Tomasz Błachowicz

Brillouin Spectroscopy in Crystal Lattices. Acoustic and Spin Waves


Silesian University of Technology Press, Gliwice 2003, PL ISSN 0072-470X







This is a book about old and new things, about phonons and magnons, about bulk excitations and surface excitations, about transparent crystals and nontransparent thin layers, about theory and experiment, and finally, about GHz acoustic and spin waves studied by Brillouin light scattering.

Over a number of years, the Brillouin scattering method became a standard tool in the research of acoustic phonons and magnons of GHz frequency. The method employs single photon on single phonon, or magnon, scattering near the origin of the first Brillouin zone. The term Brillouin light scattering (BLS) was derived from the theoretical prediction made by L. Brillouin in 1922 of possible nonelastic interaction between light and sound. The description of light scattering experiments was published in his Ph.D. dissertation. A similar prediction was made four years later in 1926 by L. I. Mandelstam. However, the first real experiment was performed by E. Gross in 1933. Spin waves, to which this book is significally related, were observed for the first time by Griffiths in 1946. However, the light scattering experiments detecting magnons were performed by Fleury in 1966. About twenty years ago, when good quality superlattices grown the Molecular Beam Epitaxy (MBE) method became available, a new era in use of the Brillouin technique begun. New sets of non-existing in nature phenomena associated with induced anisotropies in quasi-two-dimensional structures challenged to researches. Today, this is one of the fastest growing branch of the contemporary solid-state physics.

This book is focused on the following topics: (1) eigen-problem for acoustic wave propagation; (2) rotational contributions to the light scattering process; (3) a coherent-states approach; (4) acoustic phonon branch switching; (5) elastic constants; (6) acoustic waves in this layered media - the topics mostly related to theory of acoustic excitations. Then, I examine experimental methods used in Brillouin spectroscopy with special emphasis on sources of experimental errors and low-level signal analysis, and finally, spin waves in the Co/Cu metallic superlattices and in the Co/CoO exchange-bias bilayers. The section related to spin waves provides experimental results along with mostly phenomenological explanation.

The book tries to update results from both bulk and surface, phonons and magnons, Brillouin experiments. I also discuss: the switching of scattering on two transverse acoustic braches in the LiNbO3 crystal; not typical Brillouin spectrum in the the SLAO crystal; and influence of the Cu-ion doping process on acoustic wave frequencies. Chapter two introduces the theory of Brillouin light scattering. It examines acoustic-wave amplitudes and states of polarization for both bulk and quasi-two-dimensional structures. Chapter three provides a description of the experimental arrangement used by the BLS technique. The pressure-scanned system presented here was recently developed by the author as an alternative to piezoelectrically scanned Fabry-Perot’s interferometers. This part of the book provides a good occasion for the presentation of a special method for the filtration of weak and noised Brillouin signals by the use of wavelet analysis, as well as to review the deconvolution technique applied in the extraction of the real shape of spectral lines in liquids. This chapter also familiarizes the reader with the Sandercock-type Brillouin spectrometer and gives practical solutions to back-scattering geometry used mainly in nontransparent media. The Sandercock-type triple-pass Fabry-Perot interferometer has the highest available and most stable resolution power. It can distinguish 0.1 GHz frequency changes among typical frequencies of visible light of the 10000 GHz order. The fourth and the fifth chapters provide results from bulk, and surface experiments, respectively. The surface results were obtained in the Co/Cu superlattices and Co/CoO structures, both grown by the MBE method. The metallic superlattices belong to a new class of materials available recently thanks to the possibility of epitaxial growth in a very high vacuum. Applied Brillouin technique made possible measurements of frequencies and speeds of surface Rayleigh mode and of higher-order Rayleigh modes - the Sezawa modes. In the magnetic part of the measurements, spin waves coupled through dipolar fields and unidirectional Damon-Eshbach mode, of the stack as a whole, were detected. In the bilayered samples, the spin waves quantized into a surface-normal direction were measured. The fifth chapter also reports a series of in-plane results providing data about different types of magnetic anisotropies, especially those associated with exchange-biased fields. The surface-type measurements applied the Sandercock-type spectrometer in the back-scattering geometry. At the end, I provide three appendixes. They contain results from SQUID, scanning tunneling (STM) and optical microscopies, and results received by the MOKE method, the technique that utilizes the magneto-optic Kerr effect to detect easy and hard axes of magnetization and to correlate these with magnetic anisotropies measured earlier by the BLS spectroscopy.