V. L. Zhang

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.

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.

Observation of dual magnonic and phononic bandgaps in bi-component nanostructured crystals

We report on the experimental observation of dual magnonic and phononic bandgaps in bi-component nanostructured crystals. The dispersion relations of linear periodic arrays of alternating Fe (or Ni) and Ni80Fe20 nanostripes on a SiO2/Si substrate, mapped by Brillouin spectroscopy, feature distinct bandgaps. Calculations of the magnon and phonon dispersions yield good agreement with experiments. No magnon-phonon interaction is detected for the modes observed, making the structures studied a potential platform for the separate and simultaneous processing of information carried by hypersonic magnons and phonons, with no undesirable cross-talk between them.

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.

Phononic dispersion of a two-dimensional chessboard-patterned bicomponent array on a substrate

Brillouin measurements of the dispersion relations of surface acoustic- and optical-like waves along Γ-M and Γ-X symmetry directions in a two-dimensional bicomponent nanostructured crystal are reported. The sample, in the form of a periodic chessboard array of alternating Permalloy and cobalt square dots on a SiO2/Si substrate, was fabricated using high-resolution electron-beam lithographic, sputtering, etching, and lift-off techniques. The measured phononic band structures exhibit diverse features, such as a partial hybridization bandgap and unusual surface optical-like phonon branches, where there are out-of-phase vibrational characteristics between nearest-neighbor dots. Numerical simulations, based on the finite element analysis, reproduced the experimental dispersion relations.

In-plane angular dependence of the spin-wave nonreciprocity of an ultrathin film with Dzyaloshinskii-Moriya interaction

The nonreciprocal propagation of spin waves in an ultrathin Pt/Co/Ni film has been measured by Brillouin light scattering. The frequency nonreciprocity, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI), has a sinusoidal dependence on the in-plane angle between the magnon wavevector and the applied magnetic field. The results, which are in good agreement with analytical predictions reported earlier, yield a value of the DMI constant which is the same as that obtained previously from a study of the magnon dispersion relations. We have demonstrated that our magnon-dynamics based method can experimentally ascertain the DMI constant of multilayer thin films.

Band structure of magnonic crystals with defects: Brillouin spectroscopy and micromagnetic simulations

Using Brillouin spectroscopy, the first observation has been made of the band structures of nanostructured defect magnonic crystals. The samples are otherwise one-dimensional periodic arrays of equal-width Ni80Fe20 and cobalt nanostripes, where the defects are stripes of a different width. A dispersionless defect branch emerges within the bandgap with a frequency tunable by varying the defect stripe width, while the other branches observed are similar to those of a defect-free crystal. Micromagnetic and finite-element simulations performed unveil additional tiny bandgaps and the frequency-dependent localization of the defect mode in the vicinity of the defects.