M. H. Kuok

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.

Raman scattering investigations of the antiferroelectric–ferroelectric phase transition of NaNbO3

The antiferroelectric to ferroelectric phase transition in NaNbO3 was investigated by Raman scattering between 295 and 10 K. The transition occurs at 185 K on cooling and the antiferroelectric and ferroelectric phases coexist at low temperature. The transition occurs at 210 K on heating and the relatively large hysteresis is attributed to the difficulty in reversing the spontaneous polarization direction at the transition. The results agree well with low-temperature x-ray data which show that both phases coexist if the crystal samples are twinned. The transition is not considered to be of the displacive type, although two modes at 115 and 121 cm-1 were observed to soften at low temperature.

High temperature Raman study of phase transitions in antiferroelectric NaNbO3

Abstract The Raman spectra of antiferroelectric NaNbO3 have been studied at temperatures up to 480°C. Two phase transitions were observed, one at 376°C in consistence with the previous observations, and the other being a new phase transition at 190°C. There is no evident hysteresis for this transition, which indicates that the phase transition is of second order.

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.