1,721,158 research outputs found
Tailoring the magnetization reversal in antidot nanostructures using lithographically engineered inhomogeneities
No full text10.1063/1.3537948Journal of Applied Physics1097-JAPI
Frequency softening of collective modes in two-dimensional ferromagnetic antidot arrays -- Presentazione poster by R. Zivieri -- Conferenza internazionale
No full textThe softening of collective mode frequencies in two-dimensional Permalloy antidot arrays with circulat holes have been investigated. The antidots have fixed lattice constant a = 420 nm and thickness of L= 30 nm. The diameters of the holes range between 140 nm and 260 nm [1]. The magnonic
mode frequencies calculated by means of the dynamical matrix method (DMM) as a function of the in-plane
magnetic field along the y-direction at the centre of the first Brillouin zone are compared with those measured by Brillouin light scattering technique. The two lowest spin-wave mode frequencies, namely the edge mode (EM) localized at the antidot borders [2] and the F mode, whose spatial profile is confined in the horizontal channels between holes, become soft at a given critical field Hc. High-frequency mode frequencies do not soften including that of the Floc mode that is mainly confined in the horizontal rows between holes. The softening mechanism is strictly related to the rotation of the static magnetization from the hard to the easy axis marking a reorientational and continuous phase transition. By
means of an analytical model it is shown that the critical field is approximately equal and opposite to the demagnetizing field averaged over the primitive cell. The different frequency trend of the F mode and the Floc mode as a function of the aspect ratio L/s with s the antidot separation is explained in terms of the opposite demagnetizing field experienced by the two modes. This work was partially supported by MIURPRIN
2010-11 Project2010ECA8P3 "DyNanoMag"
Effect of dipolar coupling on vortex states formed in dense chains of ferromagnetic rectangular particles
No full textWe have investigated the effects of dipolar coupling on the magnetization configurations of ferromagnetic rectangular particles with rounded-corners. Two arrays of particles with lateral size of 1025×450 nm2 have been patterned by deep UV lithography, followed by the lift-off of a permalloy film of thickness 40 nm. The first array consisted of long chains of interacting nanomagnets put head-to-tail, with 85 nm interdot spacing. In the second array the interdot distance was increased to 700 nm, in order to avoid any effect of dipolar interaction. In this sample the particles could be considered as magnetically isolated and their behavior as a reference for comparison with the interacting ones.
Magneto-optical Kerr effect (MOKE) and in-field magnetic force microscopy (MFM) experiments, together with micromagnetic simulations, were performed. MFM measurements clearly showed that closure states characterized by one, two or three vortices occurred in isolated particles at remanence, together with the vortex/antivortex/vortex state. MFM performed on the same particles with a field applied along their hard direction (fig. 1) showed the detail of the nucleation and propagation process of vortices in isolated dots, in particular for the single vortex and the vortex/antivortex/vortex states. Decreasing the field intensity from positive saturation the nucleation process starts at one end of the particle below 200 Oe. The vortices cores then propagate perpendicularly to the field along the dot’s major axis toward the opposite end of the particle where they annihilate.
When the field was applied along the major axis of interacting nanomagnets put head-to-tail the cooperation of intrinsic shape anisotropy and configurational anisotropy led to the formation of a regular sequence of single domain states (fig. 2), promoted by dipolar coupling, and the vortex formation was delayed during the reversal process. It turned out that the field range in which vortices exist is much narrower in the case of interacting particles if compared with isolated ones.
An interesting result was that dipolar interaction can suppress some magnetic configurations that are present in isolated nanomagnets and favour other states. In particular, the double vortex was not observed in interacting particles, whereas the vortex/antivortex/vortex structure was found to be largely the most probable spin arrangement. A simple explanation of these findings, based on energy balance consideration, will be proposed. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°228673 (MAGNONICS)
Soft magnonic modes in two-dimensional permalloy antidot lattices
No full text10.1088/0953-8984/25/33/336002Journal of Physics Condensed Matter2533-JCOM
Metamaterial properties of arrays of rectangular magnetic dots -- Presentazione poster by R. Zivieri - Conferenza internazionale
No full textIn this paper we investigate both theoretically and from an experimental point of view the magnonic band structure of chains of Permalloy rectangular nanodots. Micromagnetic calculations and Brillouin light scattering measurements were performed for Bloch wave vector q perpendicular to the applied field H. By means of micromagnetic calculations we are able to estimate the amplitude of modes frequency oscillation (magnonic band) and the group velocity in the long wavelength limit for a vanishing separation which is an important property to identify the behavior of a one-dimensional quasi-continuous magnonic metamaterial
Collective dynamics in chains of rectangular dots -- Presentazione poster by R. Zivieri - Conferenza internazionale
No full textWe present a combined experimental and micromagnetic study of spin excitations in chains of dense magnetic dots. The samples consist of long chains of rectangular dots with rounded corners having lateral dimensions of 715 x 450 nm2 and 1025 x 450 nm2, respectively with edge-to-edge separation below 100 nm. The frequency dispersion of the spin wave excitations was measured by Brillouin Light Scattering (BLS) technique for a given applied in-plane magnetic field H and as a function of q (Bloch wave vector) directed along the chains of dots. Micromagnetic calculations, performed by using the dynamical matrix method enabled us to interpret the frequency dispersion of collective spin modes. Calculated dispersions were compared with BLS ones in the Voigt geometry (q perpendicular to H). -- Presentazione poster by R. Zivieri - Conferenza internazional
Partial frequency band gap in one-dimensional magnonic crystal
No full textCollective spin wave modes propagating in an array of magnetic stripes coupled by dynamic dipole
interaction are investigated by Brillouin light scattering. It is demonstrated that this structure
supports propagation of discrete spin waves at any angle with respect to the stripes length. The data
are interpreted using a theoretical model based on the Bloch wave approach. It is shown that, due
to the one-dimensional artificial periodicity of the medium, the gaps in the spin wave spectrum are
partial: the frequency passbands for propagation along the direction of periodicity overlap with the
stop bands for propagation along the stripes
Magnetization reversal and anisotropic magnetoresistance behavior in bicomponent antidot nanostructures
No full text10.1063/1.3474802Applied Physics Letters974-APPL
Metamaterial properties of arrays of rectangular magnetic dots -- Conferenza internazionale
No full textIn this paper we investigate both theoretically and from an experimental point of view the magnonic band structure
of chains of Permalloy rectangular nanodots. Micromagnetic calculations and Brillouin light scattering
measurements were performed for Bloch wave vector q perpendicular to the applied field H. By means of micromagnetic
calculations we are able to estimate the amplitude of modes frequency oscillation (magnonic band)
and the group velocity in the long wavelength limit for a vanishing separation which is an important property to
identify the behavior of a one-dimensional quasi-continuous magnonic metamaterial
Band gaps in 1D "magnonic" crystals: a micromagnetic study -- Presentazione orale by R. Zivieri - Conferenza internazionale
No full textThe behaviour of magnonic gaps in 1D arrays
of in-plane magnetized rectangular Permalloy
dots
with lateral dimensions 715 x 450 nm^2 and
1025 x 450 nm^2, respectively
is investigated by means of
micromagnetic simulations based on the dynamical matrix
method [L. Giovannini et al.,
Phys. Rev. B, 75 (2007) 024416].
Band gaps of
collective spin modes in the Voigt geometry
(magnetic field parallel to the easy axis and in-plane
wave vector parallel to the dot chains)
are calculated and
compared with those obtained by means of
Brillouin Light Scattering technique.
The narrowing of band gaps either in the centre
or at the border of the first Brillouin
zone for vanishing
dot separation is discussed
in terms of the collective nature
of the corresponding
spin modes.
The calculation of
the resonant mode frequency of
the corresponding stripe obtained
in the limit of zero separation is performed
and compared with the one determined by using
a simple analytical model based on the Green's
function tensor.
-- Presentazione orale by R. Zivieri - Conferenza internazional
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