C. Dugautier

Brillouin light scattering by magnetic surface waves in dot-structured permalloy layers

Abstract Brillouin light scattering was used to investigate the magnetic properties of periodic arrays of square dots of permalloy obtained by electron beam lithography and ion beam sputtering from evaporated films. The frequencies of the magnetostatic surface waves were studied for square 2 μm × 2 μm dots of thickness 30 nm separated by intervals of 1 μm, and the results were compared to those obtained from the unpatterned permalloy film. The spin wave frequencies were found to be sensitive to the size of the dots and to the direction of the applied field; at first view, the size dependence is reasonably interpreted to derive from the demagnetizing field effect. However, a closer inspection of the dependence of the frequencies versus the angle of incidence seems to show step-like variations, suggesting that the involved in-plane wave-vector could be locked and show quantized values related to the finite dimensions of the dots. Consequently, the simple model, based on the Damon-Eshbach expression modified by the demagnetizing field, appears to be insufficient, and the full description of the magnetic surface wave properties has to take into account their lateral confinement.

Spin waves in magnetic wires:: a Brillouin study

Abstract A detailed Brillouin light scattering study of the dispersion of surface spin waves in cobalt wires is reported. A comparison with the dispersion curve of the unpatterned continuos film is achieved and the effects of the size-dependent demagnetizing field are discussed. Concerning the pseudo-bulk modes appearing in the patterned film, as expected, their variation is quasi-independent from the variation of the angle of incidence. The experimental Brillouin spectra do not display any splitting of the Damon–Eshbach mode into a set of discrete lines in contrast with previously published results on permalloy. We interpret this difference as arising from the large experimental linewidth combined with the presence of an anisotropy in the case of cobalt. A qualitative model of the scattering intensity is presented in order to illustrate this difference. The involved eigenfrequencies are derived from numerical calculations which also allow us to discuss the relevant quantization conditions of the in-plane wave vector.

Brillouin scattering from the icosahedral quasicrystal Al70.4Mn8.4Pd21.2

Abstract We report on acoustic surface waves in quasicrystal Al 70.4 Mn 8.4 Pd 21.2 studied using Brillouin spectroscopy. Our data are fitted with a spectral density modelization and our results are compared to those obtained from previous inelastic neutron scattering and ultrasonic propagation measurements on the bulk samples Al 68.7 Mn 9.6 Pd 21.7 .

Shape effect on in-plane anisotropy in magnetic dots

We have studied the spin waves in square and rectangular dots by Brillouin light scattering. The effects of the demagnetizing field are investigated and commented. In square dots, the observed fourfold symmetry configurational in-plane anisotropy is qualitatively interpreted as resulting from the non-homogeneous profile of the static magnetization.

Brillouin scattering investigation of the low temperature phase transition in K2ZnCl4 and Rb2ZnCl4

Abstract In K2ZnCl4 and Rb2ZnCl4, Brillouin scattering of longitudinal acoustic phonons (C11, C22, C33) is performed as a function of temperature around the low temperature structural phase transition at To. Small and different anomalies are observed in both materials. These anomalies are compared to those observed at T3 in Rb2ZnBr4 by Horikx et al. 9 and are tentatively related to the mechanism of the transition.

Brillouin spectroscopy study of successive phase transitions in K2ZnC14

Abstract All elastic stiffness constants have been determined at room temperature in the ferroelectric phase using Brillouin spectroscopy. The temperature dependence of C11, C22, C33 and C66 is measured up to 590 K. Only very weak anomalies are observed for some stiffnesses near Tc = 403 K and T1 = 560 K which are the temperatures of transition between the ferroelectric and the incommensurate phases and the incommensurate and the normal phases, respectively. This behaviour looks quite the same as the one observed in Rb2ZnC14.

Phase transitions in tetrafluoroaluminates RbAlF4 and KAlF4: brillouin scattering and ultrasonic propagation

Abstract RbAlF4 and KAIF4 belong to the family MAIF4 (M = K, Rb, Tl) of layered tetrafluoraluminates undergoing several phase transitions starting from a real or virtual high-temperature tetragonal aristotype structure (phase I-P4/mmm, Z = 1) characterized by a two-dimensional arrangement of AlF6 octahedra in sheets separated by K+ or Rb+ cations. The main feature of both compounds is the dominant role of the rotations of the AlF6 octahedra related to softenings of normal modes which are involved in the mechanisms of the successively encountered structural transformations (Bulou and Nouet, 1982; Bulou, 1985). These transformations can be interpreted in the framework of a Landau model, taking into account the pertinent order parameters associated with the soft modes at appropriate values of the wave-vector and including in addition their coupling with the strains: this leads to predictions of the elastic behaviour in the vicinity of the phase transitions studied by measuring the temperature dependence of t...

Brillouin scattering in Co/Cu/Co and Co/Au/Co trilayers: anisotropy fields and interlayer magnetic exchange

Abstract Magnetic anisotropy and interlayer exchange have been studied by Brillouin scattering on textured sandwiched Co/Cu/Co or Co/Au/Co structures normal to a 〈111〉 (pseudo-fcc) axis or to a 〈0001〉 (pseudo-hcp) axis, obtained by ultrahigh-vacuum evaporation on oxidized silicon or on float glass substrates. The surface contribution to the uniaxial anisotropy is approximately constant in all magnetic layers ( K s ≈ 0.8 erg cm −2 ). The volume contribution is strongly sample dependent. In Co/Au/Co, an oscillatory variation of the interlayer exchange is observed with a pseudo-period of 4.1 monolayers and a damping length of 5.1 monolayers, in good agreement with previously published determinations.

Magnon and phonon Raman scattering of the planar ferromagnet Rb2CrCl4

Magnon and phonon Raman scattering is studied in the canted ferromagnet Rb2CrCl4. The phonon spectra obtained at about 8 and 200 K are discussed on the basis of the exact crystal structure (D2h18) and of the appropriate quadratic one (D4h5) previously proposed for the isostructural crystal K2CuF4. Among the twelve modes predicted in D4h5 symmetry, only one (belonging to Gamma 4+) is missing. The most important feature of this work is the first direct observation of the optic gap predicted by the theory at about 170 cm-1 and the study of its magnetic field dependence for B along the easy direction (110) and the hard direction (001). The behaviour of the line is discussed in relationship with spin wave calculations by considering a possible slight misorientation of the field off the principal axes. The acoustic magnon line, previously observed directly via ferromagnetic resonance, is also seen for B>2T along (110). Polarisation measurements on the two lines indicate that the acoustic and optic modes must be assigned to gamma 2+ and gamma 1+ (C2h) respectively. A quantitative analysis of the authors results shows that the average Lande factor is close to the spin-only value and it suggests that gperpendicular to is slightly larger than g/sub ///. It also leads to an estimate of P+2D where P and D stand for the two anisotropy parameters.

Brillouin and ultrasonic studies of phase transitions in KA1F4 and K0.94Rb0.06A1F4

Abstract The elastic behaviour of KA1F4 has been studied above the martensitic-like first order (P4/mbm - P21/m) transition at T24 = 260 K : Brillouin and ultrasonic results show significant pretransitional softenings of C33 and of C44 while the other stiffness constants are not affected. In K0.94Rb0.06AlF4, an intermediate monoclinic phase appears between T23 = 205 K and T34 = 175 K. The C33 pretransitional behaviour observed in the pure compound well extrapolates down to T23. Below T23, domain walls prevent ultrasonic measurements whereas around T23 the Brillouin spectra exhibit anomalies in C11, C22 and more drastically in C33. These variations are discussed within the framework of a Landau development of the free energy.