Juan de la Figuera | CSIC (Consejo Superior de Investigaciones Científicas-Spanish National Research Council) (original) (raw)
Papers by Juan de la Figuera
Physical Review B, 2016
Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultra-high vacuum and ... more Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultra-high vacuum and studied in-situ by low-energy electron microscopy. Annealed films present magnetic contrast in spin-polarized lowenergy electron microscopy. The energy dependence of the electron reflectivity was determined and a maximum value of its spin asymmetry of about 1% was measured. The magnetization direction of the Tb films is in-plane. Upon raising the temperature, no change in the domain distribution is observed, while the asymmetry in the electron reflectivity decreases when approaching the critical temperature, following a power law ∼ (1 -T /T C ) β with a critical exponent β of 0.39.
Oxidation Mechanism in Iron Oxide Ultrathin Films
Oxides are promising functional materials because their physical properties can be tailored by tu... more Oxides are promising functional materials because their physical properties can be tailored by tuning their stoichiometry and structure [1]. In particular, binary iron oxides range from reasonable conductors (magnetite) to insulators, and from ferrimagnets to antiferromagnets. In consequence, understanding the transformations from one phase into another, as well as the influence of external parameters on the observed changes is a long standing theme in iron oxide research, with implications in fields as diverse as corrosion, geophysics, catalysis and spintronics [2]. In the present work we aim at understanding the oxidation mechanism of bi-component iron oxide thin films. Initially, iron oxide thin films were grown on a Ru(0001) single crystal substrate using O2 -assisted molecular beam epitaxy. Under exposure to molecular oxygen at 900 K, the growth of a bi-component film is obtained. We observe nanometer-thick micron-sized, flat triangular islands of Fe3 O4 (magnetite) on a FeO (wustite) wetting layer [3, 4]. In a second step, exposure to NO2 at 600 K promotes the chemical transformation of the initially-grown film to an Fe2 O3 composition, but still in bi-component form. The triangular magnetite islands are transformed into ¿-Fe2 O3 (maghemite) and the surrounding layer is converted to ¿-Fe2 O3 (hematite). The oxidation pathway of w ¿stite to hematite and magnetite to maghemite can be explained by considering that both are topotactic transformations occurring by the diffusion of iron in octahedral sites to react with oxygen on the film¿s surface. The bi-component iron oxide thin films were characterized by low-energy electron microscopy (LEEM) and photoemission microscopy (PEEM) using a combination of laterally resolved x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD),and low-energy electron diffraction (LEED). Our study indicates the possibility of obtaining different bi-component iron oxides in ultrathin form in the same substrate by means of a choice of the experimental conditions. This behavior allows tuning the ultrathin iron oxides to exploit their catalytic and magnetic properties. [1] R. Cornell and U. Schwertmann, The Iron Oxides; (John Wiley & Sons Ltd, 1997). [2] M. Bibes, J. E. Villegas, and A. Barth ¿l ¿my, ¿Ultrathin oxide films and interfaces for electronics and spintronics¿, Adv. Phys. 60, 5 (2011). [3] B. Santos, E. Loginova, A. Mascaraque, A. Schmid, K. F. McCarty and J. de la Figuera, ¿Structure and Magnetism in Ultrathin Iron Oxides Characterized by Low Energy Electron Microscopy¿, J. Phys. Cond. Mat. 21, 314011 (2009). [4] M. Monti, B. Santos, A. Mascaraque, O. Rodr¿guez de la Fuente, M. A. Ni ¿o, T. O. Mente ¿, A. Locatelli, K. F. McCarty, J. F. Marco and J. de la Figuera, ¿Magnetism in Nanometer-Thick Magnetite¿,Phys. Rev. B 85, 020404 (2012).Peer Reviewe
A Platform for Addressing Individual Magnetite Islands Grown Epitaxially on Ru(0001) and Manipulating Their Magnetic Domains
Crystal Growth & Design, Jun 28, 2023
Effect of Ni substitution on the antiferromagnetic domains of cobalt oxide
Ultramicroscopy, Jun 1, 2023
Physical Review Letters, Sep 4, 2007
Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small se... more Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections.
Physical review, May 17, 2016
The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy ... more The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono-and bi-layers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bi-layer on a tungsten substrate has been addressed. To complete the physical picture, an iron tri-layer has also been analysed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the [1-10] direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.
Dislocation mediated layer-by-layer growth of strained Ag films on Ru(0001)
APS, Mar 1, 2003
We present experimental and theoretical investigations of multilayer Ag films grown on Ru(0001). ... more We present experimental and theoretical investigations of multilayer Ag films grown on Ru(0001). An unexpected strain relief mechanism is observed: STM measurements reveal a perfectly hexagonal 2nd layer with a lattice constant very close to bulk Ag. This 2nd layer grows on top of a first Ag layer containing a highly regular 2 dimensional array of dislocations with a density
Physical Review B, Jul 15, 2015
High aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We exp... more High aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multi-domain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design.
Hyperfine Interactions, Aug 30, 2011
Applied Surface Science, Apr 1, 2022
Thin epitaxial films of metals on insulating substrates are essential for many applications, as c... more Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al2O3(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orientation, a well-defined epitaxial relation with the substrate and a high degree of crystal quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7 nm in thickness, for which relaxation of epitaxial strain is not complete, produce a similar degree of dechanneling. The surface of the films can be prepared in a clean and ordered state in order to allow further epitaxial growth on top.
Physical review, Dec 21, 2018
Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enab... more Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in CoFe 2 O 4 /FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.
Motion of Magnetic Domain Walls and Vortices in Epitaxial Magnetite Microstructures
SSRN Electronic Journal
Efficient domain-wall pinning at chemical notches in FeNi nanowires
Combining vector XMCD-PEEM and micromagnetic simulations
Oral presentation given at the VIII AUSE Congress and III ALBA User's Meeting, held in Madrid... more Oral presentation given at the VIII AUSE Congress and III ALBA User's Meeting, held in Madrid on October 9-11th, 2017.
AMPHIBIAN CSIC experimental data CoFe2O4-PLD_ApplSurfSci2019 [Dataset]
AMPHIBIAN CSIC experimental data Spring bilayers_PhysRevB2018. AMPHIBIAN_CSIC_simulated-data_Spring-bilayers_PhysRevB2018 [Dataset]
Peer reviewe
Journal of Physics D: Applied Physics, 2020
Permanent magnets based on hard hexaferrite represent the largest family of magnets being used to... more Permanent magnets based on hard hexaferrite represent the largest family of magnets being used today by volume. They generate moderate remanence induction, but present crucial advantages in terms of availability, cost, resistance to demagnetization and corrosion and absence of eddy current losses. As a consequence, ferrites are the most logical candidate for substitution of rare-earths in selected applications that do not demand the best performing magnets. If the remanence of ferrite-based magnets was to be improved, even mildly, the door to a larger scale substitution could be opened. In this framework, we review here current strategies to improve the properties of hexaferrites for permanent magnet applications. We first discuss the potential of exploring the nanoscale. Second, progress related to controllably doping hexaferrites is revised. Third, results achieved by fabricating hard-soft magnetic composites using ferrites as the hard phase are presented. Finally, future prospects and new potential end applications for ferrite magnets are discussed.
Physical Review B, 2016
Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultra-high vacuum and ... more Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultra-high vacuum and studied in-situ by low-energy electron microscopy. Annealed films present magnetic contrast in spin-polarized lowenergy electron microscopy. The energy dependence of the electron reflectivity was determined and a maximum value of its spin asymmetry of about 1% was measured. The magnetization direction of the Tb films is in-plane. Upon raising the temperature, no change in the domain distribution is observed, while the asymmetry in the electron reflectivity decreases when approaching the critical temperature, following a power law ∼ (1 -T /T C ) β with a critical exponent β of 0.39.
Oxidation Mechanism in Iron Oxide Ultrathin Films
Oxides are promising functional materials because their physical properties can be tailored by tu... more Oxides are promising functional materials because their physical properties can be tailored by tuning their stoichiometry and structure [1]. In particular, binary iron oxides range from reasonable conductors (magnetite) to insulators, and from ferrimagnets to antiferromagnets. In consequence, understanding the transformations from one phase into another, as well as the influence of external parameters on the observed changes is a long standing theme in iron oxide research, with implications in fields as diverse as corrosion, geophysics, catalysis and spintronics [2]. In the present work we aim at understanding the oxidation mechanism of bi-component iron oxide thin films. Initially, iron oxide thin films were grown on a Ru(0001) single crystal substrate using O2 -assisted molecular beam epitaxy. Under exposure to molecular oxygen at 900 K, the growth of a bi-component film is obtained. We observe nanometer-thick micron-sized, flat triangular islands of Fe3 O4 (magnetite) on a FeO (wustite) wetting layer [3, 4]. In a second step, exposure to NO2 at 600 K promotes the chemical transformation of the initially-grown film to an Fe2 O3 composition, but still in bi-component form. The triangular magnetite islands are transformed into ¿-Fe2 O3 (maghemite) and the surrounding layer is converted to ¿-Fe2 O3 (hematite). The oxidation pathway of w ¿stite to hematite and magnetite to maghemite can be explained by considering that both are topotactic transformations occurring by the diffusion of iron in octahedral sites to react with oxygen on the film¿s surface. The bi-component iron oxide thin films were characterized by low-energy electron microscopy (LEEM) and photoemission microscopy (PEEM) using a combination of laterally resolved x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD),and low-energy electron diffraction (LEED). Our study indicates the possibility of obtaining different bi-component iron oxides in ultrathin form in the same substrate by means of a choice of the experimental conditions. This behavior allows tuning the ultrathin iron oxides to exploit their catalytic and magnetic properties. [1] R. Cornell and U. Schwertmann, The Iron Oxides; (John Wiley & Sons Ltd, 1997). [2] M. Bibes, J. E. Villegas, and A. Barth ¿l ¿my, ¿Ultrathin oxide films and interfaces for electronics and spintronics¿, Adv. Phys. 60, 5 (2011). [3] B. Santos, E. Loginova, A. Mascaraque, A. Schmid, K. F. McCarty and J. de la Figuera, ¿Structure and Magnetism in Ultrathin Iron Oxides Characterized by Low Energy Electron Microscopy¿, J. Phys. Cond. Mat. 21, 314011 (2009). [4] M. Monti, B. Santos, A. Mascaraque, O. Rodr¿guez de la Fuente, M. A. Ni ¿o, T. O. Mente ¿, A. Locatelli, K. F. McCarty, J. F. Marco and J. de la Figuera, ¿Magnetism in Nanometer-Thick Magnetite¿,Phys. Rev. B 85, 020404 (2012).Peer Reviewe
A Platform for Addressing Individual Magnetite Islands Grown Epitaxially on Ru(0001) and Manipulating Their Magnetic Domains
Crystal Growth & Design, Jun 28, 2023
Effect of Ni substitution on the antiferromagnetic domains of cobalt oxide
Ultramicroscopy, Jun 1, 2023
Physical Review Letters, Sep 4, 2007
Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small se... more Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections.
Physical review, May 17, 2016
The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy ... more The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono-and bi-layers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bi-layer on a tungsten substrate has been addressed. To complete the physical picture, an iron tri-layer has also been analysed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the [1-10] direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.
Dislocation mediated layer-by-layer growth of strained Ag films on Ru(0001)
APS, Mar 1, 2003
We present experimental and theoretical investigations of multilayer Ag films grown on Ru(0001). ... more We present experimental and theoretical investigations of multilayer Ag films grown on Ru(0001). An unexpected strain relief mechanism is observed: STM measurements reveal a perfectly hexagonal 2nd layer with a lattice constant very close to bulk Ag. This 2nd layer grows on top of a first Ag layer containing a highly regular 2 dimensional array of dislocations with a density
Physical Review B, Jul 15, 2015
High aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We exp... more High aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multi-domain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design.
Hyperfine Interactions, Aug 30, 2011
Applied Surface Science, Apr 1, 2022
Thin epitaxial films of metals on insulating substrates are essential for many applications, as c... more Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al2O3(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orientation, a well-defined epitaxial relation with the substrate and a high degree of crystal quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7 nm in thickness, for which relaxation of epitaxial strain is not complete, produce a similar degree of dechanneling. The surface of the films can be prepared in a clean and ordered state in order to allow further epitaxial growth on top.
Physical review, Dec 21, 2018
Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enab... more Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in CoFe 2 O 4 /FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.
Motion of Magnetic Domain Walls and Vortices in Epitaxial Magnetite Microstructures
SSRN Electronic Journal
Efficient domain-wall pinning at chemical notches in FeNi nanowires
Combining vector XMCD-PEEM and micromagnetic simulations
Oral presentation given at the VIII AUSE Congress and III ALBA User's Meeting, held in Madrid... more Oral presentation given at the VIII AUSE Congress and III ALBA User's Meeting, held in Madrid on October 9-11th, 2017.
AMPHIBIAN CSIC experimental data CoFe2O4-PLD_ApplSurfSci2019 [Dataset]
AMPHIBIAN CSIC experimental data Spring bilayers_PhysRevB2018. AMPHIBIAN_CSIC_simulated-data_Spring-bilayers_PhysRevB2018 [Dataset]
Peer reviewe
Journal of Physics D: Applied Physics, 2020
Permanent magnets based on hard hexaferrite represent the largest family of magnets being used to... more Permanent magnets based on hard hexaferrite represent the largest family of magnets being used today by volume. They generate moderate remanence induction, but present crucial advantages in terms of availability, cost, resistance to demagnetization and corrosion and absence of eddy current losses. As a consequence, ferrites are the most logical candidate for substitution of rare-earths in selected applications that do not demand the best performing magnets. If the remanence of ferrite-based magnets was to be improved, even mildly, the door to a larger scale substitution could be opened. In this framework, we review here current strategies to improve the properties of hexaferrites for permanent magnet applications. We first discuss the potential of exploring the nanoscale. Second, progress related to controllably doping hexaferrites is revised. Third, results achieved by fabricating hard-soft magnetic composites using ferrites as the hard phase are presented. Finally, future prospects and new potential end applications for ferrite magnets are discussed.