- Magnetism, Nanoparticles, Magnetic Hyperthermia, Magnetic Nanoparticles Used in Cancer Therapy, Hyperthermia, Density-functional theory, and 19 moreDensity Functional Theory, Density functional theory calculations, Iron Oxide Nanoparticles, Titania, Silica Coated Magnetic Nanoparticles, Rutile Structure, Magnetic Phase Transitions, Niquel Rare Earth Intermetallics, Quadrupolar Interactions, Magnetic Heating, Heat Generated by Loss Power, Magnetic Particle Properties Vs Size, Vacancy Superstructure, Materials Science, Nanomaterials, Condensed Matter Physics, Nanomaterials Characterization, Nanoscience, and Nanotechnologyedit
- Associate Professor at the Universidad Complutense de Madrid and Researcher at the Instituto de Magnetismo Aplicado. ... moreAssociate Professor at the Universidad Complutense de Madrid and Researcher at the Instituto de Magnetismo Aplicado. Her working area is magnetism, with special emphasis in nanoparticles for technological application.edit
We analyze here the magneto-optical measurements of hybrid materials containing metallic and oxide ferromagnetic phases. We show that the magnetization curves obtained with this technique can be fairly different to those obtained with... more
We analyze here the magneto-optical measurements of hybrid materials containing metallic and oxide
ferromagnetic phases. We show that the magnetization curves obtained with this technique can be
fairly different to those obtained with standard magnetometry techniques and cannot be representative of the contribution of each phase to the magnetization of the material, due to the very different optical properties of metals and oxides. However, a proper numerical analysis of the magneto-optical measurements allows estimating the fraction of each phase present in the material.
ferromagnetic phases. We show that the magnetization curves obtained with this technique can be
fairly different to those obtained with standard magnetometry techniques and cannot be representative of the contribution of each phase to the magnetization of the material, due to the very different optical properties of metals and oxides. However, a proper numerical analysis of the magneto-optical measurements allows estimating the fraction of each phase present in the material.
Research Interests:
Promising advances in nanomedicine such as magnetic hyperthermia rely on a precise control of the nanoparticle performance in the cellular environment. This constitutes a huge research challenge due to difficulties for achieving a remote... more
Promising advances in nanomedicine such as magnetic hyperthermia rely on a precise control of the nanoparticle performance in the cellular environment. This constitutes a huge research challenge due to difficulties for achieving a remote control within the human body. Here we report on the significant double role of the shape of ellipsoidal magnetic nanoparticles (nanorods) subjected to an external AC magnetic field: first, the heat release is increased due to the additional shape anisotropy; second, the rods dynamically reorientate in the orthogonal direction to the AC field direction. Importantly, the heating performance and the directional orientation occur in synergy and can be easily controlled by changing the AC field treatment duration, thus opening the pathway to combined hyperthermic/ mechanical nanoactuators for biomedicine. Preliminary studies demonstrate the high accumulation of nanorods into HeLa cells whereas viability analysis supports their low toxicity and the absence of apoptotic or necrotic cell death after 24 or 48 h of incubation. Magnetic nanoparticles have been intensively investigated for in-vivo biomedical applications within the last decades 1 , based on the harmless effects of magnetic fields to the human body and on the high biocompatibility of iron oxides. Such features motivated a plethora of research efforts in various areas, ranging from cancer diagnosis 2 to gene therapy 3 or magnetic resonance imaging 4. A particularly active research area is magnetic particle hyper-thermia (MPH), within the wide research field of the so-called magnetic nanorobotics (MNRBs). MPH refers to the heat release by magnetic nanoparticles when subjected to an external magnetic AC field (H AC) finding promising applications for cancer treatment 5 or heat-triggered drug release 6. MNRBs refer in a general way to the remotely controlled actuation of magnetic entities within the human body to trigger specific responses 7 (e.g. drug delivery or enhanced resonance imaging). Efforts to improve the efficiency of nanomagnets for MPH include fine-tuning of the size 8 /anisotropy 9 and of the interaction conditions 10 (to optimize the dose/response ratio); efforts for achieving accurate MNRBs applications include the geometrical design of optimized structures (examples are helical structures for independent positioning 11 or particle-aggregation for improved imaging contrast 12). The objective of the present work is to report on the synergy between heat release and self-induced directional reorientation that controls the magnetic behaviour of magnetic nanorods under an AC field during a hyperther-mia process. Such simultaneous –and complementary-hyperthermic and mechanical actuation can be achieved by specifically-designing the dimensions of the rods, thus conferring those nanosized magnets double function-ality as magnetic nanorobots. The shape of nanomagnets has been reported widely to be a way to control their properties due to the appearance of additional anisotropies 13. For example, shape anisotropy for magnetite is of the same order of magnitude than crystal anisotropy for nanoparticles with aspect ratios of 1.1 and increases up to two orders of magnitude for axial ratios larger than 5 14. Magnetic nanoparticles assembled into chains have been described as efficient MPH
Research Interests:
Two sets of bi-magnetic Zn 0.5 Mn 0.5 Fe 2 O 4 @Fe 3 O 4 core–shell nanoparticles were prepared by a seed-mediated modified co-precipitation method. While the first set was obtained by fast addition of the alkaline solution to grow Fe 3 O... more
Two sets of bi-magnetic Zn 0.5 Mn 0.5 Fe 2 O 4 @Fe 3 O 4 core–shell nanoparticles were prepared by a seed-mediated modified co-precipitation method. While the first set was obtained by fast addition of the alkaline solution to grow Fe 3 O 4 shells over the ferrite seeds, a slow drop-wise addition of stoichiometric Fe 2+ /Fe 3+ ion solution to the alkaline ferrite seeds solution was adopted to synthesize the second set. Samples were characterized by electron microscopy (STEM, TEM, UHRTEM) and magnetometry measurements. Viability MTT assay of the nanoparticles on L929 murine fibroblasts were performed, indicating that they are biocompatible. The coating of the Zn 0.5 Mn 0.5 FeO 4 nanoparticle by a magnetite or maghemite shell minimizes the effect of the magnetic dead layer at the core surface, improving the magnetic properties and offering thus outstanding values for biological application. Relaxometry values r 2 higher than 300 mM À1 s À1 at H 1.5 T, and cell viability at concentrations as high as 0.5 mg mL À1 means these bi-magnetic nanoparticles have a vast potential as MRI contrast agents.
Recently, potential applications of the magnetic heating for heterogeneous catalysis or organic synthesis have been reported. As these new applications are not limited by biocompatibility requirements, a wide range of possibilities for... more
Recently, potential applications of the magnetic heating for heterogeneous catalysis or organic synthesis have been reported. As these new applications are not limited by biocompatibility requirements, a wide range of possibilities for non-aqueous colloidal nanoparticles with enhanced magnetic properties is open. In this work, manganese and cobalt ferrite nanoparticles are synthesized by co-precipitation method with average particle size around 12 nm. The particles are either coated with tetramethy-lammonium hydroxide (TMAOH) and dispersed in water or with oleic acid (OA) and dispersed in hexane to produce aggregated or disaggregated nanoparticles, respectively. It is observed that the particle disaggregation improves significantly the heating efficiency from 12 to 96 W/g in the case of cobalt ferrite, and from 120 to 413 W/g for the manganese ferrite. The main responsible for this improvement is the reduction of hydrodynamic volume that allows a faster Brownian relaxation. This work also discusses the relevance of the size distribution.
Two sets of bi-magnetic Zn.5Mn.5Fe2O4@ Fe3O4 core-shell nanoparticles were prepared by a seed-mediated modified co-precipitation method. While the first set was obtained by fast addition of the alkaline solution to grow Fe3O4 shell over... more
Two sets of bi-magnetic Zn.5Mn.5Fe2O4@ Fe3O4 core-shell nanoparticles were prepared by a seed-mediated modified co-precipitation method. While the first set was obtained by fast addition of the alkaline solution to grow Fe3O4 shell over the ferrite seeds, a slow
drop-wise addition of stoichiometric Fe2+/Fe3+ ion solution to the alkaline ferrite seeds solution was adopted to synthesize the second set. Samples were characterized by electron microscopy (STEM, TEM, UHRTEM and magnetometry measurements. Viability
MTT assay of the nanoparticles on L929 murine fibroblasts were performed, indicating that they are biocompatible. The coating of Zn0.5Mn0.5FeO4 nanoparticle by a magnetite or maghemite shell minimize the effect of the magnetic dead layer at the core surface,
improving the magnetic properties and offering thus outstanding values for biological application. Relaxometry values r2 higher than 300 mM-1s
-1 at H 1.5 T, and cell viability at concentrations as high as 0.5 mg/ml render these bi-magnetic nanoparticles to have a vast potential as MRI contrast agents.
drop-wise addition of stoichiometric Fe2+/Fe3+ ion solution to the alkaline ferrite seeds solution was adopted to synthesize the second set. Samples were characterized by electron microscopy (STEM, TEM, UHRTEM and magnetometry measurements. Viability
MTT assay of the nanoparticles on L929 murine fibroblasts were performed, indicating that they are biocompatible. The coating of Zn0.5Mn0.5FeO4 nanoparticle by a magnetite or maghemite shell minimize the effect of the magnetic dead layer at the core surface,
improving the magnetic properties and offering thus outstanding values for biological application. Relaxometry values r2 higher than 300 mM-1s
-1 at H 1.5 T, and cell viability at concentrations as high as 0.5 mg/ml render these bi-magnetic nanoparticles to have a vast potential as MRI contrast agents.
Research Interests:
Research Interests:
Recently, potential applications of the magnetic heating for heterogeneous catalysis or organic synthesis have been reported. As these new applications are not limited by biocompatibility requirements, a wide range of possibilities for... more
Recently, potential applications of the magnetic heating for heterogeneous catalysis or organic synthesis have been reported. As these new applications are not limited by
biocompatibility requirements, a wide range of possibilities for non-aqueous colloidal nanoparticles with enhanced magnetic properties is open. In this work, manganese and cobalt ferrite nanoparticles are synthesized by co-precipitation method with average particle size around 12 nm. The particles are either coated with tetramethylammonium hydroxide (TMAOH) and dispersed in water or with oleic acid (OA) and dispersed in hexane to produce aggregated or disaggregated nanoparticles, respectively. It is observed that the particle disaggregation
improves significantly the heating efficiency from 12 to 96 W/g in the case of cobalt ferrite, and from 120 to 413 W/g for the manganese ferrite. The main responsible for this improvement is the reduction of hydrodynamic volume that allows a faster Brownian relaxation. This work also
discusses the relevance of the size distribution.
biocompatibility requirements, a wide range of possibilities for non-aqueous colloidal nanoparticles with enhanced magnetic properties is open. In this work, manganese and cobalt ferrite nanoparticles are synthesized by co-precipitation method with average particle size around 12 nm. The particles are either coated with tetramethylammonium hydroxide (TMAOH) and dispersed in water or with oleic acid (OA) and dispersed in hexane to produce aggregated or disaggregated nanoparticles, respectively. It is observed that the particle disaggregation
improves significantly the heating efficiency from 12 to 96 W/g in the case of cobalt ferrite, and from 120 to 413 W/g for the manganese ferrite. The main responsible for this improvement is the reduction of hydrodynamic volume that allows a faster Brownian relaxation. This work also
discusses the relevance of the size distribution.
Research Interests:
Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size,... more
Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size, hDi, equal to 40 nm. Susceptibility measurements showed the well-known Verwey transition at a temperature of 90 K. The decrease of Verwey transition temperature, with respect to the one reported in literature (125 K) was attributed to the low average crystallite size. Moreover, the spin-glass like transition was observed at 35 K. Activation energy calculated from susceptibility curves, with values ranging from 6.26 to 6.93meV, showed a dependence of spin-glass transition on frequency. Finally, hysteresis loops showed that there is not an effect of Verwey transition on magnetic properties. On the other hand, a large increase of coercivity and remanent magnetization at a temperature between 5 and 50K confirmed the presence of a magnetic transition at low temperatures.
Research Interests:
Zinc ferrite is synthesized via mechano-activation, followed by thermal treatment. Spinel ZnFe2O4 single phase is confirmed by X-ray diffraction. SEM micrographs show large particles with average particle size <Dpart> = 1 μm, with... more
Zinc ferrite is synthesized via mechano-activation, followed by thermal treatment. Spinel ZnFe2O4 single phase is confirmed by X-ray diffraction. SEM micrographs show large particles with average particle size <Dpart> = 1 μm, with particles in intimate contact. However, TEM micrographs show incrusted nanocrystallites at the particles surface, with average nanocrystallite size calculated as <Dinc> ≈ 5 nm. The blocking temperature at 118 K in the ZFC-FC curves indicates the presence of a superparamagnetic response which is
attributable to the incrusted nanocrystallites. Moreover, the hysteresis loops show the coexistence of superpara- and paramagnetic responses. The former is observable at the low
field region; meanwhile, the second one is responsible of the lack of saturation at high field region. This last behavior is related to a paramagnetisc contribution coming from well ordered crystalline microdomains. The hysteresis loops are analyzed by means of two different models. The first one is the
susceptibility model used to examine separately the para- and superparamagnetic contributions. The fittings with the theoretical model confirm the presence of the above
mentioned magnetic contributions. Finally, using the Langevin-based model, the average superparamagnetic diameter <DSPM> is calculated. The obtained value <DSPM> = 4.7 nm (~5 nm) is consistent with the average nanocrystallite size observed by TEM.
attributable to the incrusted nanocrystallites. Moreover, the hysteresis loops show the coexistence of superpara- and paramagnetic responses. The former is observable at the low
field region; meanwhile, the second one is responsible of the lack of saturation at high field region. This last behavior is related to a paramagnetisc contribution coming from well ordered crystalline microdomains. The hysteresis loops are analyzed by means of two different models. The first one is the
susceptibility model used to examine separately the para- and superparamagnetic contributions. The fittings with the theoretical model confirm the presence of the above
mentioned magnetic contributions. Finally, using the Langevin-based model, the average superparamagnetic diameter <DSPM> is calculated. The obtained value <DSPM> = 4.7 nm (~5 nm) is consistent with the average nanocrystallite size observed by TEM.
Research Interests:
The influence of magnetic interactions in assembles formed by either aggregated or dis-aggregated uniform γ-Fe2O3 particles are investigated as a function of particle size, concentration and applied field. Hyperthermia and magnetization... more
The influence of magnetic interactions in assembles formed by either aggregated or dis-aggregated uniform γ-Fe2O3 particles are investigated as a function of particle size, concentration and applied field. Hyperthermia and magnetization measurements are performed in the liquid phase of colloids consisting of 8 and 13 nm uniform γ-Fe2O3 particles dispersed in water and hexane. While hexane allows obtaining dis-agglomerated particle system, aggregation is observed in the case of water colloids. The ZFC curves shows a discontinuity in the magnetization values associated with the melting points of water and hexane. Additionally, for 13 nm γ-Fe2O3 dispersed in hexane, a second magnetization jump is observed that depends on particle concentration and shifts toward lower temperature by increasing applied field. This second jump is related to the strength of the magnetic interactions since it is only present in dis-agglomerated particle systems with the largest size, i.e. is not observed for 8 nm superparamagnetic particles, and surface effects can be discarded. Specific absorption rate (SAR) decreases with increasing concentration only for hexane colloid, whereas for aqueous colloids, SAR is almost independent on particle concentration. Our results suggest that, as consequence of the magnetic interactions, the dipolar field acting on large particles increases with concentration leading to a decrease of the SAR.
Research Interests:
In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as... more
In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as crystallite size is reduced to 15 nm. However, contrary to previously reported works, an enhancement of saturation magnetization is observed as crystallite size increases from 15 to 22 nm. This unexpected behavior is accompanied by an unusual cell volume variation that seems to induce ferromagnetic-like behavior at expense of antiferromagnetic one. Besides, field cooled hysteresis loops show exchange bias field and coercivity enhancement for increasing cooling fields, which suggest a kind of core-shell structure with AFM-FM coupling for crystallite sizes as small as 15 nm. It is expected that inner core orders antiferromagnetically, whereas uncompensated surface spins behave as spin glass with ferromagnetic-like ordering.
Research Interests:
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... 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.
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.
Research Interests:
"Iron oxides nanoparticles with different sizes are successfully synthesized using sol-gel method. X-ray diffraction (XRD) and Mössbauer spectroscopy show that the obtained nanoparticles are mainly composed of maghemite phase (-Fe2O3).... more
"Iron oxides nanoparticles with different sizes are successfully synthesized using sol-gel method. X-ray diffraction (XRD) and Mössbauer spectroscopy show that the obtained nanoparticles are mainly composed of maghemite phase (-Fe2O3). XRD and transmission electron microscopy (TEM) results suggest that the nanoparticles have sizes ranging from 14 to 30 nm, which are indeed confirmed by large magnetic saturation and high blocking
temperature. At room temperature, the observation of a on-negligible coercive field suggests that the particles are ferro/ferrimagnetic. The specific absorption rate (SAR) under an
alternating magnetic field is investigated as a function of size, frequency and amplitude of the applied magnetic field. A mean heating efficiency of 30 W/g is obtained for the smallest
particles at 110 kHz and 190 Oe, whereas further increase of particle size does not improve significantly the heating efficiency."
temperature. At room temperature, the observation of a on-negligible coercive field suggests that the particles are ferro/ferrimagnetic. The specific absorption rate (SAR) under an
alternating magnetic field is investigated as a function of size, frequency and amplitude of the applied magnetic field. A mean heating efficiency of 30 W/g is obtained for the smallest
particles at 110 kHz and 190 Oe, whereas further increase of particle size does not improve significantly the heating efficiency."
Research Interests:
Manganese and Zinc ferrites were prepared by solid state reaction. The resulting powders were pressed into pellets and heat treated at 1100 C. The samples were characterized by using X-ray diffraction, pure phases of zinc ferrite... more
Manganese and Zinc ferrites were prepared by solid state reaction. The resulting powders were pressed into pellets and heat treated at 1100 C. The samples were characterized by using X-ray diffraction, pure phases of zinc ferrite (ZnFe2O4) and manganese ferrite (MnFe2O4) were obtained. Scanning electron microscopy images showed a good contact between particles. A drop of electrical resistance was found in both samples, MnFe2O4 and ZnFe2O4, with values going from 2750 to 130X and from 1100 to 55X, respectively. Transition temperatures were determined to be TV~225K for MnFe2O4 and TV~130K for ZnFe2O4. Magnetoresistance measurements were carried out in the temperature range where R showed the transition, defined as the Verwey-like transition temperature range, DTV. No magnetoresistive effect was observed out of it. The magnetoresistive coefficient (MRC) observed at DTV reached its maximum values of 1.1% for MnFe2O4 and 6.68% for ZnFe2O4. The differences between MRC values are related to the
divalent metal element used. Finally, the magnetoresistive response indicates that the electrical transition observed is strongly influencing the magnetoresistance; where the underlying
responsible for this behavior could be a charge reordering occurring at the Verwey-like transition temperature.
divalent metal element used. Finally, the magnetoresistive response indicates that the electrical transition observed is strongly influencing the magnetoresistance; where the underlying
responsible for this behavior could be a charge reordering occurring at the Verwey-like transition temperature.
Research Interests:
This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are non-magnetic in bulk is analysed for two cases: (a)... more
This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are
non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which
were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications
non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which
were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications
Research Interests:
Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size,... more
Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size, 〈D〉, equal to 40 nm. Susceptibility measurements showed the well-known Verwey transition at a temperature of 90 K. The decrease of Verwey transition temperature, with respect to the one reported in literature (125 K) was attributed to the low average crystallite size. Moreover, the spin-glass like transition was observed at 35 K. Activation energy calculated from susceptibility curves, with values ranging from 6.26 to 6.93 meV, showed a dependence of spin-glass transition on frequency. Finally, hysteresis loops showed that there is not an effect of Verwey transition on magnetic properties. On the other hand, a large increase of coercivity and remanent magnetization at a temperature between 5 and 50 K confirmed the presence of a magnetic transition at low temperatures.
Research Interests:
deA 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-ZrO2). From the... more
deA 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-ZrO2). 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-ZrO2. 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.
Research Interests:
The specific absorption rate (SAR) of γ-Fe2O3 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... more
The specific absorption rate (SAR) of γ-Fe2O3 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.
Research Interests: Synthesis of nanoparticles, Magnetic Nanoparticles Used in Cancer Therapy, Magnetic Hyperthermia, Iron Oxide Nanoparticles, Magnetic Nanoparticles, and 6 moreNeel Relaxation Time, Magnetic Heating, Heat Generated by Loss Power, SAR Vs Size, Magnetic Particle Properties Vs Size, and Magnetic Properties As a Function of Size
Magnetic and NMR relaxivity properties of γ-Fe2O3 nanoparticles embedded into the walls of polyelectrolyte multilayer capsules and freely dispersed in a sodium borate buffer solution have been investigated. The different geometric... more
Magnetic and NMR relaxivity properties of γ-Fe2O3 nanoparticles embedded into the walls of polyelectrolyte multilayer capsules and freely dispersed in a sodium borate buffer solution have been investigated. The different geometric distribution of both configurations provides the opportunity to study the relationship of water accessibility and magnetic properties of particles on the NMR relaxivity. Changes in their blocking temperature and average dipolar field were modeled as a function of packing fraction in the ensemble of free and entrapped nanoparticles. For free nanoparticles with relatively low concentration, relaxivity values increase with packing fraction according to an increase in the dipolar field and larger water accessibility. However, for embedded nanoparticles in the capsule wall, packing fractions should be limited to optimize the efficiency of this system as magnetic resonance imaging (MRI) contrast agent.
Research Interests:
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... 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.
Research Interests:
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 HfO2 have been the subject of several earlier... 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 HfO2 have been the subject of several earlier studies, some aspects remain open. In particular, time differential perturbed angular correlation spectroscopy studies using Ta-181 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 (Ta) was performed in order to understand the second interaction observed in Ta-doped monoclinic HfO2. 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.
Research Interests:
The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferromagnetic nanoparticles of manganese oxide perovskite La0.56(SrCa)0.22MnO3 have been studied. The particles were subjected to a size... more
The effect of a high-frequency alternating magnetic field on HeLa tumor cells incubated with ferromagnetic nanoparticles of manganese oxide perovskite La0.56(SrCa)0.22MnO3 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.
Research Interests:
Colloidal magnetic iron platinum nanoparticles were embedded at different densities into the walls of polyelectrolyte multilayer capsules. Changes in their magnetic properties such as relaxivities as a function of average distances... more
Colloidal magnetic iron platinum nanoparticles were embedded at different densities into the walls of polyelectrolyte multilayer capsules. Changes in their magnetic properties such as relaxivities as a function of average distances between the magnetic nanoparticles were investigated and their properties for magnetic resonance imaging discussed.
Research Interests:
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... 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 Neel 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. (C) 2009 Elsevier Inc. All rights reserved.
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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... 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 nanoparticles-encapsulated ethosomes offer a versatile platform for the enhancement of pharmacological efficacy in transdermal and dermal delivery systems.
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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 (XRD), transmission electron... 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 (XRD), transmission electron microscopy (TEM), and perturbed angular correlation (PAC) spectroscopy. The PAC technique was used to measure the electric quadrupole interactions (QIs) 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 <= T <= 1550 K. The oxygen-metal ratio of the as-synthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4 <= x <= 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 < 5 nm, whereas bare n-HfO(2) is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO(2) is Q(A) = 32(5) 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 (m) phase up to T <= 1400 K. In coated n-ZrO(2)/Al(2)O(3), the monoclinic and tetragonal (t) phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic (t or c) 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 >= 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 < 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) similar to 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.
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In this work we present new results on spontaneous oxidation of disordered fcc FePt nanoparticles. The "as-made" oleic acid and oleylamine coated FePt nanoparticles of average size 4 nm synthesized by a high-boiling coordinating solvent... more
In this work we present new results on spontaneous oxidation of disordered fcc FePt nanoparticles. The "as-made" oleic acid and oleylamine coated FePt nanoparticles of average size 4 nm synthesized by a high-boiling coordinating solvent method were exposed to air over a period of days and characterized structurally and magnetically by means of different techniques such as XANES, XPS, EXAFS, and SQUID magnetometry. The "as-made" FePt nanoparticles stabilize in the disordered fcc structure and have a very low magnetic saturation (M(s)=11 emu/g) and a huge coercive field (H(c)=1800 Oe) compared to the low temperature bulk values of the disordered fcc FePt. We observed that the coercive field and the magnetic saturation change with the time the sample is exposed to air and these changes are associated with the oxidation or passivation of the nanoparticle surface that gives place to a core-shell structure. Indeed, the study on the electronic properties of the nanoparticles confirms the magnetic results and indicates that when the nanoparticles are exposed to air, changes in the oxidation state of both Fe and Pt occur, the oxidation state of Fe coming close to hematite. The formation of hematite tends to soften the "as-made" FePt nanoparticles as observed by the reduction of the coercive field to almost one third of the original value. Although the hematite softens the FePt nanoparticles, there is an exchange coupling at the interface of the core-shell characterized by the increase of the coercive field from 300 to 900 Oe when the sample is cooled in an applied field of 50 kOe. (C) 2008 American Institute of Physics.
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In this work, we present the magnetic properties of gold-coated FePt nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt synthesized after ligand exchange with mercaptoundecanoic acid. The particle size... more
In this work, we present the magnetic properties of gold-coated FePt nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt synthesized after ligand exchange with mercaptoundecanoic acid. The particle size determined from transmission electron microscopy (TEM) micrographs goes from 4 nm for the uncoated nanoparticles to a maximum of 10 nm for the gold-coated ones indicating that the thickness of the shell ranges from 1 to 3 nm. The magnetic characterization consists in hysteresis cycles at 10 and 300 K. The results show that, at low field and room temperature, the magnetic behavior of uncoated and coated nanoparticles are surprisingly quite similar. Because the gold-coated nanoparticles keep the magnetic properties of FePt and the presence of gold improves the functionalization of nanoparticles, the system is suitable for biological application. Mercaptoundecanoic ligand transfer was used to render water stable nanoparticles in a wide pH range. Transmission electron microscopy and dynamic light scattering (DLS) results show the nanoparticles slightly agglomerate after ligand exchange. Fourier transform infrared spectroscopy results suggest that thiol binds to the gold atoms of the surface
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In this work, the structural and magnetic properties of the gold-coated FePt nanoparticles synthesized from high- temperature solution phase are presented. The amount of gold was optimized to obtain most of the FePt particles coated. The... more
In this work, the structural and magnetic properties of the gold-coated FePt nanoparticles synthesized from high- temperature solution phase are presented. The amount of gold was optimized to obtain most of the FePt particles coated. The particle diameter increases from 4 to 10 nm as observed by TEM. The magnetic properties are largely affected by the coating. At low temperature, the coercive. field Hc of the coated nanoparticles decreases about three times respect to the uncoated and the blocking temperature reduces to the half. The changes of the magnetic behavior are discussed in terms of the effect of the gold atoms at the FePt core surface.
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Perturbed angular correlation (PAC) spectroscopy has been used to investigate the electric field gradient (EFG) at the probe nucleus In-111/Cd-111 in the paramagnetic phase of the rare earth (R)-aluminium compounds RAl2 for all R elements... more
Perturbed angular correlation (PAC) spectroscopy has been used to investigate the electric field gradient (EFG) at the probe nucleus In-111/Cd-111 in the paramagnetic phase of the rare earth (R)-aluminium compounds RAl2 for all R elements and Y and in RAl3 for R=Gd,Tm,Yb,Lu. The nuclear electric quadrupole interaction (QI) between the EFG and the Cd-111 quadrupole moment was measured as a function of temperature in the range T-C < T <= 1200 K. In the second half of the RAl2 series and in the RAl3 compounds, except for YbAl3, the quadrupole frequency nu(q) shows the monotonous decrease with increasing temperature normally observed with closed-shell probe nuclei in metallic systems. In the early members of the RAl2 series, however, nu(q)(T) passes through a maximum at T similar to 300 K. It is proposed that this unusual behavior reflects a contribution of the 4f shell of the R constituents to the EFG at the Al site which is quenched at higher temperatures by thermal averaging of the 4f quadrupole moment. In the intermediate-valence compound YbAl3 the temperature dependence of the QI exhibits a shallow maximum which can be related to the temperature variation of the 4f hole occupation. Furthermore the PAC spectra provide information on the site preference of the In-111 solutes in RAl2 for different R constituents and temperatures. In two-phase samples containing RAl2 and RAl3 with AuCu3 structure, at T < 900 K the solutes show a very pronounced preference for the Al site of RAl3, but at higher temperatures they migrate to the Al site of RAl2. Jumps of the In-111/Cd-111 probes on the Al sublattice of RAl3 compounds with AuCu3 structure (R=Tm,Yb,Lu) lead to nuclear spin relaxation of Cd-111. The temperature dependence of the relaxation rates shows an Arrhenius behavior with jump activation enthalpies E-A=1.6(1) eV for R=Tm, Lu and E-A=1.2(1) eV for R=Yb.
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In the past few years ferromagnetic-like behavior has been reported in metal gold nanoparticles coated with diverse organic surfactants. In this work we report on the effect of thermal annealing on the ferromagnetic-like behavior of oleic... more
In the past few years ferromagnetic-like behavior has been reported in metal gold nanoparticles coated with diverse organic surfactants. In this work we report on the effect of thermal annealing on the ferromagnetic-like behavior of oleic acid and oleylamine coated gold nanoparticles of about 7 nm size. The magnetic moment of the "as prepared" sample is about 3 x10(-2) emu/g and the coercive field is 200 Oe at 10 kOe and 5 K, after the annealing the behavior changes from ferromagnetic-like to paramagnetic and the magnetization at 10 kOe decreases at a factor of 10. These results are compared with those obtained for oleylamine coated gold nanoparticles, which are diamagnetic at room temperature
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Nanocrystalline ZrO2 and TiO2 (n-ZrO2, n-TiO2), synthesized in a microwave plasma, have been investigated by X-ray and electron diffraction and by perturbed angular correlation (PAC) measurements of the nuclear electric quadrupole... more
Nanocrystalline ZrO2 and TiO2 (n-ZrO2, n-TiO2), synthesized in a microwave plasma, have been investigated by X-ray and electron diffraction and by perturbed angular correlation (PAC) measurements of the nuclear electric quadrupole interaction (QI) of the probe nucleus (181) Ta residing on the cation site. The microwave synthesis produces zirconia in the cubic/tetragonal phase, titania in the anatase structure. Grain growth and phase transformations have been studied in bare and Al2O3-coated zirconia particles. Coating the nanoparticles with an amorphous Al2O3 layer obstructs grain growth and may suppress the monoclinic phase
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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... 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.
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In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption... more
In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption spectrum has a maximum at 2.35 eV. The calculated optical absorption spectrum is shifted and narrower than the experimental one, indicating that the oleic acid and oleylamine do not merely passivate the metallic nanoparticles but modify its electronic structure. These gold nanoparticles show in addition a kind of magnetic order similar to other organic passivated gold nanoparticles as thiol-capped gold nanoparticles. Although the magnetic interactions seem to be weaker than in thiol-capped ones, the magnetic behavior looks similar to that, i.e., an invariant temperature dependence of the magnetization from 5 to 300 K and a noticeable coercive field. We analyze the influence of the organic layer bonding the nanoparticles on the magnetic behavior. © 2006 American Institute of Physics. DOI: 10.1063/1.2401314
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Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus Cd-111 in ferromagnetically ordered rare earth (R)-dialuminides RAl2 as a function of... more
Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus Cd-111 in ferromagnetically ordered rare earth (R)-dialuminides RAl2 as a function of temperature for the rare earth constituents R = Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R = Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R = Pr, Nd, Tb, Dy and Ho the parameters B-4, B-6 of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The Cd-111 PAC spectrum of EuAl2 at 9 K confirms the antiferromagnetic structure of this compound
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Bare and coated TiO2 nanoparticles with particle sizes d < 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction,... more
Bare and coated TiO2 nanoparticles with particle sizes d < 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation (PAC) measurements of the electric quadrupole interaction (QI) at the probe nucleus Ta-181 on the metal site of TiO2 at temperatures 290 <= T <= 1450 K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO2 was found. Anatase appears to be the stable form of nanocrystalline TiO2 below d similar to 10 nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d < 5 nm after synthesis to d > 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO2. The data suggest a critical grain size of d similar to 10 nm for the onset of the ordering process. The spectra of coarse-grained TiO2 are reached at a particle size d >= 30 nm. In n-TiO2 coated with Al2O3 and ZrO2 both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO2 cores and in the ZrO2 coating of n-TiO2/ZrO2 at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO2 kernels and the coatings. (
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The magnetic phase transitions of pseudo-binary rare earth (R) Laves phases R(1-y)Y(x)Co(2) have been investigated by perturbed angular correlation (PAC) measurements of the magnetic hyperfine fields at the probe nucleus (111)Cd for R =... more
The magnetic phase transitions of pseudo-binary rare earth (R) Laves phases R(1-y)Y(x)Co(2) have been investigated by perturbed angular correlation (PAC) measurements of the magnetic hyperfine fields at the probe nucleus (111)Cd for R = Tb, Sm and Ho at various Y concentrations x and for R = Gd, Dy, Er, Nd and Pr at the concentration x = 0.3. First-order transitions were observed in Tb(1-x)Y(x)CO(2) and Sm(1-x)Y(x)Co(2) for x >= 0.3, in Ho(1-x)Y(x)Co(2) for x < 0.4 and in R(0.7)Y(0.3)CO(2) for R = Dy, Ho, Er, Nd and Pr. For Gd(0)(7)(.)Y(0.3)Co(2), the temperature dependence of the average magnetic hyperfine field is compatible with a second-order transition. The discontinuity of the magnetic hyperfine interaction at the first-order transitions of heavy R(1-x)Y(x)Co(2), which mainly reflects the jump of 3d magnetization of the Co subsystem at T(C), was found to increase monotonically with decreasing order temperature. The TC dependence of the normalized magnetic frequency v(M) (T(C))/v(M) (0) proportional to [1-(T(C)/T(0))(2)](1/2) with To = 203(5) K for the boundary temperature between first- and second-order transitions can be explained by the temperature dependence of the coefficient of the M(4) term of the free energy in the Wohlfarth-Rhodes-Shimizu theory of itinerant electron metamagnetism.
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RCo2The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus Cd-111 on the rare earth (R) site of the pseudobinary Laves-phase compounds R1-xYxCo2 has been investigated by perturbed angular correlation (PAC)... more
RCo2The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus Cd-111 on the rare earth (R) site of the pseudobinary Laves-phase compounds R1-xYxCo2 has been investigated by perturbed angular correlation (PAC) spectroscopy at 10 K for the rare earth R=Tb and Ho at various Y concentrations x <= 0.8 and for R=Gd, Dy, Er at the concentration x=0.3. Up to four components with different magnetic interaction frequencies nu(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 (NN) 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: nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R).The frequency nu(M)(4Y)=35(2) MHz is independent of the R constituent and of the Y concentration up to x <= 0.6. These properties identify nu(M)(4Y) as the contribution of the Co 3d moments to the hyperfine interaction at the Cd-111 site. The frequency steps Delta nu(M)[<= 0.1 nu(M)(4Y)] reflect the spin polarization directly induced by the 4f spins at the probe nucleus. From Gd to Er, the spin polarization decreases much stronger than expected from the linear variation of the 4f spin in the heavy R series. An indirect 4f contribution caused by a dependence of the Co 3d moment on the number of R neighbors can be excluded. The relation nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R) then implies that the contributions of the 3d and 4f spins to the magnetic hyperfine field in RCo2 have the same relative sign.
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AC susceptibility measurements, as a function of temperature and frequency, have been carried out in Y(1-x)Tb(x)Co(2) (0.1 less than or equal to x less than or equal to 0.9) compounds to investigate the magnetic phase transition. It is... more
AC susceptibility measurements, as a function of temperature and frequency, have been carried out in Y(1-x)Tb(x)Co(2) (0.1 less than or equal to x less than or equal to 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(1-x)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
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Rare-earth-deficient R1−xNi2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation (PAC) measurements of electric quadrupole interactions at... more
Rare-earth-deficient R1−xNi2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation (PAC) measurements of electric quadrupole interactions at the site of the probe nucleus 111Cd. Although 111Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction (QI) with frequencies νq≈265–275 MHz has been found in the paramagnetic phases of R1−xNi2 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 R1−xNi2, R=Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T>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 τC<10−6 s, have been detected. These observations can be explained consistently by two assumptions: (i) the mother isotope 111In of the PAC probe 111Cd constitutes an attractive potential for vacancies and (ii) the R vacancies in R1−xNi2 are highly mobile at temperatures T<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 R1−xNi2. For R=Pr,Nd,Sm,Gd the binding energy is in the range 0.15–0.40 eV. The activation energy EA for vacancy jumps near the probe derived from the temperature dependence of the nuclear spin relaxation at 200 K⩽T⩽300 K is small. The values observed in different samples cover a range of 0.1 eV⩽EA⩽0.23 eV. The trial frequency w0 of these jumps appears to be correlated to the activation energy: ln w0(MHz)≈58EA(eV). At high temperatures T>500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R1−xNi2. Auxiliary 111Cd PAC measurements have been carried in Sc0.95Ni2, ScNi2, ScNi0.97, Gd2Ni17, GdNi5, GdNi3, and GdNi.
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The orientation of the magnetic hyperfine field B-hf at Cd-111 in gadolinium metal relative to the hexagonal symmetry axis of the host has been investigated by perturbed angular correlation spectroscopy. Its temperature dependence agrees... more
The orientation of the magnetic hyperfine field B-hf at Cd-111 in gadolinium metal relative to the hexagonal symmetry axis of the host has been investigated by perturbed angular correlation spectroscopy. Its temperature dependence agrees with the results of mu SR and neutron diffraction measurements of the orientation of the Gd magnetic moment, indicating that the Gd anisotropy constants are not greatly altered by the impurity substitution. (C) 2003 Elsevier B.V. All rights reserved.
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The magnetic hyperfine field of Cd-111 in the C15 Laves phases RNi2 has been investigated by perturbed angular correlation (PAC) spectroscopy as a function of temperature for the rare earth constituents R=Nd, Sm, Gd, Tb, Dy, Ho, Er, and... more
The magnetic hyperfine field of Cd-111 in the C15 Laves phases RNi2 has been investigated by perturbed angular correlation (PAC) spectroscopy as a function of temperature for the rare earth constituents R=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm.
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This paper presents a perturbed angular correlation study of the magnetic and electric hyperfine interactions of Cd-111 on Gd sites of the Gd-Ni intermetallic compounds GdNi, GdNi2 GdNi3, Gd2Ni7, GdNi5 and Gd2Ni17
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The order and other properties of the magnetic phase transitions in the rare-earth (R)-cobalt Laves phases RCo2 have been studied for R=Gd, Tb, Dy, Ho, Er, Sm, Nd, and Pr by measuring the temperature dependence of the magnetic hyperfine... more
The order and other properties of the magnetic phase transitions in the rare-earth (R)-cobalt Laves phases RCo2 have been studied for R=Gd, Tb, Dy, Ho, Er, Sm, Nd, and Pr by measuring the temperature dependence of the magnetic hyperfine interaction of the nuclear probe 111Cd on the cubic R sites using the perturbed angular correlation technique. Both for heavy and light R constituents the transitions change from second order (Gd, Tb, Sm) to first order (Dy, Ho, Er, Nd, Pr) at order temperatures of 150–200 K. For heavy R constituents, the order deduced from the hyperfine interaction is in agreement with previous investigations. The observation of first order transitions in NdCo2 and PrCo2, however, is unexpected. In earlier studies the transitions in these compounds are usually classified as second order transitions. Both in the heavy and the light RCo2 the discontinuous jump of the hyperfine interaction at the first order transitions increases with decreasing order temperature. This trend implies that the Co magnetization at the transition increases with decreasing TC which can be related to the temperature dependence of the coefficient of the M4 term of the free energy in the Wohlfarth-Rhodes-Shimizu theory of itinerant electron magnetism. All compounds investigated presented a spread of the order temperature of ∼1–2K which results in a coexistence of the paramagnetic and the magnetically ordered phase near the transition and causes a critical increase of the relative linewidth of the hyperfine frequency diverging as δ∝(1-T/TC)ɛ with ɛ=-1.0(1).
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We present an ab initio study of the relaxations introduced in TiO2 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... more
We present an ab initio study of the relaxations introduced in TiO2 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 ofthe orientation of the largest EFG tensor component, V33, 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 V33 at the Cd site is parallel to either the [110] or the [11̅ 0] crystal axis.
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The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The... more
The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The effect of this point defect on Ta-181 probe in BaTi x Hf1-xO3 for x = 0.70, 0.50, 0.30, 0.10, 0.05, and 0.01, is studied. The lattice constant of these oxides at room temperature were measured using XRD technique. The quadrupole parameters corresponding to probe-defect interaction were obtained by means of Perturbed Angular Correlation (PAC) spectroscopy as functions of both, composition and temperature. A static, asymmetric and distributed quadrupole interaction was fitted to all PAC data. This interaction was observed to be strongly composition dependent. These results and those corresponding to compositions 0.75 less than or equal to x < 1 were compared to the ones obtained using the point charge method for the calculation of the electric field gradient. The proposed model took into account polarized oxygen vacancies, different covalence of Ti-O and Hf-O bonds and using computational simulation for cations and oxygen vacancies lattice positions.