Brillouin Light-scattering Experiments Research Articles

Brillouin Light Scattering from Magnetic Excitations.

Brillouin light scattering (BLS) has been established as a standard technique to study thermally excited sound waves with frequencies up to ~100 GHz in transparent materials. In BLS experiments, one usually uses a Fabry-Pérot interferometer (FPI) as a spectrometer. The drastic improvement of the FPI contrast factor over 1010 by the development of the multipass type and the tandem multipass type FPIs opened a gateway to investigate low energy excitations (ħω ≤ 1 meV) in various research fields of condensed matter physics, including surface acoustic waves and spin waves from opaque surfaces. Over the last four decades, the BLS technique has been successfully applied to study collective spin waves (SWs) in various types of magnetic structures including thin films, ultrathin films, multilayers, superlattices, and artificially arranged dots and wires using high-contrast FPIs. Now, the BLS technique has been fully established as a unique and powerful technique not only for determination of the basic magnetic constants, including the gyromagnetic ratio, the magnetic anisotropy constants, the magnetization, the SW stiffness constant, and other features of various magnetic materials and structures, but also for investigations into coupling phenomena and surface and interface phenomena in artificial magnetic structures. BLS investigations on the Fe/Cr multilayers, which exhibit ferromagnetic-antiferromagnetic arrangements of the adjacent Fe layer's magnetizations depending on the Cr layer's thickness, played an important role to open the new field known as "spintronics" through the discovery of the giant magnetoresistance (GMR) effect. In this review, I briefly surveyed the historical development of SW studies using the BLS technique and theoretical background, and I concentrated our BLS SW studies performed at Tohoku University and Ishinomaki Senshu University over the last thirty five years. In addition to the ferromagnetic SW studies, the BLS technique can be also applied to investigations of high-frequency magnetization dynamics in superparamagnetic (SPM) nanogranular films in the frequency domain above 10 GHz. One can excite dipole-coupled SPM excitations under external magnetic fields and observe them via the BLS technique. The external field strength determines the SPM excitations' frequencies. By performing a numerical analysis of the BLS spectrum as a function of the external magnetic field and temperature, one can investigate the high-frequency magnetization dynamics in the SPM state and determine the magnetization relaxation parameters.

Enhancement of Brillouin light scattering signal with anti-reflection layers on magnetic thin films

The significant enhancement of Brillouin light scattering (BLS) spectroscopy intensity in a ferromagnetic thin film with an additional dielectric anti-reflection layer is experimentally investigated. The anti-reflection layer thickness dependent BLS measurements on ferromagnetic layers are performed systematically. Consequently, we observe that BLS signals are dramatically enhanced by more than 450% at a specific dielectric layer thickness due to the pure optical effect. Because of the large signal enhancements, the errors of the spin wave resonance peak frequencies are noticeably reduced as well. Since many magnetic properties such as the saturation magnetization, the surface anisotropy, and the exchange stiffness constant are determined by the spin wave resonance frequencies from the BLS spectra, the additional anti-reflection layer can help to improve the reliability of BLS experiments. Especially, the BLS signal improvement plays a crucial role in the precise determination of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) energy density, since the iDMI energy density is calculated from the difference of Stokes and anti-Stokes resonance frequencies, which is typically order of 1 GHz.

Spin dynamics in thin nanometric elliptical Permalloy dots: A Brillouin light scattering investigation as a function of dot eccentricity

Brillouin light scattering (BLS) spectra have been measured in arrays of cylindrical Permalloy dots with elliptical cross section, $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wide, $15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick, and eccentricities from 1 to 3. Several spin modes are observed and their frequencies tracked as a function of the direction of the applied $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ magnetic field $\mathbit{H}$. The experimental data are interpreted within the framework of the recently introduced dynamical matrix method to calculate spin excitations in magnetic particles. We find that the mode frequencies strongly depend on the eccentricity of the dots and on the direction of the applied field. For fields along the principal axes the solutions can be classified into: (i) modes localized near the particle ends, (ii) modes with nodal lines perpendicular to $\mathbit{H}$ (backwardlike modes), (iii) modes with nodal lines parallel to $\mathbit{H}$ (Damon-Eshbach-like modes) and (iv) modes with both parallel and perpendicular nodal lines. In cases where the frequencies of two modes in different families are similar, some hybridization between the modes is observed. For $\mathbit{H}$ along nonsymmetry directions only the modes of type (i) remain reasonably well defined, other modes can at best be described as hybrids of modes in the above categories. Determining which of the modes is active in BLS experiments leads to excellent agreement with the experimental results.

Elastic constants of the SrLaAlO 4 and SrLaGaO 4 crystals measured at ultrasonic and hypersonic acoustic frequencies

Elastic constants measured by Brillouin light-scattering experiments were compared with results estimated from the pulse–echo–overlap method for the ultrasonic frequency to find dispersion in their values. In this way Brillouin hypersonic experiments complete the ultrasonic results. The present investigation includes a careful analysis of the accuracy of the results. The measured values of elastic constants suggest a weak dispersion in acoustic wave velocities. However, given the magnitude of the experimental errors, it is difficult to conclude about the dispersion.

Shear-elasticity anomaly in Ce/Fe multilayers

Brillouin light-scattering experiments have been performed at room temperature to determine the shear modulus C{sub 44} of Ce/Fe multilayers (in their reference frame) as a function of the modulation wavelength {Lambda}. The study is complementary to a previous investigation of the flexural modulus E{sub F} of this system in the sense that, while E{sub F} is dominated by the Fe layers, C{sub 44} largely reflects the elasticity of the more compliant Ce component. In the heterostructures, the Ce layers exhibit a laminated two-phase structure composed of low-volume {alpha} and high-volume {gamma}-like Ce, with a thickness-dependent relative fraction. Furthermore, structural transitions occur from an amorphous phase to the crystalline phases of bcc Fe and fcc Ce, for Fe abruptly within the entire sublayer volume at a critical thickness near 20 {Angstrom}, for Ce more gradually with increasing layer thickness starting near 40 {Angstrom}. These structures and phase transitions are reflected in the variation of C{sub 44} with {Lambda}. A large portion of this variation can be ascribed to the {Lambda}-dependent {alpha}-{gamma}-like composite structure of the Ce layers. A superposed softening of C{sub 44} is essentially the signature of the amorphous-to-crystalline transition in the Ce sublayers. {copyright} {ital 1997} {ital The Americanmore » Physical Society}« less

Elastic constants of WC-a-C:H composite films studied by Brillouin spectroscopy.

Brillouin light-scattering experiments have been performed on tungsten-carbide containing amorphous-hydrogenated-carbon (WC\char21{}a-C:H) films which were prepared on (111) silicon wafers by a reactive rf sputtering process. From the measured velocity of the Rayleigh surface modes the shear modulus G is calculated as a function of the tungsten-carbide volume fraction, reflecting a phase transition of the films from an a-C:H-dominated to a crystalline \ensuremath{\beta}-WC-dominated WC\char21{}a-C:H mixture. The elastic constants determined agree well with results obtained from depth-sensing indentation measurements. Theoretical calculation of the elastic constant of the composite was also performed as a function of WC volume fraction. The results show a small deviation from the experimental data in the range of low WC fraction, which is probably caused by the phase change of the a-C:H matrix. The relatively large discrepancy of the theoretical calculation in the high-WC-fraction region may be a result of the invalidity of the theory.

Investigation of the dependence of BLS frequencies on angle of incidence for thin iron films

Brillouin light-scattering experiments have been done at various angles of incidence, θ, for four specimens prepared by molecular-beam epitaxy. The specimens were single ultrathin films of Fe deposited on single-crystal Ag substrates. Dependence of magnon frequency on θ is easily resolvable in all specimens. We find that the magnitude of this dependence is in good agreement with a theoretical calculation that takes into account magnetic anisotropies, dipole-dipole, and exchange interactions. Our results imply that magnetic excitations in these specimens are correlated over distances of at least 5000 Å.

Spin-wave propagation on imperfect ultrathin ferromagnetic films.

The effects of localized imperfections on spin-wave propagation in very thin ferromagnetic films are examined. The imperfections are assumed to be localized to a few lattice sites and cause local changes in anisotropy and exchange fields. These imperfections may be due to thickness variations or other geometrical imperfections. We find that in very thin films the lifetime of long-wavelength spin waves is relatively insensitive to scattering from even large numbers of imperfections, and therefore cannot explain large observed linewidths observed in Brillouin light-scattering experiments. On the other hand, we find that a band of long-wavelength spin-wave modes can exist in an inhomogeneous film with a distribution of effective anisotropy fields. It is possible to have large bands with bandwidths on the order of 10 GHz in rough ultrathin films due to the sensitive dependence of effective anisotropy fields on thickness.

Studies of Exchange Coupling in Fe (001) Whisker/Cr/Fe Structures using BLS and Rheed Techniques.

ABSTRACTThe conditions for an almost perfect growth of smooth Cr (001) films on an iron whisker substrate have been investigated by means of reflection high energy electron diffraction (RHEED). The exchange interaction between 20 Monolayer thick Fe (001) films separated from a bulk whisker Fe (001) substrate by a variable number of Cr (001) Monolayers (ML) has been investigated by means of Brillouin light scattering experiments (BLS). These experiments show unambiguously that the exchange coupling strength between the iron film and the iron whisker can be described by a short wavelength oscillatory term superposed on a slowly varying antiferromagnetic background. The BLS data enabled one to separate the bilinear and the biquadratic contributions to the antiferromagnetic exchange coupling terms. Both the bilinear and the biquadratic coupling strengths exhibited a short period oscillatory dependence on the Cr interlayer thickness (∼2 Monolayers). Maxima in the bilinear antiferromagnetic coupling strength occur for an odd number of Cr Monolayers. This observation is not in agreement with first principles calculations. The first phase inversion has been found to occur between 4 and 5 ML of Cr.

Evidence for collective exchange modes in Co/Pd multilayers observed by Brillouin light scattering (abstract)

Recently, exchange-dominated collective spin-wave excitations have been predicted for multilayered structures consisting of alternating magnetic and nonmagnetic layers.1 These types of collective excitations exist for nonzero propagation wave vectors if the magnetic layers are exchange coupled across the spacer layers.1 The modes are the collective counterpart to the optic dipolar-exchange mode observed in magnetic double layers by Vohl, Barnás and Grünberg.2 For a multilayer with N periods of bilayer thickness Λ there exist N modes which, in the absence of exchange coupling, form the well-known band of dipolar collective spin-wave excitations. The modes are dominantly surface-mode-like at the upper band edge and mostly bulk-mode-like at the lower band edge. With the onset of exchange coupling the stack bulk modes are shifted to higher frequencies and become exchange-mode-like in character. In the full exchange coupling limit these modes become the so-called standing spin waves of the total multilayer stack, and their frequencies depend on the stack thickness NΛ. The stack surface mode in this limit becomes the surface Damon–Eshbach mode of a single film of thickness NΛ. Brillouin light-scattering experiments have been performed on magnetron-sputtered Co/Pd multilayers.3,4 The number of atomic layers (AL) is the same in each Co and Pd layer. The number of periods was chosen to be N=8 and 30, with Λ varying between 4 and 140 Å. In the case of N=30 a strong increase of spin-wave frequencies for all but the stack surface mode is observed for d<8 AL. Decreasing d0 increases the interlayer exchange coupling and the band of collective bulklike modes increases in frequency to become a band of exchange-dominated modes. Depending on the strength of the coupling (i.e., spacer layer thickness), this band can have frequencies higher than that of the stack surface mode. From the crossing regime the maximum exchange coupling length is estimated to be 10 Å. For samples with N=8, collective exchange modes were not observed. Model calculations show that for N=8 the expected frequency increase near the onset of interlayer exchange coupling is much steeper than in the N=30 case, which might prohibit their experimental observation. Also, the cross section is reduced by a factor of 4 due to the smaller total stack thickness.

Brillouin light-scattering experiments on exchange-coupled ultrathin bilayers of iron separated by epitaxial copper (001).

The magnetic properties of bilayers of bcc Fe(001) separated by bcc Cu(001) films of variable thickness have been investigated by means of Brillouin light scattering. For copper thicknesses less than 9 monolayers (ML) the coupling between the iron films was found to be ferromagnetic; for thicknesses between 9 and 12 ML the coupling was found to be antiferromagnetic. The data are compared with the results of a model calculation that assumes a uniform static magnetization across each film: The magnetizations of the two films, ${\mathit{M}}_{\mathit{A}}$ and ${\mathit{M}}_{\mathit{B}}$, are assumed to be coupled by an interaction energy of the form J${\mathbf{M}}_{\mathit{A}}$\ensuremath{\cdot}${\mathbf{M}}_{\mathit{B}}$/(\ensuremath{\Vert}${\mathit{M}}_{\mathit{A}}$\ensuremath{\Vert}\ensuremath{\Vert}${\mathit{M}}_{\mathit{B}}$\ensuremath{\Vert}), where J is a surface energy. For small applied fields and for antiferromagnetic coupling the magnetizations in the films need not be parallel. The magnetic-field dependence of frequencies calculated using this model display features that are similar to the observations.

Experimental Evidence for Brillouin Asymmetry Induced by a Temperature Gradient

Very-low-angle Brillouin light-scattering experiments have been performed in liquid water subjected to a temperature gradient. The line intensities are unequal, and both the sign and the order of magnitude of this effect agree with recent theoretical calculations for nonequilibrium systems.