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Angular Band Diagrams for Multidirectional Spin Wave Propagation in Square Antidot Lattices
No full textBy means of a joint experimental and theoretical investigation, we propose an alternative way of describing band properties of collective spin waves (SWs) when considering their propagation direction across a magnonic crystal (in our case, a square antidot lattice, ADL): to build up an angular band diagram, in which frequency is plotted as a function of the angle of SW propagation. Similarly to conventional band diagrams, even in this case different dispersions of different modes give origin to allowed/forbidden bands, related not only to the specific in-plane angle at which they are considered, but also to the SW wavevector magnitude and the ADL constant. We performed Brilluoin light scattering (BLS) measurements on a Permalloy ADL (with a lattice constant of 440 nm), mounted on a two-axis goniometer, which allows us to choose a specific angle of incidence of light as well as to rotate the sample around the surface normal (azimuthal rotation). By changing these angles, it is possible to change the magnitude and/or the in-plane direction of the wavevector of light (and, as a consequence, of the detected SW) [1]. In Fig. 1, we show an example of angular band diagram for a SW with k=1.06×2π/a (corresponding to a light incidence angle of 40 degrees): full circles are the experimental data, lines are the dispersions calculated by means of the dynamical matrix method (DMM) [2]. As apparent in Fig. 1, a minimum gap between the most intense SW modes occurs close to φG=28 degrees: however, this value is critically dependent on the wavevector magnitude (and the lattice constant, if different ADLs are compared). We derive and prove this functional dependence with analitical considerations. We believe that this results are of interest for magnon-spintronic applications where a straight antenna on an ADL is used to emit SWs in any direction (omnidirectional SW emission).
References:
[1] G. Gubbiotti, F. Montoncello,S. Tacchi, M. Madami, G. Carlotti, L. Giovannini, J. Ding and A. O. Adeyeye, Applied Physics Letters 106, 262406 (2015).
[2] L. Giovannini, F. Montoncello, and F. Nizzoli, Physical Review B 75, 024416 (2007)
Asymmetric frequency dispersions of equivalent spin wave modes measured along symmetry directions of a hexagonal magnonic crystal
No full textMagnonic crystals have been receiving special attention in front-line research on magnetism and magnetic materials because of their outstanding physical properties and potential
technological applications: in particular, the collective mode propagation can be easily controlled in these systems by an external field. This gives the possibility of tuning, from one side, the width and the frequency range of the allowed/forbidden spin wave bands and, from the other, the group velocity of the spin waves so that information could be stored or delivered with little effort within the same device, which can operate either as a memory or a waveguide. In this paper, we report a Brillouin light scattering (BLS) investigation on a hexagonal array of Permalloy interacting disks, fabricated via etched nanosphere lithography technique.
BLS spectra were measured in the Voigt configuration for two different orientation of the applied magnetic field, i.e., parallel and perpendicular to the direction of adjacent dots [1]. Measurements highlight the occurrence of non-monotonic dispersions, with maximum/minimum occurring inside the first Brillouin zone. The dynamical matrix method [2] was used to interpret the experimental results, and the thorough understanding of all dispersions, in all their complexity, was found straightforward within the effective wavevector model [3]. Asymmetry of mode propagation is considered a challenging feature of these systems: equivalent modes can propagate in different directions with different bandwidth and different group velocity, and in some cases different dispersion slope: this means that they can carry different binary digits, a crucial property useful, for example, in spin-logic devices.
This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n. 228673 (MAGNONICS) and by MIUR-PRIN 2010-11 Project 2010ECA8P3 "DyNanoMag".
References:
[1] F. Montoncello, L. Giovannini, S. Tacchi, M. Madami, G. Gubbiotti, G. Carlotti, E. Sirotkin, E. Ahmad, F. Y. Ogrin and V. V. Kruglyak, Applied Physics Letters 102, 202411 (2013).
[2] L. Giovannini, F. Montoncello and F. Nizzoli, Physical Review B 75, 024416 (2007).
[3] S. Tacchi, F. Montoncello, M. Madami, G. Gubbiotti, G. Carlotti, L. Giovannini, R. Zivieri, F. Nizzoli, S. Jain, A. O. Adeyeye and N. Singh, Phys. Rev. Lett. 107, 127204 (2011)
Spin wave band structure of a two-dimensional ferromagnetic antidot array -- Presentazione orale by R. Zivieri - Conferenza internazionale
No full textThe spin wave band structure of a two-dimensional square array of NiFe circular antidots (ADs) has been investigated both experimentally and theoretically by using Brillouin light scattering (BLS) technique and micromagnetic calculations, carried out by means of the dynamical matrix method (DMM) with implemented periodic boundary conditions [1]. Sample consists of 22 nm NiFe film with etched circular having diameter of 120 nm and periodicity of 800 nm. As depicted in the inset to Fig.1, the external magnetic field is applied along y direction, while the transferred wave vector is along x direction. Both the experimental measurements and the calculated spin wave dispersion provide evidence for either extended or localized magnonic modes having a propagative nature. Extended modes spreading in the “horizontal” channels comprised between adjacent rows of ADs have a non-vanishing precession amplitude also along the horizontal rows of holes. These spin-wave modes are labelled as DEnBZ (black curves) where nBZ denotes a given Brillouin zone with n=1,2,.. Instead, the other kind of spin-wave modes, mainly localized along the horizontal rows of antidots, the so-called localized modes [2], are labelled as DElocnBZ (red curves). Both families of modes exhibit bandgaps at Brillouin zone boundaries predicted by the DMM calculations. Opening of bandgaps is interpreted in terms of Bragg diffraction of spin waves from the AD lattice and a quantitative explanation of this effect is given by studying the behavior of the mean internal field. The calculated mean internal field experienced by the two kind of modes is strongly inhomogeneous and is larger in correspondence of ADs. The DE2BZ mode exhibits its maximum precession amplitude where the internal field is larger (smaller) and has thus a larger (smaller) frequency. Band gaps are also calculated within the analytical model according to a perturbation approach. The eigenfunctions representing frequency modes at the BZs boundaries belonging to n-th and (n+1)-th band (sin (k π / a) x and cos (k π / a) x with k = 1,2,.., respectively) are interchanged with respect to those of electrons in electronic bands studied within the nearly-free electron model. This can be understood taking into account that the periodic mean internal field has its maxima in correspondence of ADs, while the periodic electronic potential is minimum close to the nuclei and vice versa. A comparison between the bandgap measured by BLS and the values calculated by means of the analytical model and by using the DMM is shown in Table 1. According to the analytical model, it is found that the relevant scattering potential for Bragg reflection is not provided by the holes themselves, but by the concomitant internal field inhomogeneity between holes [3]. This is in contrast to antidots in photonics and electronics where the back-reflection is directly caused by the presence of holes. The results of this study are important also for the potential applications of these patterned structures that can be used in magnonic devices. Indeed, AD behaves not only as waveguide for spin waves, but the presence of bandgaps permits to filter the frequency of travelling excitations. In this way, AD can be used also as a filter for spin waves.
The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n228673 (MAGNONICS).
[1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007)
[2] S. Tacchi, M. Madami, G. Gubbiotti, G. Carlotti, A.O. Adeyeye, S. Neusser, B. Botters, and D. Grundler,
IEEE Trans. Magn. 46, 172 (2010)
[3] R. Zivieri, S. Tacchi, F. Montoncello, L. Giovannini, F. Nizzoli, M. Madami, G. Gubbiotti, G. Carlotti, S.
Neusser, G. Duerr, and D. Grundler, “Bragg diffraction of spin waves from a two dimensional antidot lattice
”, in press in Physical Review B --
Presentazione orale by R. Zivieri - Conferenza internazional
Polyion-Induced Cluster Formation in Different Colloidal Polyparticle Aqueous Suspensions
No full textThe formation of aggregates in polyion-induced charged colloidal particles in aqueous suspension is characterized, under appropriate conditions, by two complementary effects, known as re-entrant condensation and charge inversion, which are considered as proof for the existence of a cluster phase. In this paper, we extend our previous investigation to a set of aqueous colloidal particle suspensions, such as polystyrene spheres, colloidal gold particles, and polylactic acid particles. These systems are characterized by the evolution of the average size of the aggregates and their surface electrical charge (charge inversion) by means of dynamic light-scattering measurements and laser Doppler electrophoretic techniques. The results, together with the previous ones concerning liposome particles, support the notion of a common behavior of this group of complex colloids characterized by short-ranged attractive interactions. The study provides some insights into these structures, which are potentially useful in biotechnological applications, such as multicompartmental carriers in nonviral drug delivery
Brillouin scattering study of spin dynamics in patterned nanoelements: from single-layer to multilayered structures
No full text
Band structure of a two-dimensional ferromagnetic antidot lattice
No full textThe spin wave band structure of a two-dimensional square array of NiFe circular
antidots having diameter of 120 nm and periodicity of 800 nm has been investigated by
using Brillouin light scattering technique and micromagnetic calculations based on the
dynamical matrix method [1]. The external magnetic field was applied in the plane and
perpendicularly to the transferred wave vector. Extended and localized spin modes
having a propagative nature were found. Opening of bandgaps is interpreted in terms of
Bragg diffraction of spin waves from the antidot lattice and this effect is explained by
studying the behaviour of the internal field as shown in Fig.1. The mean internal field is
larger along the vertical rows of antidots and smaller between the antidots (see panel (a)
for extended modes and (c) for localized modes). By developing an analytical model
according to which the mean internal field is represented by means of a rectangular step
function characterized by a region 1 corresponding to vertical rows of antidots and a
region 2 between the antidots (see panels (b) and (d)), the relevant scattering potential
for Bragg reflection is not provided by the holes themselves, but by the concomitant
internal field inhomogeneity between holes [2]. This is in contrast to antidots in
photonics and electronics where the back-reflection is directly caused by the presence of
holes. The research leading to these results has received funding from the European
Community's Seventh Framework Programme (FP7/2007-2013) under Grant
Agreement n228673 (MAGNONICS).
[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 83, (2012)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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