G. Carlotti

Direct observation of a propagating spin wave induced by spin-transfer torque

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Brillouin light scattering studies of planar metallic magnonic crystals

The application of Brillouin light scattering to the study of the spin-wave spectrum of one- and two-dimensional planar magnonic crystals consisting of arrays of interacting stripes, dots and antidots is reviewed. It is shown that the discrete set of allowed frequencies of an isolated nanoelement becomes a finite-width frequency band for an array of identical interacting elements. It is possible to tune the permitted and forbidden frequency bands, modifying the geometrical or the material magnetic parameters, as well as the external magnetic field. From a technological point of view, the accurate fabrication of planar magnonic crystals and a proper understanding of their magnetic excitation spectrum in the gigahertz range is oriented to the design of filters and waveguides for microwave communication systems.

Acoustic investigation of the elastic properties of ZnO films

The elastic properties of ZnO films deposited by rf magnetron sputtering on Al2O3 substrates have been analyzed by means of an acoustic investigation technique. The phase velocities of a spectrum of acoustic modes propagating along the layered structure have been measured and the results exploited for determining the complete set of elastic constants of the film. The effective constants of the film are lower than those of the bulk material by amounts which depend on the elastic constant considered and range from −1.2% for c33 to −24.8% for c11. The values obtained were used for determining the dispersion curves of acoustic modes propagating along ZnO layers deposited on fused quartz and silicon and showed good agreement with experimental results.

Collective spin modes in monodimensional magnonic crystals consisting of dipolarly coupled nanowires

Magnetization dynamics of dipolarly coupled nanowire arrays has been studied by Brillouin light scattering. Measurements performed in uniformly magnetized wires as a function of the transferred wave vector demonstrated the existence of several discrete collective modes, propagating through the structure with a periodic dispersion curve encompassing several Brillouin zones relative to the artificial spatial periodicity. This experimental evidence has been quantitatively explained by a theoretical model which permits the calculation of the dispersion relation for collective modes in patterned arrays through the numerical solution of an eigenvalue problem for an integral operator.

Partial frequency band gap in one-dimensional magnonic crystals

Collective spin wave modes propagating in an array of magnetic stripes coupled by dynamic dipole interaction are investigated by Brillouin light scattering. It is demonstrated that this structure supports propagation of discrete spin waves at any angle with respect to the stripes length. The data are interpreted using a theoretical model based on the Bloch wave approach. It is shown that, due to the one-dimensional artificial periodicity of the medium, the gaps in the spin wave spectrum are partial: the frequency passbands for propagation along the direction of periodicity overlap with the stop bands for propagation along the stripes.

Magnetic field dependence of quantized and localized spin wave modes in thin rectangular magnetic dots

The magnetic field dependences of the frequencies of standing spin-wave modes in a tangentially magnetized array of thin rectangular permalloy dots (800 × 550 nm) were measured experimentally by a Brillouin light scattering technique and calculated theoretically using an approximate size-dependent quantization of the spin-wavevector components in the dipole-exchange dispersion equation for spin waves propagating in a continuous magnetic film. It was found that the inhomogeneous internal bias magnetic field of the dot has a strong influence on the profiles of the lowest spin-wave standing modes. In addition, the dynamic magnetization distributions found for both longitudinally and transversely magnetized long magnetic stripes gives a good approximation for mode distributions in a rectangular dot magnetized along one of its in-plane sides. An approximate analytic theory of exchange-dominated spin-wave modes, strongly localized along the dot edge that is perpendicular to the bias magnetic field, is developed. A good quantitative agreement with the results of the BLS experiment is found.

Brillouin scattering determination of the whole set of elastic constants of a single transparent film of hexagonal symmetry

In this work it is shown that the Brillouin light scattering technique can be successfully applied for determining the five effective elastic constants of a single transparent film of hexagonal symmetry, in the micron range of thicknesses. Measurements have been performed on a polycrystalline ZnO film, about 1.3 mu m thick, supported by a Si substrate. A major result of this work is that the elastic constant c66 is selectively determined for the first time from detection of the shear horizontal mode travelling parallel to the film surface. Similarly, a selective determination of c11 is attained from observation of the longitudinal mode guided by the film. The three remaining elastic constants, namely c13, c33, and c44, can be then obtained from detection of the Rayleigh surface mode and of the longitudinal bulk wave propagating at different angles from the surface normal.

Elastic properties of silicon dioxide films deposited by chemical vapour deposition from tetraethylorthosilicate

Abstract Dielectric silicate glass films have been obtained from tetraethylorthosilicate using both low-pressure chemical vapour deposition and plasma-enhanced chemical vapour deposition. The elastic properties of these films, which are deposited on (100)-Si wafers, have been studied by means of the Brillouin light scattering technique. The phase velocity of both the surface Rayleigh wave and the longitudinal wave in the film material have been measured and the two independent elastic constants c 11 and c 44 evaluated. This permitted us to derive the values of Youngs modulus and Poissons ratio which are useful quantities for the modelling of the elastic properties of multilevel structures used in electronics. Moreover, the substrate curvature method has been exploited in the case of the PECVD film, to study the evolution of the stress during subsequent thermal cycles. The values of the thermal expansion coefficient and of the water diffusion coefficient have thus been determined.

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO/sub 2/ substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.<<ETX>>

Comparative study of the elastic properties of polycrystalline aluminum nitride films on silicon by Brillouin light scattering

Abstract Brillouin light scattering (BLS) has been used to investigate the elastic properties of polycrystalline AIN films, about 1 μ m thick, grown by DC-reactive magnetron sputtering on Si 3 N 4 coated (100)-Si substrates. Taking advantage from the detection of a number of different acoustic modes, a complete elastic characterization of the films has been achieved. The elastic constants c 11 and c 66 have been selectively determined from detection of the longitudinal and of the shear horizontal bulk modes, respectively, travelling parallel to the film surface. The three remaining elastic constants, namely c 44 , c 33 and c 13 , have been obtained from detection of the Rayleigh surface wave and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AIN films depend on the deposition conditions and on the microstructural properties of the films, especially oxygen contamination and quality of texture. In the case of the films with the best degree of texture and the lowest oxygen content, the values of the elastic constants are rather close to those previously determined in epitaxial A1N films grown at high temperature by MOCVD. This demonstrates that sputter deposition at relatively low temperature can be used to grow high quality A1N films on Si and is of great importance in view of the integration of these films in the technology of IC semiconductors.