Brillouin-Mandelstam scattering from thermal and excited magnons

Brillouin-Mandelstam scattering (BMS) is the scattering of light from acoustical quasiparticles (phonons, magnons and others). The frequency shift under BMS is 10–100 GHz. The observation of BMS from magnons became possible only after J. Sandercock had designed a multi-pass Fabry-Perot interferometer with a high contrast (1971). BMS from magnons has, by now, been observed in CrBr₃ (Sandercock), YIG, FeBO₃ (Jantz, Sandercock, Wettling), CoCO₃ (Borovik-Romanov, Jotikov, Kreines), EuO, EuS (Grünberg, Metawe), Ni, Fe (Sandercock, Wettling), metglasses (Chang, Malozemoff, Grimsditch, Senn, Winterling). In this review the main results of the above works are presented.The dispersion laws of magnons were studied by BMS in the energy range (inaccessible for neutron diffraction) where the contributions due to three types of interaction: magnetic, dipole-dipole, and exchange can be separated.Investigation of BMS in EuO and metals has led to the discovery of surface magnons. BMS from standing spin waves has been observed in thin films of metglasses.By observing BMS, it is possible to study quasiparticles pumped by microwave power. It was found that under ferro- or antiferromagnetic resonance an excess of quasiparticles arises, these quasiparticles being magnons with the frequency equal to that of microwave power and phonons with half the microwave frequency. Scattering of light from parametrically excited magnons has also been observed. This opens new possibilities for studying relaxation processes in magnetic materials.     

Spin reorientation transition in Fe/CeH2 multilayers probed by soft X-ray resonant magnetic scattering

The magnetic domain configurations of Fe 3d spins in Fe/CeH₂ multilayers were measured by soft X-ray resonant magnetic scattering. The interface region could be probed by setting up X-ray standing waves due to the multilayer periodicity. By resolving first- and second-order magnetic scattering contributions, we show that the latter probe directly the magneto-crystalline anisotropy which is dominated by the Fe interface layers causing a spin reorientation transition when the temperature is lowered. Received: 30 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Structure and orbital ordering of ultrathin LaVO3 probed by atomic resolution electron microscopy and Raman spectroscopy

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO₃ films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature‐dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm^–1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO₃ films at about 130 K.     

Giant enhancement of the Raman scattering by local magnon modes in FeF2:Mn2+

A study of the local magnon mode in FeF₂ crystals with Mn²⁺ impurities by inelastic Raman light scattering is presented. Though the interaction between the radiation and the Mn spins is negligible the scattering by the s₀ local mode is very strong. The data of the frequency and the scattering intensity are analyzed in terms of a magnetization coupled mode approach.     

Brillouin scattering studies of rare gas solids

Abstract

The scattering of light by elementar excitations in the matter is results in two phenomena, discriminated by the zero wavevector frequency of the excitation: if this frequency is zero, one deals with Brillouin scattering, and with Raman scattering in the other case. Brillouin scattering results from the interaction of light with thermal excitations (acoustic phonons in a crystal) of a material, or, from a classical point of view, with density waves. Contrary to Raman scattering, the selection rules allow always the observation of at least one mode. It is a powerful technic in the study of rare gases under pressure: at ambient temperature, rare gases crystallize in the face centered cubic structure (except helium which structure was recently found to be hexagonal) and are therefore Raman and infrared inactive.

Experimental results will be reviewed on rare gases and rare gas mixtures in the fluid phase, like He-Ne and He-H₂. These results will be discussed in relation with recent measurements of the frequency of global oscillations of Jupiter.     

Brillouin scattering study of multilayer cobalt-niobium films

Spin waves which are characteristic of periodic structures of thin ferromagnetic Co films alternating with nonmagnetic Nb films have been investigated by means of Brillouin light scattering. The dependence of the magnon frequencies on the magnetic inplane field and on the wave-vector was measured for several samples with different numbers and thicknesses of the layers. The experimental data are in good agreement with a theory of magnetostatic surface spin waves in such media elaborated by Grünberg and Mika. The amplitudes of the transverse magnetization in the different layers of the stack have been calculated. The highest frequency branch resembles the Damon-Eshbach surface magnon. With decreasing frequency the branches acquire volume mode character. In addition, a number of phonon branches has been observed which are interpreted as plate modes of the combined Co-Nb layer on the Si substrate.     

Doppler-free resonances of the second-order raman scattering

The possibility of the observation of Raman scattering resonances completely free from the influence of the Doppler effect has been examined for the first time. The phenomenon is based on the excitation of a Raman oscillation standing wave in a gas by two standing light waves, whose frequency difference is equal to half the Raman frequency. The complete compensation of Doppler shifts results from the simultaneous interactions between atomic particles and two pairs of counter-propagating waves. Doppler-free resonances of the second-order Raman light scattering appear in the number of particles excited to the upper Raman level and in the radiation at the Stokes and anti-Stokes frequencies. The amplitude estimate for the resonance in the number of particles is given for the example of neon.     

Resonant scattering of phonons by adsorbates on clean silicon surfaces

We used Phonon Surface Scattering Spectroscopy (PSSS) to study excitations of adsorbates in the far infrared region from 100 eV up to 3 meV. We investigated the scattering of pulses of monochromatic phonons by thin films of H₂O, D₂O and Ne deposited in situ onto laserannealed Si(100)-surfaces. For all substances we observed a broad scattering at coverages below one monolayer and a strong resonant scattering at about 3 monolayers. The resonance frequency decreased with increasing coverage. The reasons for the resonances are discussed. Because of the lack of any observable isotope effect between H₂O and D₂O, the possibility of tunneling states in these layers can be excluded. Instead, we conclude that a lateral vibrational mode of the adlayer is responsible for the resonance effect. Corresponding calculations by means of continuum elasticity theory describe the scattering behaviour very well if similar elastic constants are used for all film substances.     

Spin–orbit scattering effect on electron–electron interactions in disordered metals

We have measured the low-temperature resistivities of a series of bulk crystalline disordered Ti_73−xAl₂₇Sn_x alloys (x≲5) as well as the sheet resistances of a number of thin ferromagnetic Ni films (≈120 Å thick) sandwiching an ultrathin Ag or Au (≲5 Å) layer. The level of impurities (concentration of Sn in the former case, and thickness of Ag or Au in the latter case) is progressively increased in order to enhance the spin–orbit scattering in a controllable manner. The influence of the spin–orbit scattering on the electron–electron interaction effects is studied from the temperature dependence of resistivities (sheet resistance) at low temperatures. We find that the electron–electron interaction contribution to the resistivities (sheet resistances) increases slightly with increasing spin–orbit scattering. Our observation is discussed in terms of the current theoretical concept for the electron–electron interactions in disordered metals.     

Spin solitons and waves in chiral molecular ferrimagnets

Resonance modes corresponding to a spin-soliton resonance have been found in the electron spin resonance spectra of [Cr(CN)₆][Mn(S)-pnH-(H₂O)]H₂O two-dimensional (2D) chiral single crystals and [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}₂(H₂O)][Cr(CN)₆]₂ chiral single crystals with a 3D magnetic order. It is also established that the chiral crystals of both types exhibit a spin-wave resonance analogous to the excitation of standing spin waves in thin magnetic films. At the same time, racemic crystals of the first type do not exhibit spin-soliton resonance. The entire body of experimental data indicates that the chirality of crystals influences the spin excitations (standing spin waves and solitons) in these media.     

Low frequency Raman scattering of anatase titanium dioxide nanocrystals

TiO₂ nanoparticle of size 7.8 nm are synthesized by wet chemical route and characterized by low-frequency Raman scattering (LFRS), transmission electron microscopy (TEM) and X-ray diffraction. The low frequency peaks in the Raman spectra have been explained using the Lamb's theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low-frequency Raman scattering originates from the spherical (l=0) and quadrupolar vibrations (l=2) of the spheriodal mode due to the confinement of acoustic vibrations in TiO₂ nanoparticles. In addition to the low-frequency peak due to the vibrational quadrupolar and spheriodal modes, a band is also observed, which is assigned to the Raman forbidden torsional l=2 mode originating from the near spherical shape of the TiO₂ nanoparticles. The size distribution is also obtained from LFRS, which is in good agreement with TEM.     

Study of Raman spectrum of uranium using a surface‐enhanced Raman scattering technique

Raman spectroscopic investigation on weak scatterers such as metals is a challenging scientific problem. Technologically important actinide metals such as uranium and plutonium have not been investigated using Raman spectroscopy possibly due to poor signal intensities. We report the first Raman spectrum of uranium metal using a surface‐enhanced Raman scattering‐like geometry where a thin gold overlayer is deposited on uranium. Raman spectra are detected from the pits and scratches on the sample and not from the smooth polished surface. The 514.5‐ and 785‐nm laser excitations resulted in the Raman spectra of uranium metal whereas 325‐nm excitation did not give rise to such spectra. Temperature dependence of the B_3g mode at 126 cm⁻¹ is also investigated. Copyright © 2011 John Wiley & Sons, Ltd.     

Au nanoplate/polypyrrole nanofiber composite film: preparation,characterization and application as SERS substrate

Without any other additives or additional energy, Au nanoplates have been successfully prepared and integrated simultaneously with the dedoped polypyrrole nanofiber film via the in situ reduction of AuCl₄^– on the film surface. The morphology and structure of the as‐prepared composite film are characterized, and its application for surface‐enhanced Raman scattering is also investigated. It has been found that the morphology of as‐prepared Au nanoplates is dependent on the reaction duration, while the density is dependent on the concentration of AuCl₄^– ions in the reaction process. It is suggested that polypyrrole plays dual reducing and structure‐directing roles during the formation of Au nanoplates. Surface‐enhanced Raman scattering study shows that the Au nanoplates give an intensive and enhanced Raman scattering when 4‐aminothiophenol is used as a probing molecule. The employed approach may shed some light on simultaneously fabricating and immobilizing other noble metal micro/nanostructures with unique morphology. Copyright © 2011 John Wiley & Sons, Ltd.     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

X-ray diffraction, vibrational and photoluminescence studies of the self-organized quantum well crystal H3N(CH2)6NH3PbBr4

We have prepared new semiconductor H₃N(CH₂)₆NH₃PbBr₄ crystals which are self-assembled organic-inorganic hybrid materials. The grown crystals have been studied by X-ray diffraction, infrared absorption and Raman spectroscopy scattering. We found that the title compound, abbreviated 2C₆PbBr₄, crystallizes in a two-dimensional (2D) structure with a P2₁/a space group. In the inorganic semiconductor sub-lattice, the corner sharing PbBr₆ octahedra form infinite 2D chains. The organic C₆H₁₈N₂⁺ ions form the insulator barriers between the inorganic semiconductor layers. Such a packing leads to a self-assembled multiple quantum well structure. Raman and infrared spectra of the title compound were recorded in the 50-500 and 400-4000 cm⁻¹ frequency regions, respectively. The assignment of the observed Raman lines was performed by comparison with the homologous compounds. Transmission measurements on thin films of 2C₆PbBr₄, obtained by the spin coating method, revealed a strong absorption peak at 380 nm. Luminescence measurements showed an emission line at 402 nm associated with radiative recombinations of excitons confined within the PbBr₆ layers. The electron-hole binding energy is estimated at 180 meV.     

Enhancement of MR ratios using thin oxide layers in PtMn and α-Fe2O3-based spin valves

Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/Ta spin valves. MR ratio of 13.9% and considerably large sheet ΔR of 2.55 Ω were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet ΔR of 1.3 Ω were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. α-Fe₂O₃ based spin valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in α-Fe₂O₃/CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR ratios may be attributed to the specular scattering effect of the conduction electrons by the thin oxide layers.     

High-pressure Raman study of osmium and rhenium up to 200 GPa and pressure dependent elastic shear modulus C44

High-pressure Raman scattering from hexagonal close-packed (HCP) metals Os and Re have been extended up to 200 GPa, and the pressure-dependent shear modulus C₄₄ has been deduced from the Raman-active mode E_2g, which is generated from the adjacent vibration of atoms in hexagonal planes, providing the valuable information about the elastic properties for HCP metals under high pressure. Combined with the available data of HCP metals from previous works, a further study indicates that the $C_{44}^\prime/C_{44}$ ratio would be close to a constant value, 0.01, with increasing atomic number of metals. The results obtained from high-pressure Raman scattering will allow us to probe the elastic anisotropy of the HCP metals at very high pressure.     

Raman scattering in quasi-one-dimensional ZrTe5

Raman scattering experiments on the compound ZrTe₅ isostructural with HfTe₅ have been performed at temperatures between 300–313 K. The Raman spectra of ZrTe₅ resemble those of HfTe₅ excepting two high frequency modes which should include the most characteristic diatomic mode. In spite of structural similarity with the transition-metal trichalcogenides, the Raman scattering activity cannot be directly compared. These unexpected results and the unusual temperature dependence of the Raman intensities suggest the possibility of a structural instability in HfTe₅ and/or ZrTe₅ in relation with the anomalies in the transport properties and in the X-ray forbidden reflections.     

Crystal field and magnetoelastic interactions in Tb2Ti2O7

In terms of a semiphenomenological exchange charge model, we have obtained estimates of parameters of the crystal field and parameters of the electron-deformation interaction in terbium titanate Tb₂Ti₂O₇ with a pyrochlore structure. The obtained set of parameters has been refined based on the analysis of spectra of neutron inelastic scattering and Raman light scattering, field dependences of the forced magnetostriction, and temperature dependences of elastic constants.     

Linear and nonlinear spin wave resonances in a very thin amorphous wire

Linear and nonlinear FMR spectra of a very thin amorphous wire are investigated. A phenomena called “spin wave ripple” is reported here. It is observed on the low field side of the linear resonance curve and is attributed to the excitation of radial standing spin waves with the energy corresponding to the incident microwave frequency. The fine structure of the nonlinear resonance curves, recently observed on thin amorphous wires, is then explained by the dipole-exchange models of a cylinder parametrically excited by Suhl's first-order process. This explanation is consistent with the value of the exchange stiffness constant A determined from the spin wave ripple.