Z. K. Wang

Nanostructured Magnonic Crystals with Size-Tunable Bandgaps

Just as a photonic crystal is a periodic composite composed of materials with different dielectric constants, its lesser known magnetic analogue, the magnonic crystal can be considered as a periodic composite comprising different magnetic materials. Magnonic crystals are excellent candidates for the fabrication of nanoscale microwave devices, as the wavelengths of magnons in magnonic crystals are orders of magnitude shorter than those of photons, of the same frequency, in photonic crystals. Using advanced electron beam lithographic techniques, we have fabricated a series of novel bicomponent magnonic crystals which exhibit well-defined frequency bandgaps. They are in the form of laterally patterned periodic arrays of alternating cobalt and permalloy stripes of various widths ranging from 150 to 500 nm. Investigations by Brillouin light scattering and computer modeling show that the dispersion spectrum of these crystals is strongly dependent on their structural dimensions. For instance, their first frequency bandgap is found to vary over a wide range of 1.4-2.6 gigahertz. Such a functionality permits the tailoring of the bandgap structure which controls the transmission of information-carrying spin waves in devices based on these crystals. Additionally, it is observed that the bandgap width decreases with increasing permalloy stripe width, but increases with increasing cobalt stripe width, and that the bandgap center frequency is more dependent on the stripe width of permalloy than that of cobalt. This information would be of value in the design of magnonic crystals for potential applications in the emerging field of magnonics.

Ferromagnetic and antiferromagnetic spin-wave dispersions in a dipole-exchange coupled bi-component magnonic crystal

The magnon dispersion relations of the ferromagnetic and antiferromagnetic phases in a dipole-exchange coupled one-dimensional magnonic crystal comprising alternating cobalt and Permalloy nanostripes have been mapped by Brillouin spectroscopy. To elucidate the magnetization dynamics at the interfaces between stripes, the experimental data are analyzed based on a macroscopic theory under Hoffmann-type boundary conditions. Good agreement is obtained between theory and experiment for both the ferromagnetic and antiferromagnetic phases. Results suggest the existence of strong exchange coupling across the cobalt-Permalloy interfaces, comparable with the exchange coupling within each component material.

Brillouin observation of bulk and confined acoustic waves in silica microspheres

Brillouin scattering from loose silica microspheres has been investigated as a function of their diameter (140 nm to 4 μm). The measured linear dependence of the confined acoustic mode frequencies on the inverse sphere diameters accords well with Lamb’s theory. Bulk acoustic modes are also observed in the larger microspheres, suggesting that the limiting size of a particle for these modes to be observable is about three times their wavelength. Internal consistency in the analyses of the bulk and confined mode data shows the validity of this Brillouin technique in the evaluation of the elastic properties of microspheres. Additionally it also affords a means of sizing the diameters of microspheres.

Hypersonic Vibrations of Ag@SiO2 (Cubic Core)−Shell Nanospheres

The intriguing optical and catalytic properties of metal-silica core-shell nanoparticles, inherited from their plasmonic metallic cores together with the rich surface chemistry and increased stability offered by their silica shells, have enabled a wide variety of applications. In this work, we investigate the confined vibrational modes of a series of monodisperse Ag@SiO(2) (cubic core)-shell nanospheres synthesized using a modified Stöber sol-gel method. The particle-size dependence of their mode frequencies has been mapped by Brillouin light scattering, a powerful tool for probing hypersonic vibrations. Unlike the larger particles, the observed spheroidal-like mode frequencies of the smaller ones do not scale with inverse diameter. Interestingly, the onset of the deviation from this linearity occurs at a smaller particle size for higher-energy modes than for lower-energy ones. Finite element simulations show that the mode displacement profiles of the Ag@SiO(2) core-shells closely resemble those of a homogeneous SiO(2) sphere. Simulations have also been performed to ascertain the effects that the core shape and the relative hardness of the core and shell materials have on the vibrations of the core-shell as a whole. As the vibrational modes of a particle have a bearing on its thermal and mechanical properties, the findings would be of value in designing core-shell nanostructures with customized thermal and mechanical characteristics.

Band gap parameters of one-dimensional bicomponent nanostructured magnonic crystals

We investigate theoretically the lowest-frequency spin waves in one-dimensional periodic arrays of alternating cobalt and permalloy nanostripes to analyze their recently measured first band gap parameters. Based on insights from numerical calculations, we formulate simple equations relating center frequencies and widths of the first band gaps of these magnonic crystals to their geometric and magnetic parameters. Analytical calculations based on these equations accord well with both the measured and the numerically generated size-dependences of the band gap parameters. Our approach permits the easy and convenient evaluation of the band gap parameters of bicomponent magnonic crystals comprising one-dimensional arrays of nanostripes of alternating magnetic materials.

Band structures of exchange spin waves in one-dimensional bi-component magnonic crystals

We present the micromagnetic study of magnonic band structures for exchange spin waves propagating in one-dimensional magnonic crystals. The crystals are of laterally patterned periodic arrays of alternating cobalt and nickel stripes. Large magnonic bandgaps with widths of tens of GHz are observed. It is found that the higher-order transmission bands and bandgaps have wider widths than those of the lower-order bands and bandgaps. Another interesting feature is that the widths of the first two bandgaps are independent of the applied field, in contrast with an earlier report of decreasing bandgap widths with increasing applied field observed for dipolar spin waves.

Interfacial magnetization dynamics of a bi-component magnonic crystal comprising contacting ferromagnetic nanostripes

An investigation of the interfacial magnetization dynamics of a magnonic crystal comprising alternating, contacting cobalt and Permalloy nanostripes is reported. Recently measured spin-wave dispersion data of the crystal in its ferromagnetic and antiferromagnetic phases are interpreted in terms of the effective spin pinning at its cobalt-Permalloy interfaces based on generalized Guslienko-Slavin boundary conditions. Calculations using a macroscopic theory well reproduce the experimental data. Numerically generated mode profiles reveal considerable effective pinnings at the interfaces. The obtained pinning parameters have values significantly larger than the values calculated at the lateral edges of individual stripes, suggesting that the strong interfacial coupling, reported in our previous study, contributes significantly to the effective pinning at the interfaces between adjacent stripes.

Magnetic-field dependence of spin waves in ordered permalloy nanowire arrays in two dimensions

The longitudinal magnetic-field dependence of quantized spin waves in ordered permalloy nanowire arrays in two dimensions (2D) has been studied by Brillouin spectroscopy. The results were analyzed on the basis of collective spin waves in a hexagonal 2D nanowire array and compared with the results obtained from an analysis of spin waves in an isolated wire. It is found that only the lowest-frequency spin-wave mode of an isolated nanowire is marginally influenced by the presence of neighboring nanowires. These findings are significant for the theoretical understanding and the future technological utilization of magnetic nanowires.

Hypersonic confined eigenvibrations of gold nano-octahedra

The size-dependence of the vibrational mode frequencies of octahedron-shaped gold nanocrystals has been measured by micro-Brillouin spectroscopy. A finite element analysis reveals that the nine well-resolved peaks observed are due to confined acoustic modes with each peak arising from more than one mode. The elastic constants of the nanocrystals are found to be comparable to those of bulk gold crystals. Findings suggest that the eigenfrequencies of any free regular-shaped homogeneous object always scale with its inverse linear dimension. Additionally, this universal relationship is valid for such objects of any size in the classical regime and is independent of elastic properties.

Synthesis and magnetic properties of large-area ferromagnetic cylindrical nanoshell and nanocup arrays

Large-area arrays of magnetic Ni80Fe20 cylindrical nanoshells, nanocups, and perforated nanocups were synthesized using oblique deposition into topographical templates patterned using laser interference lithography. The geometry of the template and the tilt angle of the sample during deposition provide versatile control over the final geometry and dimension of nanostructures with thickness below 10 nm. Decreasing shell thickness led to a magnetization switching path between onion (bidomain) and reverse onion states, bypassing the vortex (flux-closed) state. The variation of magnetization reversal processes with geometry was characterized using vibrating sample magnetometry, and the results were in good agreement with micromagnetic simulations.