Papers by Luis Fernández Barquín
Physical Review B, 2008
Measurements of specific heat, dc magnetic susceptibility, and Yb L II and L III x-ray absorption... more Measurements of specific heat, dc magnetic susceptibility, and Yb L II and L III x-ray absorption near-edge structure ͑XANES͒ and extended x-ray absorption fine structure ͑EXAFS͒ on YbAl 3 milled alloys are reported. X-ray diffraction patterns are consistent with a reduction in particle size down to 10 nm and an increase in the lattice strain up to 0.4% for 120 h of milling time. A decrease in the mean valence from 2.86 for the unmilled alloy to 2.70 for 120 h milled YbAl 3 is obtained from the analysis of XANES spectra. From the analysis of spectra in the EXAFS region, an increase in the mean-square disorder of neighbor distance with milling time is detected in good agreement with the results of x-ray diffraction. Size effects strongly influence the magnetic and thermal properties. The value for the maximum of the magnetic susceptibility decreases around 30% for 120 h milled alloy and an excess specific heat, with a peak around 40 K in the milled samples, is derived. These changes in the physical properties along the milled YbAl 3 alloys are associated with the reduction in particle size. Such a reduction leads to the existence of a large number of Yb 2+ atoms at the surface with respect to the bulk affecting the overall electronic state.
Journal of Physics: Conference Series, 2010
Measurements of the magnetization and specific heat on nanocrystalline TbAl 2 , TbCu2 and GdAl2 w... more Measurements of the magnetization and specific heat on nanocrystalline TbAl 2 , TbCu2 and GdAl2 with mean particle diameters below 40 nm show a different behavior from those made on bulk alloys. A reduction of the saturation magnetization quantified in TbAl 2 from 8.4 µ B /mol for the bulk to 5.8 µ B /mol for a 14 nm alloy is attributed to a spin disordered layer on the particle surface. In the specific heat, the lambda anomaly associated to the ferro (∆c= 6.6 J/molK, 9.8 J/molK) and to the antiferromagnetic transitions (7.1 J/molK) in the bulk samples, respectively, is smeared out due to finite size effects.
Journal of Magnetism and Magnetic Materials, 2004
Results concerning the identification of a spin-glass transition in perovskites using the IN10 ba... more Results concerning the identification of a spin-glass transition in perovskites using the IN10 backscattering spectrometer at ILL are presented. The observed temperatures of these transitions agree with the previously obtained ones by means of DC and AC susceptibility measurements. Such a behaviour has been interpreted in terms of a transition from a polaronic liquid to a polaronic glass state.
Physical Review B, 2006
The effect of Fe doping on the magnetic properties of La 0.7 Pb 0.3 Mn 1−x Fe x O 3 ͑0 ഛ x ഛ 0.2͒... more The effect of Fe doping on the magnetic properties of La 0.7 Pb 0.3 Mn 1−x Fe x O 3 ͑0 ഛ x ഛ 0.2͒ colossal magnetoresistance perovskites is studied by muon spin rotation ͑SR͒ spectroscopy and macroscopic static magnetization measurements. The latter quantities show a cusp at temperatures below that signaling magnetic ordering ͑Curie temperature T C ͒ as well as a kink at lower temperatures which are taken as signatures of magnetic irreversibility. Random substitution of Mn by Fe ions gives rise to antiferromagnetic couplings mediated by conventional superexchange interaction. This manifests itself by the decrease of Curie temperatures and a concomitant decrease of the spontaneous magnetization with increased doping. The muon relaxation data under zero and applied longitudinal fields is interpreted in terms of progressive spin freezing concomitant with a reduction of the ferromagnetic component as Fe substitution is increased. The spin dynamics as T C is crossed from above shows clear departures from those expected for exchange-coupled Heisenberg or Ising systems but rather it displays features common to magnetically interacting-cluster systems. This effect leads to the appearance of a peak in the muon relaxation rate versus temperature curves that is located at temperatures below that signaling magnetic ordering ͑T C ͒. Finally, direct evidence of the existence of antiferromagnetic and ferromagnetically short-range-ordered regions within the x = 0.2 sample is provided by polarized neutron diffraction. The picture that emerges from our study identifies competing positive ͑double exchange͒ and negative ͑superex-change͒ interactions at random sites as the entities responsible for the low-temperature freezing monitored by using SR spectroscopy.
Journal of Non-Crystalline Solids, 2008
... Javier Bermejo a , Luis Fernández Barquín b , Jon Gutiérrez a and Jose Manuel Barandiarán a ,... more ... Javier Bermejo a , Luis Fernández Barquín b , Jon Gutiérrez a and Jose Manuel Barandiarán a , Corresponding Author Contact Information , E-mail The ... We want to thank I. Orúe (SGIKER of the UPV/EHU), S. Cottrell (ISIS), C. Mondelli and P. Fouquet (ILL) for their assistance. ...
Comparable d c as S1 but agglomerates.
In November 2013, a new and innovative EU FP7 NMP-Large project on Nanometrology Standardization ... more In November 2013, a new and innovative EU FP7 NMP-Large project on Nanometrology Standardization Methods for Magnetic Nanoparticles (NanoMag) has started. The NanoMag project will continue until November 2017, and the NanoMag consortium involves research institutes, universities, companies and metrology institutes. The NanoMag project objectives are to improve and redefine existing analyzing methods and to develop new analyzing methods for magnetic nanostructures. Using improved manufacturing technologies we synthesize magnetic nanoparticles with specific properties that are being analyzed with a multitude of characterization techniques (focusing on both structural and magnetic properties) and bring the experimental results together to obtain a self-consistent picture which describes how structural and magnetic properties are interrelated. This extensive survey will then be used to define standard measurements and techniques which are necessary for defining a magnetic nanostructure ...
International Journal of Inorganic Materials, 1999
Journal of Physics: Condensed Matter, 1997
We report x-ray, EXAFS and neutron diffraction experiments on stress-relieved (SR) and field-anne... more We report x-ray, EXAFS and neutron diffraction experiments on stress-relieved (SR) and field-annealed (FA) ribbons of Fe 78 Si 9 B 13 using optimized geometries to measure the structural and magnetic anisotropy to unprecedented accuracy. A peak of magnitude ∼5% of the main peak height is found in the S(Q) pattern from three independent neutron experiments on the FA ribbons, at Q ∼ 3Å −1 . We establish that this peak is of magnetic origin, arising from moments that to some extent lie perpendicularly to the anneal field direction, and is associated with local structure. We show that this anisotropy is consistent with a recently proposed disturbed exchange model, suggesting that the result represents the first direct observation of the existence of antiferromagnetically coupled moments within an amorphous ferromagnetic alloy.
Journal of Materials Chemistry, 1999
Fine particle Fe±Zr±B alloys have been prepared by sodium borohydride reduction of aqueous soluti... more Fine particle Fe±Zr±B alloys have been prepared by sodium borohydride reduction of aqueous solutions of iron(II) and zirconium(IV) sulfate. A comprehensive survey of reaction conditions was completed and optimum parameters found for the preparation of precipitated products. These included rapid stirring, controlled pH in the range 5±6, using degassed solvents, Fe : Zr ratios between 4 : 1 and 20 : 1 and sodium borohydride concentrations between 0.1 and 1.2 M. Scanning electron microscopy showed that the products comprised spherical particles of 0.1 mm diameter with metal compositions, determined by atomic absorption, electron microprobe and EDAX, which mirrored the initial starting solutions. The products fell into two categories: amorphous Fe±Zr±B alloys when the Fe : Zr ratio was near 9 : 1, and nanocrystalline alloys, comprising a small amount of nanometre scale a-Fe crystallites embedded in amorphous Fe±Zr±B alloy, when the Fe : Zr ratio was either higher or lower than 9 : 1. Minor impurities in the form of Fe(II) complex and Fe(III) oxide phases were also detected in the products. Full structural and magnetic characterisations were performed on selected samples, including X-ray diffraction, Mo È ssbauer spectroscopy, differential scanning calorimetry, magnetometry and Zr-edge EXAFS measurements.
Physica B: Condensed Matter, 2006
Temperature-dependent zero-, longitudinal-and transverse-field mSR spectra have been recorded in ... more Temperature-dependent zero-, longitudinal-and transverse-field mSR spectra have been recorded in nanometric Fe 13 Cu 10 Ag 77 obtained by mechanical alloying. The alloy comprises Fe(Cu) nanoparticles (5 nm) in a Ag matrix formed by 16 nm particles, as deduced from the Rietveld refinement of the X-ray diffraction pattern. The muon depolarization can be described by a Dynamical Lorentzian Kubo-Toyabe function. This is in agreement with a high degree of interparticle dipolar interactions, giving rise to a cluster spin glass-like behavior below 100 K. The intra particle interactions are also probed by the mSR, and influence the thermal variation of the static width. r
Journal of Non-Crystalline Solids, 2001
We discuss the properties of the spin glass phase observed in the CeNi 1Àx Cu x system. We have m... more We discuss the properties of the spin glass phase observed in the CeNi 1Àx Cu x system. We have measured the AC susceptibility, ®eld cooled and zero ®eld cooled magnetization, hysteresis loops, magnetic relaxation and neutron scattering. An inhomogeneous magnetic arrangement, namely spin glass-like state, is proposed. A description consisting of nanometric magnetic clusters embedded in a non-magnetic matrix is likely for the richer Ni compounds. We support this hypothesis as a consequence of the coexistence of chemical disorder in the Ni/Cu crystallographic sites, the random and coherent anisotropy, the competing magnetic interactions and large 4f-conduction band hybridization. Ó
Journal of Alloys and Compounds, 2010
The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compo... more The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compositions and Curie temperatures between 230 and 300 K has been studied. Albeit the maximum entropy change is low (|!S M | max ~ 3 J kg -1 K -1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads out over a broad temperature interval (!T ~ 200 K). The estimated values for the relative cooling power are close to that of pure Gd. From the applied magnetic field dependence of the magnetic entropy change a master curve valid for these Fe-rich FeZrB amorphous alloys has been found. However, two reference temperatures of the |!S M | (T) are needed in order to get a collapsing curve for all the maximum applied magnetic field values.
Intermetallics, Jan 1, 2010
Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amor... more Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amorphous ribbons of two different compositions, Fe91Zr7B2 and Fe88Zr8B4, with respective Curie temperature values of 230 and 285 K have been studied. Although the maximum magnetic entropy change is relatively moderate (|ΔSM|max ∼ 3 J K−1 kg−1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads over a broad temperature interval (∆T ∼ 200 K), giving rise to a large refrigerant capacity loss (RC ∼ 435 J kg−1) without any hysteresis. The Curie temperature can be easily tuned between 200 and 350 K by changing the boron content. Therefore, the MCE can be controlled over a wide temperature interval, thus making these amorphous alloys promising candidates for magnetic refrigeration near room temperature.
Journal of Magnetism and Magnetic Materials, 2007
The magnetic behaviour of a Fe 50 Ag 50 granular thin film has been studied by means of AC and DC... more The magnetic behaviour of a Fe 50 Ag 50 granular thin film has been studied by means of AC and DC magnetic measurements. Exchange coupling between magnetic nanoparticles appears at Tp200 K decreasing the coercive field of the sample. Additionally, an exchange bias is observed at low temperature related to the existence of a spin disordered interface around the nanoparticles. r
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Papers by Luis Fernández Barquín