1,721,011 research outputs found
Linear and Autonomous Magnetization Dynamics in Spin-Torque Auto-Oscillators: a Lagrangian Approach -- Presentazione orale by R. Zivieri - Conferenza internazionale
No full textWe present the generalization of the Dynamical Matrix Method developed for magnetic systems in the presence of conservative forces [1] to the dissipative case. To this aim, in order to include the effects arising from non-conservative contributions, we adopt a Lagrangian formalism based upon generalized Lagrange equations. We first derive the explicit expressions for the kinetic and the potential energies for a magnetic system in which each spin describes, during its precessional motion, an elliptical orbit about the equilibrium configuration. Then we deduce, from simple physical arguments, the dissipation functions corresponding to the intrinsic Gilbert damping torque and to the one connected with the spin-transfer torque. The analysis is performed in the “linear” regime, valid under the assumption of small oscillations of the spins around their equilibrium positions. Therefore, the energy terms appearing in the generalized Lagrange equations are expressed as Taylor series truncated at the second perturbative order. Within this framework, we can characterize the linear and autonomous magnetization dynamics of a magnetic auto-oscillatory system subject to the action of an external bias dc field and a dc spin-polarized electric current. The resulting linearized Lagrange equations are recast as a complex generalized non-Hermitian Eigenvalue problem which has to be solved numerically. According to this analysis it is possible to identify the eigenmodes which become unstable under the action of a spin-polarized current by taking into account the change of sign of their imaginary part. Within this formalism it is also possible to provide an accurate estimation (error less than 1%) of the value of the excitation threshold current, which fully agrees with results of both standard finite-differences micromagnetic codes and analytical theory [2].
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] M. Grimsditch, L. Giovannini, F. Montoncello, F. Nizzoli, G.K. Leaf, and H.G. Kaper, Phys. Rev. B 70, 054409 (2004).
[2] G. Consolo, G. Gubbiotti, L. Giovannini, and R. Zivieri, Appl. Math. Comp. 217, 8204 (2011).
-- Presentazione orale by R. Zivieri-Conferenza internazional
Symmetry properties and invariance of vortex-state linearized equations of motion in ferromagnetic dots -- Presentazione poster by R. Zivieri - Conferenza internazionale
No full textWe prove some discrete space and time symmetries of linearized equations of motion in vortex-state ferromagnetic dots. The space symmetries are strictly related to the vortex-state configuration of the magnetization characterized by a chirality c= ±1 (counterclockwise or clockwise rotation of the magnetization) and a polarity p = ±1 (outward or inward core magnetization) and to its nature of axial vector. For a purely conservative dynamics, the
linearized equation of motion in vortex-state ferromagnetic dots is written in the approximated form. First, we discuss the space symmetries. By placing the vortex parallel to the reflection plane (xy plane) and by introducing the space reflection operator σxy, the following symmetry transformation holds: σxy c= -c. The c
switching leads to a change of sign of the in-plane static magnetization component (M →-M) and of the in-plane dynamic one, namely δmu→-δmu which leads to a change of sign of the
corresponding dynamic effective field component δhu→-δhu. Moreover, there is also the switching δ mvC → −δ mvC of the core (C) component δ mvC = −δ mz cosθ with θ the polar angle.
These transformations lead to the invariance of the equation of motion. By placing the vortex perpendicularly to the reflection plane (xy plane), we get σxy p= - p. The p switching (vortex core switching) leads to δmu→-δmu and to δmz→-δmz leaving again the equation of motion invariant. A remarkable physical effect on spin dynamics is the interchange kv→-kv of the phases of azimuthal vortex modes characterized by the azimuthal number kv = ±1, ±2, .. obtained by using a reflection operator xy σ in a quantum mechanical scheme We now discuss the time-reversal T symmetry, namely t →-t. Under this symmetry transformation both the static and dynamic quantities change sign, due to the reversal of angular moment of electrons, leaving Eq.(1) invariant like in the case of space symmetries. Within a quantum mechanical scheme, according to Wigner anti-unitary operator [2]
the effect is equivalent to that of xy σ (kv→-kv) and leads to a change of the sense of rotation of azimuthal vortex modes including the gyrotropic. Instead, the dynamics of radial modes (kv = 0), is not affected by the T symmetry.
[1] R. Zivieri and F. Nizzoli, Phys. Rev. B 75 (2005), 014411-1-5; ibidem Phys. Rev. B 74 (2006), 219901(E).
[2] E.P. Wigner, Group theory and its application to the quantum mechanics, New York, Academic Press (1959), p.325
Hamiltonian and Lagrangian Dynamical Matrix Approaches Applied to Magnetic Nanostructures
No full textTwo micromagnetic tools to study the spin dynamics are reviewed. Both approaches are based upon the so-called dynamical matrix method, a hybrid micromagnetic framework used to investigate the spin-wave normal modes of confined magnetic systems. The approach which was formulated first is the Hamiltonian-based dynamical matrix method. This method, used to investigate dynamic magnetic properties of conservative systems, was originally developed for studying spin excitations in isolated magnetic nanoparticles and it has been recently generalized to study the dynamics of periodic magnetic nanoparticles. The other one, the Lagrangian-based dynamical matrix method, was formulated as an extension of the previous one in order to include also dissipative effects. Such dissipative phenomena are associated not only to intrinsic but also to extrinsic damping caused by injection of a spin current in the form of spin-transfer torque. This method is very accurate in identifying spin modes that become unstable under the action of a spin current. The analytical development of the system of the linearized equations of motion leads to a complex generalized Hermitian eigenvalue problem in the Hamiltonian dynamical matrix method and to a non-Hermitian one in the Lagrangian approach. In both cases, such systems have to be solved numerically
Excitation of magnetic normal modes by spin-torque: a Lagrangian approach
No full textIn this paper the spin-wave normal modes in a nanopillar with circular cross-section, excited by the simultaneous action of an external magnetic field and a spin-polarized current, are investigated. The study is performed by using a micromagnetic approach based upon a linearized Lagrangian formalism which requires to solve a complex generalized nonHermitian eigenvalue problem. In all the studied configurations it is shown that, in the linear regime and independently of the orientation of the external bias field, a spatially uniform spin-polarized current excites the lowest-frequency mode, which is not necessarily the quasi-uniform mode of the spectrum. The obtained numerical results are also confirmed by finite-difference time-domain numerical simulations
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
Removing Numerical Instabilities in the Preisach Model Identification Using Genetic Algorithms
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