89,664 research outputs found
Metamaterial Properties of Two-Dimensional Magnonic Crystals -- Presentazione poster by R. Zivieri - Conferenza nazionale
No full textThe metamaterial properties of two-dimensional magnonic crystals (MCs) are studied according to micromagnetic and analytical calculations. Micromagnetic calculations were performed by using the Dynamical Matrix Method (DMM) extended to periodic systems [1]. The nanometric systems are composed by periodic square arrays of circular antidots (ADs) (holes) embedded into a Permalloy ferromagnetic film [2]. In the calculations the diameter of the holes varies between 10 nm and 120 nm, the array periodicity is a = 800 nm and the thickness is L = 22 nm. The geometry studied is with the external field H perpendicular to the Bloch wave vector K in the sample plane. Since holes are much smaller than the characteristic mode wavelength, the dynamics is described in terms of an effective medium approximation. This description allows us to highlight the metamaterial properties of this class of two-dimensional MCs [3]. Due to the periodicity, frequencies of collective spin modes having Damon-Eshbach-like (DE) character exhibit a band structure and the dispersion shows opening of band gaps at Brillouin zone boundaries. For each collective spin mode an effective wavelength commensurable with the array periodicity is defined. The effective wavelength is related to the scattering from the ADs of the given spin-wave mode. Holes have thus the role of point defects affecting spin dynamics. It is also shown that the effective wavelength and the associated small wave vector are not necessarily equal to their corresponding Bloch quantities.
[1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007).
[2] R. Zivieri, S. Tacchi, F. Montoncello, L. Giovannini, F. Nizzoli, M. Madami, G. Gubbiotti, G. Carlotti, S. Neusser, G. Duerr, and D. Grundler, Phys. Rev. B 85, 012403 (2012).
[3] R. Zivieri and L. Giovannini, “Metamaterial properties of ferromagnetic antidot lattices” in press in Metamaterials
LENGUAJES DE LA POLÍTICA. MÁS ALLÁ DE LAS PALABRAS
No full textA través de sus representaciones, los discursos políticos, tan presentes en nuestra sociedad moderna, contribuyen a crear el espacio público y, de manera más o menos explícita, intentan difundir tendencias ideológicas. Esta es una de las razones por las que los lenguajes permiten ser analizados desde múltiples dimensiones. La escritura y la edición de este libro responde al creciente interés que se está desarrollando alrededor de este tema y nace de la convicción de que en un mundo que cambia continuamente es necesario enfocar la evolución de la cuestión. Ya que la temática aborda un campo muy amplio, en esta breve introducción es preciso distinguir cuáles son los varios sentidos con los que identificamos la expresión del título Lenguajes de la política. En el volumen se quiere trazar un plan según el cual el lenguaje por una parte se considera como el uso lingüístico propio de las actividades políticas (incluso de la política lingüística) y por otra, se considera también como lo que es propiamente político y se presenta como dimensión peculiar del lenguaje mismo: lenguaje de la política y política del lenguaje, así como política y lenguaje caracterizados por su germen manipulativo. En la ideación de una estructura coherente, el plano del libro se estructura en dos grandes bloques: sección de lengua y sección de literatura. Se amplía el discurso sobre la relación lengua y política al campo literario, en 8 Introducción obras en las cuales la conexión entre los dos términos se convierte en elemento determinante. Dentro de las diferentes facetas en las que se puede declinar el tema elegido como hilo conductor del presente volumen, caben varios aspectos que participan de la indisoluble relación entre palabra, política y literatura. En este caso, focalizamos el significado del término ‘política’ a través de su raíz etimológica -πολις-, es decir, en el aspecto más bien relacionado con la esfera pública, social y, consecuentemente, historicizada, del discurso político mismo, y, por eso, trasmisible a través de diferentes ‘medios de comunicación’ social -la palabra escrita en los periódicos, en las obras literarias, en los guiones teatrales y cinematográficos-. Nos enfrentamos con obras y autores que utilizan la palabra, el lenguaje, a partir de sus posiciones éticas -pues políticas-, para informar, denunciar, testimoniar del abuso político actuado sobre grupos disidentes en determinados momentos históricos; para reivindicar el papel social de grupos o segmentos sociales excluidos de su contexto activo; para reflexionar e incluso proponer un análisis diacrónico desde dentro, es decir, una lectura más objetiva que busca evidenciar las responsabilidades de los cuadros directivos de la militancia de izquierda que cooperaron a su derrota política, sus errores y su falta de una visión realística de la realidad en su momento, sus divisiones internas que quitaron fuerza a la oposición armada en contra del terrorismo de Estado, en países de habla castellana, de un lado a otro del Atlántico. Nada más, ni menos, que una variada muestra de las diferentes matizaciones que adquiere el lenguaje en su contexto cívico, colectivo, dentro del concepto amplio y moderno de πολις, donde la experiencia personal –de militancia política, de detención forzada, de lucha intelectual para la justicia social- pretende abarcar toda una sociedad en un preciso momento histórico. Todo eso se funda en la palabra. Una palabra que se hace expresión privilegiada de un momento social ben definido, implicando así el recurso a un léxico altamente historicizado que se carga, a menudo si no siempre, de un enorme valor simbólico, capaz de movilizar masas o de crear consensos políticos, hasta cambiar profundamente la visión ético-política misma de una sociedad. Hay, incluso, quien se interroga sobre la sobrevivencia de esa carga simbólica con el paso del tiempo y en contextos históricos mudados, si la fuerza argumentativa de la palabra expresamente codificada por grupos políticos en un ‘aquí’ y ‘ahora’ definidos, se guarde más allá de aquel momento y lugar en los que ella se impuso como Palabra Política con mayúsculas
Università e Scuola per la formazione iniziale degli insegnanti
No full textPartendo dagli elementi di scenario sulla formazione iniziale degli insegnanti e dall'analisi dei recenti documenti della Commissione europea, viene proposto un rapporto dinamico teoria-pratica. Esso richiede un modello di insegnamento/apprendimento di tipo esperienziale che ricalca il modello di indagine scientifica e comporta, tra l'altro, l'esigenza di non trascurare le teorie e le credenze, spesso ingenue, del soggetto che apprende. Tale impostazione confligge con la scelta ministeriale di ricondurre la formazione professionale di scuola secondaria di secondo grado in un anno aggiuntivo dopo la laurea magistrale
Band structure and properties of vortex modes in a 2-D magnonic crystal
No full textMagnonic crystals are artificial materials with periodic modulation of the magnetic properties that have recently received large interest from the scientific community. Indeed, the possibility of tuning the propagating properties, speed and allowed/forbidden band width of magnetic collective excitations with an external field has attracted special attention on these systems. However, while in the saturated case the band diagram has been extensively investigated [1], only a few incomplete studies on collective modes in the vortex state have been reported [e.g. Ref. 2]. Here we present a thorough investigation on this subject: employing the dynamical matrix method [3], we performed calculations on a squared 2D lattice of dots in the vortex state, varying the in-plane wavevector components to investigate the first Brillouin zone. We computed the dispersion relations for gyrotropic, azimuthal and radial modes (Fig. 1). Dynamics in vortex states is a complex matter, since the coupling is mainly due to the fluctuations of the magnetization, which is a second order effect, but is more interesting because a slight change of the external field can have dramatic consequences on the information carriers (“magnons”), which can be slowed down even to zero speed: in this way information could be stored or delivered with little energy effort within the same device, which operates either as a memory or a waveguide. We discuss the dynamical coupling of modes with different cell wavefunctions, the corresponding mode dispersion and bandwidth, the effects of interdot coupling on the circular polarization on the modes, the Brillouin Light Scattering cross section of the principal modes. The investigation extends also to vortex states in presence of a nonzero applied field, where the vortex core is no more in the center of the disk, and to the corresponding symmetry breaking in the dispersion relations. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°233552 (DYNAMAG).
[1] S. Tacchi, F. Montoncello, M. Madami, G. Gubbiotti, G. Carlotti, L. Giovannini, R. Zivieri , F. Nizzoli, S. Jain, A. O. Adeyeye, and N. Singh, Physical Review Letters, in press (2011).
[2] A. Yu. Galkin, B. A. Ivanov and C. E. Zaspel, Physical Review B, 74, 144419 (2006).
[3] L. Giovannini, F. Montoncello, and F. Nizzoli, Physical Review B 75, 024416 (2007).
Spin waves in configurational antiferromagnetic states: transitions to and from ferromagnetic states
No full textWe study collective spin waves in transversally magnetized chains of thin elliptical dots initially arranged into an antiferromagnetic (AF) configuration. The dispersion relations of the SW as well as the corresponding magnetization precession profiles (eigenvectors) are computed with the dynamical matrix method [1]. We report the peculiar features of AF optical and acoustical spin modes within the effective wavevector model [2], which has been adapted to the present geometry. We investigate the effects of the application of a transverse external field, leading
the system from a perfect AF configuration to the ferromagnetic (FM) state, at a specific transition field. We discuss the mode localization and symmetry, and in particular, we focus on the AF soft mode, its symmetry and its relationship with the instability leading to AF-to-FM transition. We discuss the importance and potential application of the SW propagative properties suddenly changed at the switching from AF to FM. Finally, we present the calculations of spin dynamics concerning a FM-to-AF transition in a chain of ellipses with different width: in this case, the different shape anisotropy of the two ellipses results in different switching fields and hence in a field interval where the AF state is spontaneously attained. We show how this AF field interval can also be tuned by an effective uniaxial anisotropy of one out of two ellipses: this is indicative of any other source of anisotropy (like inverse-magnetostriction, particularly promising for voltage driven magnetic transitions in multiferroics): in this way, we investigate the AF-to-FM transition spin dynamics by varying the effective anisotropy constant of the material. Finally, we show and comment on thepeculiar and remarkable change of slope of the mode dispersion across the AF-to-FM transition, with its implications in the field of magnon-spintronics devices.
References: [1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007).
[2] F. Montoncello, S. Tacchi, L. Giovannini, M. Madami, G. Gubbiotti, G. Carlotti, E. Sirotkin, E. Ahmad, F. Y. Ogrin, and V. V. Kruglyak, Appl. Phys. Lett. 102, 202411 (2013)
Stopping field for collective spin waves at the edge of magnetization reversal
No full textSimilarly to other magnetic systems, even magnonic crystals are characterized by soft modes with a vanishing frequency at the critical field of any given magnetic transition. The profile of these modes has a symmetry that depends on the symmetry change between the initial and final magnetic configurations[1]. The knowledge of the soft mode is not a theoretical-only issue, but can have technological implications, especially in the field of magnonic- and spin-logic devices, where collective spin waves are used for information storage and delivery[2]. Actually, it has been recently demonstrated[3] that the bandwidth of the mode that softens at the critical field, undergoes dramatic variations even when just approaching this critical field. This fact can result either in a band broadening of modes usually non-dispersive (like some end modes), or, vice versa, in a strong band reduction for modes with usually a large bandwidth (as the fundamental mode). In some cases, it is possible to design the magnonic crystal to be characterized by one or another soft mode with the desired symmetry in order to use its bandwidth variation close to the transition field for a specific purpose. We apply this concept to a rectangular array of interacting elliptical dots of Permalloy (i.e., a 2-D magnonic crystal), magnetized along the major axis, and, by calculations with the dynamical matrix method, find out the behavior of the soft mode dispersion at the edge of magnetization reversal. We discuss the correlation among different curves characterizing the magnetic system close to reversal: the magnetization curve, the soft mode frequency vs. field curve, and the frequency vs. wavevector curve. We investigate different aspect ratios for the ellipses, and different magnetic configurations. We show how the soft mode is characterized by a bandwidth that goes to zero at a magnetic field quite distinct from the critical transition field, and we call this field stopping field, because at this field the collective soft mode turns into non-dispersive (stationary). We believe that this feature can be used to design versatile devices, in which information can be stored or delivered at the energy costs of a small magnetic field variation.
[1] F. Montoncello, L. Giovannini, F. Nizzoli, P. Vavassori, and M. Grimsditch, Phys. Rev. B 77, 214402 (2008).
[2] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, M. P. Kostylev, V. S. Tiberkevich, and B. Hillebrands, Phys. Rev. Lett. 108, 257207 (2012). [3] F. Montoncello and L. Giovannini, Applied Physics Letters 104, 242407 (2014)
Collective Vortex Modes in Magnonic Crystals: Multipolar Effects on Dispersion Curves
No full textIt has been recently demonstrated through calculations [1] how collective vortex modes in magnonic crystals can dramatically change their propagation properties as soon as a bias field is applied. Clearly, this fact is potentially helpful in the perspective of magnonic-spin-logic devices, in which the propagation/steadiness of the information carrier (“magnon”) can be given a different binary digit. Employing the dynamical matrix method, we performed calculations on a squared 2D lattice of dots in the vortex state, varying the in-plane
wavevector components to investigate the first Brillouin zone. We computed the dispersion relations for gyrotropic, azimuthal and radial modes. We discuss the dynamical coupling of modes with different cell wavefunctions, which is not purely dipolar as for the modes in saturated states: multipolar contributions are often necessary, and determine the dependence of the bandwidth of a given mode on the lattice parameter. We also discuss how the circular polarization of the modes depends on the Bloch wavevector k and changer as k is changed. When considering a bias magnetic field, in the equilibrium configuration the vortex core sets off the center of the disk: for a class of modes, propagation along the direction of the applied field is slowed down, while perpendicular to the applied field is speeded up. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°n°228673 (MAGNONICS).
[1] F. Montoncello and L. Giovannini, Appl. Phys. Lett. 100, 182406 (2012)
Information carrier bandwidth and speed tunability in magnonic crystals in the vortex state
No full textMagnonic crystals are artificial materials with periodic modulation of the magnetic properties that have recently received special attention due to the fact that slight changes of the external field can have dramatic consequences on the information carrier (“magnon”) propagation, which can be boosted or delayed even to steadiness: in this way the same device can operate either as a memory or a waveguide. Employing the dynamical matrix method [1], we performed calculations on a squared 2D lattice of dots in the vortex state, varying the in-plane wavevector components to investigate the first Brillouin zone. We computed the dispersion relations for gyrotropic, azimuthal and radial modes. We discuss the dynamical coupling of modes with different cell wavefunctions, which is not purely dipolar as for the saturated states. We discuss how the circular polarization on the modes depends on the Bloch wavevector. We considered also the effects of application of a magnetic field, which moves the vortex core off the center of the disk: for a class of modes, propagation perpendicular to the direction of the applied field is speeded up, while parallel to the applied field is slowed down. These results can be important for designing versatile magnetic filters, in which variation of the applied field direction and intensity can turn the device from a waveguide into a memory, but also for spin logic devices, in which propagation or steadiness of the information carrier along a desired direction can be associated to different binary digits.
[1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007)
LENGUA DE LA MILITANCIA, LENGUA DEL RECUERDO: MEMORIAS AUTOBIOGRÁFICAS DE UNA HIJA DE MONTONEROS, "LA CASA DE LOS CONEJOS" DE LAURA ALCOBA
No full textLa novela de Laura Alcoba se proprone como memoria autobiográfica de una hija de militantes Montoneros, en la Argentina de los 70. Es la historia, contada por la propia protagonista después de , de los meses en los que la pequeña Laura, de siete años, tuvo que vivir en clandestinidad junto a su madre y a otros compañeros de militancia, en la casa en las afueras de La Plata donde se imprimía clandestinamente la revista montonera Evita Montonera. Ahora la casa Mariani-Testuggi es patrimonio de la memoria, porque, poco después de que Alcoba y su medre se fueron, fue atracada por los militares argentinos que mataron a todos los militantes y robaron la nena Alehi Testuggi de solo tres meses, hasta ahora todavís desaparecida. Mi trabajo profundiza la relación entre yo autobiográfico y los diferentes tiempo de la novela, el de lo narrado y el de la narración, tema central para la búsqueda identitaria que los hijos de militantes intentan lograr a través de la escritura de aqulel doliente pasado. El léxico de la militancia, ya en deshuso, se hace metáfora de la imposibilidad por parte de los hijos de entender de manera plena las motivaciones que llevaron sus padres a sacrificar su familia y su vida ‘normal’ para entrar en la lucha armada y en la clandestinidad, sabiendo que así se iba arriesgando su vida y de sus seres queridos. Palabras de la política que ahora, en el momento de la escritura, se revelan antiguas, casi arcaicas, que en cambio, para los jóvenes de los 70, constituían el andamio sobre el cual habían construido su vida
Dalla Germania dell’Est all’Italia: Paul Gompitz/Klaus Müller sulle tracce di J. G. Seume in “La passeggiata da Rostock a Siracusa” di F. C. Delius
No full textAttraverso il personaggio di Paul Gompitz, il racconto La passeggiata da Rostock a Siracusa (1995) di F. C. Delius narra la reale esperienza di Klaus Müller che, dopo una lunga e accurata pianificazione, nel 1988 fugge temporaneamente dalla Repubblica Democratica Tedesca per ripercorrere il viaggio in Italia effettuato a piedi, nel 1802, dal conterraneo scrittore J. G. Seume. Il testo di Delius consente non solo di mettere a fuoco la funzione del viaggio di formazione setto-ottocentesco nel contesto geo-politico della Germania dell’Est, ma anche di indagare interessanti questioni sul piano spaziale, temporale e narrativo. L’Italia, meta agognata da Gompitz/Müller sin dall’infanzia, si configura nel testo soprattutto come parte integrante dell’Occidente libero e capitalista, la cui raffigurazione letteraria mostra in che misura rappresentare l’Altro implichi rappresentare se stessi
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