperthermia treatment is lower than 0.5°C.

J. Phys. Chem. C, 2011, 115 (13), pp 5500–5508

FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon thermal annealing, magnet... more FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon thermal annealing, magnetic nanocomposites with combined magnetic properties. Although the pre- and postannealed samples are usually well-characterized, a detailed investigation during annealing is necessary to reveal the role of intermediate processes to produce a desirable composite. We present an alternative method using in situ XANES to investigate the thermal evolution of oleic acid- and oleylamine-coated Pt-rich FePt/Fe3O4 heterodimers. As the temperature increases, a progressive reduction of Fe3O4 to FeO occurs helped by the thermolysis of the surfactants, while above 550 K Fe3Pt starts to be formed. At 840 K an abrupt increase of FeO further drives the phase transformation to stabilize the iron platinum soft phase. Thus, the Fe3O4 reduction acts as catalyst that promotes the Fe and Pt interdiffusion between the Pt-rich FePt and Fe3O4/FeO to form Fe3Pt instead of exchange-coupled FePt/Fe3O4 with hard magnetic properties. In addition, the role of the interface of the heterodimer ends is discussed. The pre- and postannealed samples were also characterized by TEM, XRD, EXAFS, magnetometry, and Mössbauer spectroscopy.

The Journal of Physical Chemistry B, 2009

Gold nanoparticles have been synthesized inside ethosomes, vesicles composed of phospholipid, eth... more Gold nanoparticles have been synthesized inside ethosomes, vesicles composed of phospholipid, ethanol, and water, which could be very efficient not only in delivery probes to the skin but also as diagnostic and therapeutic multimodal agents. High efficiency encapsulation of gold nanoparticles is achieved by a simple strategy: the nanoparticles synthesis occurs simultaneously with the ethosomes formation in the absence of any undesirable reducing agents. A three-dimensional reconstruction of a gold-embedded ethosome generated by cryoelectron tomography reveals that the gold particle is localized inside the lipid bilayer, leaving the ethosome surface and core free for further functionalization. The resulting gold nanoparticles are homogeneous in size and shape and, depending on synthesis temperature, the size ranges from 10 to 20 nm, as revealed by TEM. The ethosome-nanoparticles hybrids' size has been investigated by means of dynamic light scattering and has been found to vary with temperature and gold salt concentration from 700 to 400 nm. Gold nanoparticlesencapsulated ethosomes offer a versatile platform for the enhancement of pharmacological efficacy in transdermal and dermal delivery systems.

Physical Review Letters, 2002

We present an ab initio study of the relaxations introduced in TiO 2 when a Cd impurity substitut... more We present an ab initio study of the relaxations introduced in TiO 2 when a Cd impurity substitutes a Ti atom and an experimental test of this calculation by a perturbed-angular-correlation (PAC) measurement of the orientation of the electric-field gradient (EFG) tensor at the Cd site. The ab initio calculation predicts strong anisotropic relaxations of the nearest oxygen neighbors of the impurity and a change of the orientation of the largest EFG tensor component, V 33 , from the 001 to the 110 direction upon substitution of a Ti atom by a Cd impurity. The last prediction is confirmed by the PAC experiment that shows that V 33 at the Cd site is parallel to either the 110 or the 110 crystal axis.

Physical Review B, 2008

Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanopartic... more Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO 2 and ZrO 2 synthesized in a microwave-plasma process have been investigated by x-ray diffraction ͑XRD͒, transmission electron microscopy ͑TEM͒, and perturbed angular correlation ͑PAC͒ spectroscopy. The PAC technique was used to measure the electric quadrupole interactions ͑QIs͒ of the nuclear probes 181 Ta and 111 Cd in nanocrystalline HfO 2 and ZrO 2 as a function of temperature. For comparison, the QI of 181 Ta in the bulk oxides was determined in the same temperature range 300 K ഛ T ഛ 1550 K. The oxygen-metal ratio of the as-synthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4ഛ x ഛ 1.8. A hydrate surface layer with a hydrogen content of 5 -10 wt %, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by 1 H magic-angle spinning NMR. XRD and TEM show that bare n-ZrO 2 , Al 2 O 3 -coated n-ZrO 2 , and Al 2 O 3 -coated n-HfO 2 are synthesized in the tetragonal or cubic modification with a particle size d Ͻ 5 nm, whereas bare n-HfO 2 is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO 2 is Q A =32͑5͒ kJ/ mol. Coating with Al 2 O 3 stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While TEM micrographs of the native nanoparticles reveal a well-ordered cation sublattice, the observation of a broad QI distribution in the PAC spectra suggests a high degree of disorder of the oxygen sublattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature QI measurements. Hafnia nanoparticles persist in the monoclinic ͑m͒ phase up to T ഛ 1400 K. In coated n-ZrO 2 / Al 2 O 3 , the monoclinic and tetragonal ͑t͒ phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic ͑t or c͒ n-ZrO 2 presents a sharp m ↔ t transition. A "defect" component involving 30%-40% of the probe nuclei appears in the 181 Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ജ 1200 K. The temperature dependence of this component can be reproduced by assuming that Ta impurities in hafnia and zirconia may trap electrons at low temperatures. The observation that the defect component appears only in nanoparticles with diameter d Ͻ 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies ͑V O ͒ and/or V O -hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion is supported by the absence of a size effect for 111 Cd probes in HfO 2 and ZrO 2 . The temperature dependence of the 181 Ta defect fraction is consistent with a Ta + impurity level at E d ϳ 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.

Physical Review B, 2012

A combination of experiments and ab initio quantum-mechanical calculations has been applied to ex... more A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine electronic, structural, and hyperfine interactions in pure and Ta-doped zirconium dioxide in its monoclinic phase (m-ZrO 2 ). From the theoretical point of view, the full-potential linear augmented plane wave plus local orbital (APW + lo) method was applied to treat the electronic structure of the doped system including the atomic relaxations introduced by the impurities in the host in a fully self-consistent way using a supercell approach. Different charge states of the Ta impurity were considered in the study and its effects on the electronic, structural, and hyperfine properties are discussed. Our results suggest that two different charge states coexist in Ta-doped m-ZrO 2 . Further, ab initio calculations predict that depending on the impurity charge state, a sizeable magnetic moment can be induced at the Ta-probe site. This prediction is confirmed by a new analysis of experimental data.

Physical Review B, 2005

The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus 111 Cd on the rar... more The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus 111 Cd on the rare earth ͑R͒ site of the pseudobinary Laves-phase compounds R 1−x Y x Co 2 has been investigated by perturbed angular correlation ͑PAC͒ spectroscopy at 10 K for the rare earth R = Tb and Ho at various Y concentrations x ഛ 0.8 and for R = Gd, Dy, Er at the concentration x = 0.3. Up to four components with different magnetic interaction frequencies M i could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor ͑NN͒ R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n R of NN R atoms and the magnetic hyperfine frequencies:

Physical Review B, 2010

A combination of experiments and ab initio quantum-mechanical calculations has been applied to ex... more A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine hyperfine interactions in Ta-doped hafnium dioxide. Although the properties of monoclinic HfO 2 have been the subject of several earlier studies, some aspects remain open. In particular, time differential perturbed angular correlation spectroscopy studies using 181 Ta as probe atom revealed the coexistence of two hyperfine interactions in this material but an explanation was only given for the more populated one. Until now, no models have been proposed that explain the second interaction, and it has not yet been associated with a specific crystallographic site. In this work, a detailed study of the different charge states for the impurity-probe atom ͑Ta͒ was performed in order to understand the second interaction observed in Ta-doped monoclinic HfO 2 . The combination of experiments and theory suggests that two different charge states coexist in this compound. Further, ab initio calculations predict that, depending on the impurity charge state, a sizeable magnetic moment can be induced at the probe site. This is confirmed by a new analysis of experimental data.

Physical Review B, 2004

Rare-earth-deficient R 1−x Ni 2 Laves phases, which reportedly crystallize in a C15 superstructur... more Rare-earth-deficient R 1−x Ni 2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation ͑PAC͒ measurements of electric quadrupole interactions at the site of the probe nucleus 111 Cd. Although 111 Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction ͑QI͒ with frequencies q Ϸ 265-275 MHz has been found in the paramagnetic phases of R 1−x Ni 2 with R = Pr, Nd, Sm, Gd. This interaction is not observed for the heavy R constituents R = Tb, Dy, Ho, Er. The fraction of probe nuclei subject to the QI in R 1−x Ni 2 , R =Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T Ͼ 300 K and 500 K for R = Sm, Gd and R = Pr, Nd, respectively. At T = 100 K the QI is static within the PAC time window, but at T = 200 K fluctuations with correlation times C Ͻ 10 −6 s, have been detected. These observations can be explained consistently by two assumptions: (i) the mother isotope 111 In of the PAC probe 111 Cd constitutes an attractive potential for vacancies and (ii) the R vacancies in R 1−x Ni 2 are highly mobile at temperatures T Ͻ 300 K, which is incompatible with a static vacancy superstructure. The measurements indicate a decrease of the vacancy-probe binding energy from the light to the heavy R constituents of R 1−x Ni 2 . For R = Pr, Nd, Sm, Gd the binding energy is in the range 0.15-0.40 eV. The activation energy E A for vacancy jumps near the probe derived from the temperature dependence of the nuclear spin relaxation at 200 K ഛ T ഛ 300 K is small. The values observed in different samples cover a range of 0.1 eVഛ E A ഛ 0.23 eV. The trial frequency w 0 of these jumps appears to be correlated to the activation energy: ln w 0 ͑MHz͒Ϸ58E A ͑eV͒. At high temperatures T Ͼ 500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R 1−x Ni 2 . Auxiliary 111 Cd PAC measurements have been carried in Sc 0.95 Ni

Physical Review B, 2007

In their analysis of our perturbed angular correlation ͑PAC͒ study of the magnetic phase transiti... more In their analysis of our perturbed angular correlation ͑PAC͒ study of the magnetic phase transitions of RCo 2 ͓Phys. Rev. B 68, 014409 ͑2003͔͒, Herrero-Albillos et al. ͓Phys. Rev. B 73, 134410 ͑2006͔͒ come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos et al. on the order of the magnetic phase transitions of PrCo 2 and NdCo 2 . In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo 2 and NdCo 2 undergoes a discontinuous, first-order phase transition at T C . In a recent differential scanning calorimetry ͑DSC͒ study of the magnetic phase transitions of the rare earth ͑R͒ Laves phases RCo 2 Herrero-Albillos et al. 1,2 report that the magnetic phase transitions of the light R compounds PrCo 2 and NdCo 2 are second-order transitions ͑SOT͒. This conclusion is in conflict with the results of a perturbed angular correlation ͑PAC͒ investigation 3 of RCo 2 according to which the magnetization of PrCo 2 and NdCo 2 undergoes a first-order transition ͑FOT͒. As documented in Ref. 3, earlier studies of PrCo 2 and NdCo 2 have not been conclusive as to the order of the phase transition of these compounds.

Physica B: Condensed Matter, 2004

AC susceptibility measurements, as a function of temperature and frequency, have been carried out... more AC susceptibility measurements, as a function of temperature and frequency, have been carried out in Y 1Àx Tb x Co 2 (0.1p x p 0.9) compounds to investigate the magnetic phase transition. It is known that YCo 2 exhibits very strongly enhanced Pauli paramagnetism, while TbCo 2 orders magnetically with a Co moment induced by the Tb molecular field. The phase transition of TbCo 2 at 231 K is a second-order phase transition. We found that in Y 1Àx Tb x Co 2 the transition changes from first-to second-order as x increases. The increase in the order temperature with Tb concentration is observed and the type of the phase transition is analyzed in the framework of the Inoue-Shimizu model. r

Journal of Solid State Chemistry, 2009

We present a study on the magnetic properties of naked and silica-coated Fe 3 O 4 nanoparticles w... more We present a study on the magnetic properties of naked and silica-coated Fe 3 O 4 nanoparticles with sizes between 5 and 110 nm. Their efficiency as heating agents was assessed through specific power absorption (SPA) measurements as a function of particle size and shape. The results show a strong dependence of the SPA with the particle size, with a maximum around 30 nm, as expected for a N eel relaxation mechanism in single-domain particles. The SiO 2 shell thickness was found to play an important role in the SPA mechanism by hindering the heat outflow, thus decreasing the heating efficiency. It is concluded that a compromise between good heating efficiency and surface functionality for biomedical purposes can be attained by making the SiO 2 functional coating as thin as possible.

Journal of Solid State Chemistry, 2001