S. C. Ng

Observation of frequency band gaps in a one-dimensional nanostructured magnonic crystal

We report the experimental observation of band gaps in a synthetic nanostructured magnonic crystal composed of two different magnetic materials. The sample, in the form of a one-dimensional periodic array comprising alternating Permalloy and cobalt nanostripes, has been fabricated using advanced lithographic techniques. Dispersion relations of spin waves in the magnonic crystal have been mapped by Brillouin spectroscopy. The center frequency and width of the band gaps observed are tunable by an applied magnetic field. Dispersion relations calculated based on the finite element method accord with the measured data.

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.

Direct observation of the Dzyaloshinskii-Moriya interaction in a Pt/Co/Ni film.

The interfacial Dzyaloshinskii-Moriya interaction in an in-plane anisotropic Pt(4  nm)/Co(1.6  nm)/Ni(1.6  nm) film has been directly observed by Brillouin spectroscopy. It is manifested as the asymmetry of the measured magnon dispersion relation, from which the Dzyaloshinskii-Moriya interaction constant has been evaluated. Linewidth measurements reveal that the lifetime of the magnons is asymmetric with respect to their counter-propagating directions. The lifetime asymmetry is dependent on the magnon frequency, being more pronounced, the higher the frequency. Analytical calculations of the magnon dispersion relation and linewidth agree well with experiments.

Micromagnetic study of spin wave propagation in bicomponent magnonic crystal waveguides

The propagation of spin waves in bicomponent magnonic crystal waveguides has been investigated by micromagnetic simulations. The nanostructured waveguides studied are regular square lattice arrays of circular Fe dots embedded in an yttrium iron garnet matrix. Our simulations show that the waveguides exhibit wide magnonic band gaps of the order of 10 GHz. Band gap tunability, arising from variations in the filling fraction, lattice constant, and applied magnetic field has been demonstrated. Our findings would be of value to the efficient transmission and processing of microwave signals on the nanoscale by means of spin waves.

Asymmetric spin-wave dispersion due to Dzyaloshinskii-Moriya interaction in an ultrathin Pt/CoFeB film

Employing Brillouin spectroscopy, strong interfacial Dzyaloshinskii-Moriya interactions have been observed in an ultrathin Pt/CoFeB film. Our micromagnetic simulations show that spin-wave nonreciprocity due to asymmetric surface pinning is insignificant for the 0.8 nm-thick CoFeB film studied. The observed high asymmetry of the monotonic spin wave dispersion relation is thus ascribed to strong Dzyaloshinskii-Moriya interactions present at the Pt/CoFeB interface. Our findings should further enhance the significance of CoFeB as an important material for magnonic and spintronic applications.

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 frequency and damping anomalies in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3

Brillouin scattering from relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 has been measured between 296 and 780 K. Heating of the virgin Pb(Zn1/3Nb2/3)O3 crystal produced results that are different from those of subsequent heating or cooling processes. An unusually broad softening of the longitudinal acoustic-mode frequency, due to the diffuse ferroelectric-phase transition, is observed. The attendant phonon damping is attributed to order-parameter fluctuations. A huge temperature hysteresis, which eluded observation by other techniques, has been found.

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.

Theory of non-radiative capture of carriers by multiphonon processes for deep centres in semiconductors

A quantum-mechanical calculation of the non-radiative transition rate by multiphonon processes is performed by employing a more direct mathematical approach than those used by previous workers. Based on the trap potential model proposed by Lucovsky and the optical deformation form of electron-phonon interaction, the analytical expression for the carrier capture cross section is brought to a transparent form for easy comparison with experiments after some transcriptions. The effect due to the charge state of the deep centre is also discussed. Both the absolute magnitude and the temperature-dependent behaviour of the capture cross section predicted in our calculations are well supported by the experimental results of various deep centres in semiconductors. In particular, good fits are obtained for the temperature dependence of the experimental electron capture cross sections reported by Henry and Lang (1977) for B and A centres in GaAs. The accuracy of the Huang-Rhys factor and the phonon energy obtained for the B centre is corroborated by the good fittings obtained for the photoionization cross section data reported by Wang et al (1984). The results of our theory are also shown to be useful in identifying more accurately the charge state of a deep centre.