Person:
Pérez García, Lucas

First Name

Lucas

Last Name

Pérez García

URI

https://hdl.handle.net/20.500.14352/75170

Affiliation

Universidad Complutense de Madrid

Faculty / Institute

Ciencias Físicas

Department

Física de Materiales

Area

Física Aplicada

Identifiers

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Now showing 1 - 8 of 8

Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN
(Applied physics letters, 2006) Dhar, S.; Kammermeier, T.; Ney, A.; Pérez García, Lucas; Ploog, K. H.; Melnikov, A.; Wieck, A. D.
The structural and the magnetic properties of Gd-focused ion-beam-implanted GaN layers are studied. Gd^(3+) ions were uniformly implanted in molecular beam epitaxy rown GaN layers at room temperature with an energy of 300 keV at doses ranging from 2.4x10^(11) to 1.0x10^(15) cm^(-2) which corresponds to an average Gd concentration range of 2.4x10^(16)-1.0x10^(20) cm^(-3). The implanted samples were not subjected to any annealing treatment. No secondary phase related to Gd was detected by x-ray diffraction in these layers. Magnetic characterization with superconducting quantum interference device reveals a colossal magnetic moment of Gd and ferromagnetism with an order temperature above room temperature similar to that found in epitaxially grown Gd-doped GaN layers. The effective magnetic moment per Gd atom in these samples is, however, found to be an order of magnitude larger than that found in epitaxially grown layers for a given Gd concentration which indicates that the defects play an important role in giving rise to this effect.
Materiales magnéticos blandos obtenidos por electrodeposición : aplicaciones en sensores integrados
(2005) Pérez García, Lucas; Sánchez Trujillo, María del Carmen; Aroca Hernández-Ros, Claudio
En esta tesis doctoral se estudia la elaboración de aleaciones de CoNiFe y CoP por electrodeposición, la caracterización magnética de las mismas y su utilización para fabricar heteroestructuras y sensores magnéticos integrados.Hemos realizado un análisis del proceso de electrodeposición de las aleaciones de CoNiFe, de la estabilidad de los electrolitos y de la electroquímica del sistema, así como de la influencia de los parámetros de la electrodeposición en la composición y homogeneidad de las mismas. Hemos estudiado también la relación entre composición, estructura cristalina y propiedades magnéticas de las aleaciones obtenidas. Con todo ello, hemos establecido las condiciones óptimas para la obtención de aleaciones de CoNiFe magnéticamente blandas. Además, hemos estudiado su estructura de dominios magnéticos y la dependencia de ésta con el espesor de las películas. En el caso de las aleaciones amorfas de CoP, comprobamos que es posible controlar la anisotropía mediante la fabricación de multicapas formadas por capas ferromagnéticas con distinta imanación de saturación. Las aleaciones así obtenidas tienen un campo coercitivo muy pequeño (HC < 10 A/m), lo que hace que sean muy interesantes como núcleo en aplicaciones de tipo inductivo. Realizamos también un estudio de la dependencia del campo coercitivo con la frecuencia.Como aplicación, hemos estudiado heteroestructuras semiconductor / ferromagnético y piezoeléctrico / magnetostrictivo además de fabricar dos tipos de sensor fluxgate. Destaca el desarrollo de un fluxgate toroidal basado en tecnología PCB. La utilización de la electrodeposición facilita la fabricación e integración de este tipo de dispositivos además de mejorar su funcionamiento. Hemos desarrollado también un modelo teórico para calcular la sensibilidad de este tipo de fluxgates.
Magnetic transitions in alpha-Fe_2O_3 nanowires
(Journal of Applied Physics, 2009) Díaz-Guerra Viejo, Carlos; Pérez García, Lucas; Piqueras de Noriega, Javier; Chioncel, M. F.
Magnetic transitions in single-crystal alpha-F_2O_3 (hematite) nanowires, grown by thermal oxidation of iron powder, have been studied in the range of 5-1023 K with a superconducting quantum interference device below room temperature and with a vibrating sample magnetometer at higher temperatures. The broad temperature range covered enables us to compare magnetic transitions in the nanowires with the transitions reported for bulk hematite. Morin temperatures (T-M) of the nanowires and of hematite bulk reference powder were found to be 123 and 263 K, respectively. Also the Neel temperature (T-N) of the nanowires, 852 K, was lower than the bulk T-N value. Measurements of the magnetization as a function of temperature show an enhanced signal in the nanowires, which suggests a decrease in the anti ferromagnetic coupling. A coercive field observed below T-M in the hysteresis loops of the nanowires is tentatively explained by the presence of a magnetic phase.
Room-temperature ferromagnetism in the mixtures of the TiO₂ and Co₃O₄ powders
(Physical review B, 2009) Serrano, A., A.; Plaza, M.; Pérez García, Lucas; Venta, J. de la; LLopis, J.; García, M. A.
We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO₂ and Co₃O₄ powders despite the antiferromagnetic and diamagnetic characters of both oxides, respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO₂ in anatase structure; no FM is found for identical samples prepared with rutile-TiO². Optical spectroscopy and x-ray absorption spectra confirm a surface reduction of octahedral Co^(+3) -> Co^(+2) in the mixtures which is in the origin of the observed magnetism.
Magnetic phases and anisotropy in Gd-doped GaN
(Physical review B, 2006) Pérez García, Lucas; Lau, G. S.; Dhar, S.; Brandt, O.; Ploog, K. H.
In this work we present a detailed study of the magnetic properties of GaN:Gd layers with different Gd content (6x10^(15) to 1x10^(19) cm^^ (-3)) grown by reactive molecular beam epitaxy. The temperature dependence of the magnetic properties suggests the existence of at least two ferromagnetic phases with different order temperatures. The coexistence of two ferromagnetic phases is explained within the framework of the phenomenological model, introduced previously by Dhar [Phys. Rev. Lett. 94, 037205 (2005)]. The layers are also found to exhibit a magnetic anisotropy, with the hard axis along the growth direction and an easy plane parallel to the surface. Moreover, the saturation magnetization shows a dependence on the orientation of the magnetic field, which may result from the anisotropy in the polarization induced in the GaN matrix by the internal and external magnetic fields.
Electrodeposited amorphous CoP multilayers with high permeability
(Journal of magnetism and magnetic materials, 2000) Pérez García, Lucas; de Abril, O.; Sánchez Tujillo, M. C.; Aroca, C.; López, E.; Sánchez, P.
Multilayer films composed of Co_(x)P_(1-x) ferromagnetic layers with different composition (0.74 < x < 0.86) have been obtained by varying the electrolytic current during the deposition process. These samples exhibit planar anisotropy, high permeability and a very low coercive force (similar to 5 A/m). The magnetic properties of these samples have been compared with the properties of Co_(x)P_(1-x) multilayers consisting of magnetic and non-magnetic (x < 0.7) layers.
Effect of annealing on the magnetic properties of Gd focused ion beam implanted GaN
(Applied physics letters, 2007) Khaderbad, M. A.; Dhar, S.; Pérez García, Lucas; Ploog, K. H.; Melnikov, A.; Wieck, A. D.
The authors have studied the effect of annealing on the magnetic and the structural properties of Gd focused ion beam implanted GaN samples. Molecular beam epitaxy grown GaN layers, which were implanted with 300 keV Gd^(3+) ions at room temperature at doses 2.4x10^(11) and 1.0x10^(15) cm^(-2), are rapid thermally annealed in flowing N₂ gas up to 900 ⁰C for 30 s. X-ray diffraction results indicate the presence of Ga and N interstitials in the implanted layers. Their densities are also found to reduce upon annealing. At the same time, magnetic measurements on these samples clearly show a reduction in the saturation magnetization as a result of the annealing for the lowest Gd incorporated sample, while in the highest Gd incorporated sample it does not change. These findings suggest that Gd might be inducing magnetic moment in Ga and/or N interstitials in giving rise to an effective colossal magnetic moment of Gd and the associated ferromagnetism observed in Gd:GaN.
Gd-doped GaN: A very dilute ferromagnetic semiconductor with a Curie temperature above 300 K
(Physical review B, 2005) Dhar, S.; Pérez García, Lucas; Brandt, O.; Trampert, A.; Ploog, K. H.; Keller, J.; Beschoten, B.
We present a systematic study of growth, structural, and magnetic characterization of GaN:Gd layers grown directly on 6H-SiC(0001) substrates by reactive molecular-beam epitaxy with a Gd concentration ranging from 7x10^(15) to 2x1019 cm^(−3). The structural properties of these layers are found to be identical to those of undoped GaN layers. However, the magnetic characterization reveals an unprecedented effect. The average value of the magnetic moment per Gd atom is found to be as high as 4000 μ_(b) as compared to its atomic moment of 8 μ_(b). Such a colossal magnetic moment can be explained in terms of a long range spin polarization of the GaN matrix by the Gd atoms which is reflected by the circular polarization of magnetophotoluminescence measurements. Moreover, the material system is found to exhibit ferromagnetism well above room temperature in the entire concentration range under investigation. We propose a phenomenological model to understand the macroscopic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd. D