Publications (25)51.42 Total impact
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Article: Breakdown of Hund's third rule in amorphous Co-W nanoparticles and crystalline Co_ {3} W alloys
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ABSTRACT: We present x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) measurements performed at the Co K and W L2,3 edges, on amorphous Co-W alloy nanoparticles, and a comparison with those on a bulk Co3W alloy. A strong hybridization between the 4p and 3d orbitals in Co and the 5d band in W are observed, resulting in an induced magnetic moment in the W atoms. The orbital to spin moment ratio in W of all these Co-W systems is positive, suggesting a parallel orientation of the two moments. This is opposite to the expected antiparallel coupling for atoms with a less-than-half-filled 5d band, according to Hund's third rule. These findings are supported by calculations of the electronic density of states projected at the Co 3d and W 5d orbitals, as well as XANES spectra and XMCD signals at the Co K and W L2,3 edges in a Co3W system.Phys. Rev. B. 05/2013; 86(6):064428. -
Article: SiO2 coating effects in the magnetic anisotropy of Fe3-xO4 nanoparticles suitable for bio-applications.
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ABSTRACT: We present radio frequency transverse susceptibility (TS) measurements on oleic acid-coated and SiO2-coated Fe3-xO4 magnetite nanoparticles. The effects of the type of coating on the interparticle interactions and magnetic anisotropy are evaluated for two different particle sizes in powder samples. On the one hand, SiO2 coating reduces the interparticle interactions as compared to oleic acid coating, the reduction being more effective for 5 nm than for 14 nm diameter particles. On the other hand, the magnetic anisotropy field at low temperature is lower than 1 kOe in all cases and independent of the coating used. Our results are relevant concerning applications in biomedicine, since the SiO2 coating renders 5 and 14 nm hydrophilic particles with very limited agglomeration, low anisotropy, and superparamagnetic behavior at room temperature. The TS technique also allows us to discriminate the influence on the anisotropy field of interparticle interactions from that of the thermal fluctuations.Nanotechnology 03/2013; 24(15):155705. · 3.98 Impact Factor -
Article: Strong Paramagnetism of Gold Nanoparticles Deposited on a Sulfolobus acidocaldarius S Layer.
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ABSTRACT: Magnetic properties of Au nanoparticles deposited on an archaeal S layer are reported. X-ray magnetic circular dichroism and superconducting quantum interference device magnetometries demonstrate that the particles are strongly paramagnetic, without any indication of magnetic blocking down to 16 mK. The average magnetic moment per particle is M_{part}=2.36(7) μ_{B}. This contribution originates at the particle's Au 5d band, in which an increased number of holes with respect to the bulk value is observed. The magnetic moment per Au atom is 25 times larger than any measured in other Au nanoparticles or any other configurations up to date.Physical Review Letters 12/2012; 109(24):247203. · 7.37 Impact Factor -
Article: Fabrication and magnetic characterization of cobalt antidot arrays: effect of the surrounding continuous film.
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ABSTRACT: We have performed an experimental study on the influence of a ferromagnetic continuous film in the magnetization reversal processes in discrete submicrometric antidot arrays fabricated on it. In order to compare the magnetic properties, two sets of antidot arrays have been fabricated over a cobalt thin film: embedded in the continuous film, and isolated by a trench surrounding the array. X-ray photoemission electron microscopy images of the virgin state show the same magnetic domain distribution in both sets of samples, finding no evidence of any effect of the surrounding film. This result is supported by the hysteresis loops measured with magneto-optical Kerr effect, as isolated and non-isolated arrays present almost coincident loops. A huge increase of the coercivity of the film is achieved, and the expected dependence on the geometrical parameters of the array is found, connecting the previous studies on the micro- and nanometric scales.Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7437-41. · 1.56 Impact Factor -
Article: X-ray photoemission electron microscopy studies of local magnetization in Py antidot array thin films
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ABSTRACT: Permalloy antidot thin films were grown by sputtering onto anodic alumina templates, replicating their hexagonal order inside micrometric geometric domains. The advanced high-spatial and sensitive x-ray photoemission electron microscopy technique under an applied magnetic field has enabled magnetic domain structure imaging and quantitative hysteresis loop analysis inside nanoscale regions with geometric order and at border regions. The study has been complemented by vibrating sample magnetometry and magneto-optic Kerr effect measurements. The magnetization process is clearly determined by the geometry characteristics of the antidot arrays. Inside geometric ordered domains, the strength of effective in-plane magnetic anisotropy depends on the antidot diameter–to–film thickness ratio, which determines the partial balance between stray fields generated by magnetic charges at the lateral surface of the antidots and those at the upper–bottom film surface. In addition, the border regions between geometric domains act as pinning centers for magnetization reversal and eventually generate a harder magnetic region.Physical Review B 04/2012; 184427(18-85):184427. · 3.69 Impact Factor -
Article: Magnetic nanoparticles with bulk-like properties
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ABSTRACT: The magnetic behavior of $Fe_{3-x}O_4$ nanoparticles synthesized either by high-temperature decomposition of an organic iron precursor or low-temperature co-precipitation in aqueous conditions, is compared. Transmission electron microscopy, X-ray absorption spectroscopy, X-ray magnetic circular dichroism and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulk-like magnetic and electronic properties, while nanoparticles synthesized by co-precipitation show much poorer crystallinity and particle-like phenomenology, including reduced magnetization, high closure fields and shifted hysteresis loops. The key role of the crystal quality is thus suggested since particle-like behavior for particles larger than about 5 nm is only observed when they are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or bio-distribution studies.Journal of Applied Physics 04/2011; 109:07B524. · 2.17 Impact Factor -
Article: Magnetic nanoparticles with bulklike properties (invited)
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ABSTRACT: The magnetic behavior of Fe3−xO4 nanoparticles synthesized by either high-temperature decomposition of an organic iron precursor or low-temperature coprecipitation in aqueous conditions is compared. Transmission electron microscopy, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulklike magnetic and electronic properties, while nanoparticles synthesized by coprecipitation show much poorer crystallinity and particlelike phenomenology, including reduced magnetization, high closure fields, and shifted hysteresis loops. The key role of the crystal quality is thus suggested, because particlelike behavior for particles larger than about 5 nm is observed only when the particles are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or biodistribution studies.Journal of Applied Physics 04/2011; 109(7):07B524-07B524-6. · 2.17 Impact Factor -
Article: Magnetic switching and magnetic transitions in ErCo2 probed by radio-frequency transverse susceptibility
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ABSTRACT: ErCo2 represents a typical example of magnetism of itinerant electron systems and metamagnetic processes and has been the subject of extensive research. We present here the first study of radio-frequency transverse susceptibility (TS) of bulk ErCo2 using a self-resonant tunnel-diode oscillator technique. TS measurements reveal the collective magnetic switching of the Er moments at temperatures below the ferrimagnetic transition temperature, Tc ∼ 32 K, and the existence of Co nanoclusters with short-range correlations at Tc<T<Tf (Tf denoted as the flipping temperature). The difference in the magnetic configuration between the ferrimagnetic, parimagnetic, and paramagnetic states, as well as the change from the paramagnetic to parimagnetic regime upon varying dc magnetic fields are also probed by TS experiments. These findings are discussed in the context of our previous investigations using other different techniques which provide further insights into the magnetism and the so-called parimagnetism phenomenon in ErCo2.Journal of Applied Physics 03/2011; 109(7):07E118-07E118-3. · 2.17 Impact Factor -
Article: Breakdown of Hund's third rule for intrinsic magnetic moments
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ABSTRACT: Detailed X-ray magnetic circular dichroism measurements as a function of temperature are reported in ErCo2 at the Co L2, 3 edges. The application of magneto-optical sum rules allows for the determination of the Co orbital and spin magnetic moments both in the ferrimagnetic and paramagnetic phases. In agreement with the predictions of Hund's third rule, Co orbital and spin moments are parallel throughout the overall temperature range, except in a narrow temperature window within the paramagnetic phase in which, contrary to what is expected, they are overall coupled antiparallel. This is the first time that such behavior is observed in an intrinsic magnetic moment. A qualitative consideration of the balance between the interatomic and intra-atomic interactions acting on the cobalt atoms explains the phenomenon.EPL (Europhysics Letters) 01/2011; 93(1):17006. · 2.17 Impact Factor -
Article: Surface anisotropy broadening of the energy barrier distribution in magnetic nanoparticles.
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ABSTRACT: The effect of surface anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the Tln(t/τ(0)) scaling approach. The comparison between the distributions of the anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core anisotropy, and of the total anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable Tln(t/τ(0)), enables the determination of the surface anisotropy as a function of the particle size. We show that the contribution of the particle surface to the total anisotropy energy can be well described by a size-independent value of the surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe(3-x)O(4) particles of 4.9 nm average size and x about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective surface anisotropy constant K(s) = 2.9 × 10(-2) erg cm(-2) gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.Nanotechnology 11/2008; 19(47):475704. · 3.98 Impact Factor -
Article: X-Ray Circular Magnetic Dichroism (XCMD) as a Probe of the Dynamics of the Spin Reorientation Transitions in Nd2Fe14B and Er2Fe14B Systems
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ABSTRACT: We present the first experimental observation of a spin reorientation phase transition (SRT) with the X-ray circular-magnetic-dichroism (XCMD) technique. The XCMD signal, related to the projection of the rare-earth moment on the direction of the X-ray beam, has been measured as a function of temperature for both, a first-order SRT in Er2Fe14B (abrupt reorientation by 90°) and a second-order one in Nd2Fe14B (continuous tilt of the magnetization from the c-axis). The feasibility of this technique to study SRTs and the capability to better understand the magnetic behaviour of intermetallic compounds exhibiting coexistence of itinerant and localized magnetism are demonstrated.EPL (Europhysics Letters) 07/2007; 28(2):135. · 2.17 Impact Factor -
Chapter: MAGNETIC DYNAMICS OF CO NANOSPHERES: ORIGIN OF THE ENHANCED ANISOTROPY
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ABSTRACT: The present work deals with the observation of enhancement of the magnetic anisotropy of Co nanoparticles and its origin. The samples were granular multilayer samples prepared by sequential deposition, by sputtering, of amorphous Al2O3 and Co layers on a Si substrate. Co nanoparticles are selforganized in a quasi-regular spatial order of approximately hexagonal closepacked symmetry. The particles studied range in average diameter 〈D〉 between 0.7 nm and nearly 5 nm, with a narrow size distribution. This well controlled morphology has enabled us to circumvent ambiguities in sample configuration and, by means of a simple model for fluctuating moments, explain the dynamics of the Co particle moments in terms of an activation energy with contributions from anisotropy Keff, dipole-dipole interactions Edip, and a bias magnetic field H. The anisotropy is enhanced by one to two orders of magnitude with respect to the bulk fcc Co due to strong pinning of the surface Co magnetic moments anisotropy, and increase as 1/D as the particle diameter decreases. The origin of this enhancement is related to an increase of the orbital magnetic moment at the surface atoms. Capping the Co nanospheres with a Cu film increases further the particle anisotropy and the orbital magnetic moment of the surface atoms.05/2006: pages 1-25; -
Article: Enhancement of the magnetic anisotropy of Co clusters by Au capping
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ABSTRACT: We study the magnetic properties of Co nanoparticles, prepared by sputtering, with diameters ranging from 1 to 3.5 nm. The effective anisotropy, which determines the activation energy for the magnetization reversal, increases with decreasing particle’s size, showing the dominant role played by surface atoms. We find that the superparamagnetic blocking temperature and the coercive field are significatively enhanced when the clusters are capped by a thin Au layer. This enhancement is probably caused by the bonding or hybridization of Co and Au atoms at the interface between the two metals. It provides thus a method to modify the magnetic anisotropy, which can be of interest for the applications of magnetic nanoparticles.Journal of Applied Physics 04/2006; 99(8):08G705-08G705-3. · 2.17 Impact Factor -
Article: Competitive effects of dipolar interactions and a bias magnetic field on the magnetic relaxation times of Co clusters
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ABSTRACT: We have investigated, by means of ac susceptibility experiments, the magnetic relaxation of layers of Co nanoclusters (D≃2.6 nm) embedded in Al2O3. Superparamagnetic blocking takes place at higher temperatures as the number of layers increases. These results are interpreted using a simple model in which dipole–dipole interactions between nearest neighbor particles increase the relaxation time. The influence of interactions is affected by the application of bias magnetic field H. As the magnetic moments of the particles become polarized by H, the blocking temperature approaches the behavior expected for noninteracting particles. © 2003 American Institute of Physics.Journal of Applied Physics 05/2003; 93(10):7032-7034. · 2.17 Impact Factor -
Article: Magnetic relaxation of interacting co clusters: crossover from two- to three-dimensional lattices.
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ABSTRACT: The influence that dipole-dipole interactions exert on the dynamics of the magnetization of nanometer-sized Co clusters has been studied by means of ac and dc susceptibility experiments. These clusters grow in a quasiordered layered structure, where all relevant parameters can be tailored and measured independently. Our data show without ambiguity that the magnetic relaxation becomes slower as the degree of interaction increases. The effective activation energy increases linearly with the number of nearest neighbor clusters, evolving from the value for a 2D layer to the fully 3D behavior, which is nearly reached for five layers. The experimental results agree quantitatively with the predictions of a simple model.Physical Review Letters 06/2002; 88(21):217205. · 7.37 Impact Factor -
Article: Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: Influence of the surface and of interparticle interactions
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ABSTRACT: We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25–7000 atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal process is shown to proceed classically down to the lowest temperature investigated (1.8 K).Phys. Rev. B. 02/2002; 65(9). -
Article: Fe K-edge x-ray magnetic circular dichroism study in R6Fe23 (R=Ho and Y) compounds near compensation temperature
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ABSTRACT: We present a x-ray magnetic circular dichroism (XMCD) study performed at the Fe K edge in R6Fe23 compounds exhibiting magnetic compensation (R=Ho and Y). The Fe K-edge XMCD signal has been identified as due to both Fe itself and rare-earth contributions. Following a simple two-sublattices model for the analysis of the dichroic signal the contribution of the rare-earth sublattice to total Fe K-edge XMCD signal has been extracted and proven to be directly correlated to the R magnetic moment. © 2000 American Institute of Physics.Journal of Applied Physics 06/2000; 88(1):336-338. · 2.17 Impact Factor -
Article: X-ray magnetic circular dichroism at the rare earth L2,3 edges in R2Fe14B intermetallics
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ABSTRACT: We present a systematic x-ray magnetic circular dichroism (XMCD) study performed at the rare-earth (R) L2,3 edges on the R2Fe14B series. The identification of multipolar contributions to the signal allows the application of sum rules to the obtained XMCD spectra. The results show that both orbital and spinorial components of the R(5d) magnetic moment are proportional to the 4f ones. Hence, the magnetic moment of 5d shell derived from a sum-rules analysis evidences an important nonquenched orbital contribution. This result is in contradiction with Campbell’s model for R–Fe intermetallic compounds. Consequently, both XMCD sum rules and Campbell’s model have to be revised in the atomic level.Journal of Applied Physics 05/2000; 87:5884. · 2.17 Impact Factor -
Article: Strong noncollinearity between nonequivalent Nd magnetic moments in Nd2Fe14B at low temperature
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ABSTRACT: A temperature-dependent x-ray resonant magnetic scattering study through the spin reorientation transition (SRT) that Nd2Fe14B undergoes below TSRT=135 K is presented. The experimental results evidence a strong noncollinearity between the magnetic moments of Nd atoms in 4f and 4g crystallographic sites in the low-temperature phase. © 2000 American Institute of Physics.Journal of Applied Physics 04/2000; 87(9):4762-4764. · 2.17 Impact Factor -
Article: Magnetic polarization of noble metals by Co nanoparticles in M-capped granular multilayers (M= Cu, Ag, and Au): An x-ray magnetic circular dichroism study
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ABSTRACT: X-ray magnetic circular dichroism spectra have been recorded at the L2,3 edges of Co, Cu, Ag, and Au, and at the K edges of Co and Cu for a series of multilayer systems of partially self-assembled Co nanoparticles, both capped with alumina and with different metals (Cu, Ag, and Au). The orbital and spin moments mL and mS and their ratio mL/mS, for Co, Cu, Ag, and Au, have been determined from those measurements. The trend of increasing mL/mS for Co is the same as found for increasing the effective anisotropy Keff alumina <Cu<Ag<Au capped. It is shown that the relative increase in the surface contribution for both quantities, (mL/mS)surf and KS, is directly correlated. It is argued that the increase in mL with metal capping actually reflects an increase in the anisotropy of the orbital moment for the Co surface atoms. The redistribution of the number of holes in the noble metal bands by hybridization with the Co surface atom 3d band and the polarization of the nd band by exchange interaction with the Co bands give rise to the magnetic moments derived for the three noble metals investigated.Phys. Rev. B. 77(18).
Top co-authors
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Institutions
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2012
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Spanish National Research Council
- Instituto de Ciencia de Materiales de Madrid
Madrid, Madrid, Spain
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2000–2012
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Universidad de Zaragoza
- Departamento de Física de la Materia Condensada
Zaragoza, Aragon, Spain
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