Search

Magnonic crystals are structures with periodically varied magnetic properties that are used to control collective spin-wave excitations. With micromagnetic simulations, we study spin-wave spectra in a 2D antidot lattice based on a multilayered thin film with perpendicular magnetic anisotropy (PMA). We show that the modification of the PMA near the antidot edges introduces interesting changes to the spin-wave spectra, even in a fully saturated state. In particular, the spectra split into two types of excitations: bulk modes with amplitude concentrated in a homogeneous part of the antidot lattice and edge modes with an amplitude localized in the rims of reduced PMA at the antidot edges. Their dependence on the geometrical or material parameters is distinct, but at resonance conditions fulfilled, we found strong hybridization between bulk and radial edge modes. Interestingly, the hybridization between the fundamental modes in bulk and rim is of magnetostatic origin, but the exchange interactions determine the coupling between higher-order radial rim modes and the fundamental bulk mode of the antidot lattice. [ABSTRACT FROM AUTHOR]

A theory of three-dimensional (3D) magnonic crystal is developed and applied to explain the existence of the spin-wave frequency gap, recently revealed by neutron scattering experiments. A confrontation of the theory with experimental results proves that certain manganites could be regarded as 3D magnonic crystals existing in the nanoscale in nature.

We have investigated multifunctional periodic structures in which electromagnetic waves and spin waves can be confined in the same areas. Such simultaneous localization of both sorts of excitations can potentially enhance the interaction between electromagnetic waves and spin waves. The system we considered has a form of one dimensional photonic-magnonic crystal with two types of magnetic layers (thicker and thinner ones) separated by sections of the dielectric photonic crystals. We focused on the electromagnetic defect modes localized in the magnetic layers (areas where spin waves can be excited) and decaying in the sections of conventional (nonmagnetic) photonic crystals. We showed how the change of relative thickness of two types of the magnetic layers can influence on the spectrum of spin waves and electromagnetic defect modes, both localized in magnetic parts of the system. [ABSTRACT FROM AUTHOR]

We investigate the properties of a photonic-magnonic crystal, a complex multifunctional one-dimensional structure with magnonic and photonic band gaps in the GHz and PHz frequency ranges for spin waves and light, respectively. The system consists of periodically distributed dielectric magnetic slabs of yttrium iron garnet and nonmagnetic spacers with an internal structure of alternating TiO2 and SiO2 layers which form finite-size dielectric photonic crystals. We show that the spin-wave coupling between the magnetic layers, and thus the formation of the magnonic band structure, necessitates a nonzero in-plane component of the spin-wave wave vector. A more complex structure perceived by light is evidenced by the photonic miniband structure and the transmission spectra in which we have observed transmission peaks related to the repetition of the magnetic slabs in the frequency ranges corresponding to the photonic band gaps of the TiO2/SiO2 stack. Moreover, we show that these modes split to very high sharp (a few THz wide) subpeaks in the transmittance spectra. The proposed novel multifunctional artificial crystals can have interesting applications and be used for creating common resonant cavities for spin waves and light to enhance the mutual influence between them. [ABSTRACT FROM AUTHOR]

We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon-Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film, the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity, and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range. [ABSTRACT FROM AUTHOR]

We present the observation of a complete bandgap and collective spin wave excitation in two-dimensional magnonic crystals comprised of arrays of nanoscale antidots and nanodots, respectively. Considering that the frequencies dealt with here fall in the microwave band, these findings can be used for the development of suitable magnonic metamaterials and spin wave based signal processing. We also present the application of a numerical procedure, to compute the dispersion relations of spin waves for any high symmetry direction in the first Brillouin zone. The results obtained from this procedure have been reproduced and verified by the well established plane wave method for an antidot lattice, when magnetization dynamics at antidot boundaries are pinned. The micromagnetic simulation based method can also be used to obtain iso-frequency contours of spin waves. Iso-frequency contours are analogous of the Fermi surfaces and hence, they have the potential to radicalize our understanding of spin wave dynamics. The physical origin of bands, partial and full magnonic bandgaps have been explained by plotting the spatial distribution of spin wave energy spectral density. Although, unfettered by rigid assumptions and approximations, which afflict most analytical methods used in the study of spin wave dynamics, micromagnetic simulations tend to be computationally demanding. Thus, the observation of collective spin wave excitation in the case of nanodot arrays, which can obviate the need to perform simulations, may also prove to be valuable. [ABSTRACT FROM AUTHOR]

We investigate spin waves in two-dimensional superlattices in the form of an infinite ferromagnetic slab of finite thickness with periodically embedded ferromagnetic inclusions. One of the reasons of the interest in magnonic superlattices is the possibility of tailoring their spectrum to produce magnonic gaps, crucial for the prospective applications. The objective of this study is to find the optimal values of structural parameters for which low-frequency magnonic gaps will occur and have a maximum width. We provide some guidelines for designing superlattices with a desired spectrum. We focus on the role of the symmetry of the lattice versus other geometrical factors, including the filling fraction and the superlattice constant. We compare the spectra of superlattices with inclusions of circular, hexagonal, and square cross sections. [ABSTRACT FROM AUTHOR]

We present the results of our theoretical investigation of the effect of a periodic interface modulation on the longitudinal vibrational modes in one-dimensional phononic crystals. Based on the plane wave method and the finite difference time domain method, our calculations show that the bottom and top of the first phononic gap can be controlled independently by adjusting the modulation introduced. The effective acoustic impedance of the modulated layers is demonstrated to be of key importance for the opening of phononic energy gaps. Band gaps for longitudinal modes can be expected to occur in a large range of interface inhomogeneity. The shape of the modulation proves not to have any significant effect on the investigated properties. [ABSTRACT FROM AUTHOR]

We study spin-wave excitations in multilayered magnetic nanograins composed of a stack of planes with perpendicular magnetic anisotropy. The inhomogeneity is modeled through the multiple repetition of a unit cell composed of layers of two different magnetic materials. The magnetic inhomogeneity along the central axis is found to split the frequency spectrum of magnetostatic excitations into two bands, while into the number of bands in the spectrum of exchange spin waves. We show that this difference in behavior is a result of the underlying long- and short-range interactions, respectively. We describe a way of increasing the role of the dipolar interactions in the formation of magnonic bands in patterned magnetic multilayers with perpendicular anisotropy, which can allow one to obtain ferromagnetic resonance spectra with two strong absorption peaks in low and high frequencies up to the sub-THz range. Our findings open a new area for modeling the spin-wave spectra of patterned magnetic multilayers with potential applications, and for studying the role of the microscopic magnetic structure in forming spin-wave spectra. [ABSTRACT FROM AUTHOR]

We present the results of plane wave method-based calculations adopted to magnonic band structures for exchange spin waves propagating in three-dimensional magnonic crystals (MCs) composed of two ferromagnetic metals. The crystals under consideration consist of a system of ferromagnetic spheres arranged in sites of a hexagonal lattice and embedded in a ferromagnetic material. Having analyzed all the possible combinations of magnonic crystal component materials from: Co, Ni, Fe, and Py (for spheres and matrix), we find material configurations for which either absolute or partial magnonic gaps occur in the spin-wave spectrum of the MC. We also demonstrate that the opening of a magnonic gap necessitates a sufficiently large contrast of magnetic parameters, and find the exchange length contrast to be the best measure of the capacity of the MC to produce a magnonic gap. Wider magnonic gaps are obtained in MCs in which the exchange length in the sphere material is larger than in the matrix. Among the MCs considered in this study, an absolute magnonic gap is obtained in a crystal with Ni spheres embedded in Fe. [ABSTRACT FROM AUTHOR]

We have computed the electronic and hole spectra of a three-dimensional superlattice consisting of layers of GaAs rods of finite height arranged in a hexagonal lattice and embedded in an AldGa1-dAs matrix, alternating with spacer layers of homogeneous AlAs. The spectra are calculated in the envelope function approximation, with both light-hole and heavy-hole subbands and hole spin degeneracy taken into account. The application of thick spacers allows to investigate the band structure of isolated layers of cylindrical rods. We estimate the ultimate efficiency of solar energy conversion in a solar cell based on an array of cylindrical quantum dots versus the dot height, and determine the optimal value of this parameter. [ABSTRACT FROM AUTHOR]

We study the effect of cross-sectional geometry and size on the spectrum of magnetostatic modes in dipole-coupled magnetic nanorods of circular or square cross section. The scope of this study is confined only to excitations propagating along the central axis of the nanorod; at the same time, the direction of the central axis is assumed to correspond to that of the magnetization of the sample. We find that a modification of the cross-sectional geometry results in nonuniform changes in the local field profile and the effect proves especially strong in the range of nanometer lateral dimensions. Moreover, the effect of cross-sectional geometry on the magnetostatic spectrum of a nanorod is found to be the most significant in peripheral regions of elongated rods. This is due to the occurrence of dipolar local field wells in these very regions. The shape of these peripheral wells affects first of all the bulk-dead and surface modes, which are “stuck” in them; frequencies of these modes prove to increase when the cross-sectional shape changes from square to circle. [ABSTRACT FROM AUTHOR]

Properties of purely dipolar and dipolar-exchange modes in finite circular rods are investigated numerically, with the sample static magnetization assumed to be uniform and parallel to the rod axis; we study magnetic excitations propagating along this direction. In particular, we demonstrate size-exchange effects induced by modifying the rod aspect ratio (the diameter to length ratio) and the strength of the exchange coupling. Untypical groups of purely dipolar modes are shown to occur in smooth rods: bulk-dead modes, characterized by a dead-amplitude region inside the rod, and comb modes, in which a nearly “dead” region is found at the sample borders, while numerous (“comb-like”) oscillations take place in the bulk. These two mode groups fade to disappear when the sample geometry evolves toward the thin-film one, or when the exchange becomes dominant. The existence of these untypical modes is related to local demagnetizing field profile nonhomogeneity, particularly to two edge wells found in the field profile. The bulk-dead modes are low-frequency modes confined to the well regions only (and therefore double-degenerated), while modes with higher frequency values (subjected to an interference within the barrier) become fast-oscillating comb modes. When the contribution of the exchange grows to prevail, the “dipolar” nonhomogeneity becomes negligible with respect to the total (exchange and dipolar) energy, and only the typically exchange-related bulk and surface modes are found to subsist. [ABSTRACT FROM AUTHOR]