Spin waves are the elementary excitations of the spin system in a magnetically ordered material state and magnons are their quasi particles. In the following article, we will discuss Brillouin light scattering (BLS) spectroscopy which is now a well-established tool for the characterization of spin waves. BLS is the inelastic scattering of light from spin waves and confers several benefits: the ability to map the spin-wave intensity with spatial resolution and high sensitivity as well as the potential for the simultaneous detection of frequency and wave vector. For several decades, the field of spin waves gained huge interest by the scientific community due to its relevance regarding fundamental issues in spin dynamics. Recently, the ongoing research in the field of magnonics has put particular emphasis on the high potential of spin waves regarding information technology. Opposed to charge-based schemes in conventional electronics and spintronics, magnons are charge-free currents of angular momentum, and, therefore, less subject to dissipative scattering processes. These ideas have propelled the quest for concepts to guide and manipulate spin-wave transport as well as for the miniaturization of spin-wave conduits towards sub-micrometer dimensions. For the further development of potential spin-wave-based devices, the ability to directly observe spin-wave propagation with spatial resolution is crucial. As an optical technique, BLS allows for a sub-micron space resolution by the implementation of a microscope objective in the optical setup. Over the last decade, this micro-focus BLS technique has become an established method for the investigation of spin waves in microstructured magnetic elements and proved its value in particular regarding magnonics. In this article, we will discuss the basic principles of BLS and illustrate the experimental optical setup. Particular emphasis will be put on the implementation of the high spatial resolution of BLS microscopes as well as on their computer based operation and automated sample positioning. Owing to these improvements in ease of use as well as experimental applicability, the BLS technique has maintained its relevance for investigations on spin waves in miniaturized magnetic structures as will be illustrated by a selection of experiments.

A characterization of the magnetic properties of amorphous Co40Fe40B20 thin films, developed for low damping applications in magnon spintronics, using vector network analyzer ferromagnetic resonance (VNA-FMR) and the magneto-optical Kerr effect is presented. Our films show a very weak uniaxial anisotropy and a low Gilbert damping parameter (α=0.0042). The saturation magnetization MS extracted from the FMR measurements is 1250 kA/m. The frequency dependence of the first perpendicular standing spin waves mode on the applied magnetic field is used to determine the exchange constant A for this alloy resulting in a value of 1.5×10−11 J/m. These values are discussed in comparison to literature values for different CoFeB compositions and other related alloys.

We present the experimental observation of parallel parametric amplification of selected thermal spin-wave modes in a transversally magnetized Ni81Fe19 microstripe. By employing Brillouin light scattering microscopy, we identify the dominant group, i.e., the spin-wave mode that is preferentially amplified. Due to the existing spin-wave quantization in the system, it is possible to select one specific mode to be parametrically excited by changing the bias magnetic field. This gives access to transversal spin-wave eigenmodes of the stripe which are promising for spin-wave information processing and also to modes localized at the stripe edges.

This work studies the influence of crystallographic alignment onto magnetization reversal in partially epitaxial Co films. A reproducible growth sequence was devised that allows for the continuous tuning of grain orientation disorder in Co films with uniaxial in-plane anisotropy by the controlled partial suppression of epitaxy. While all stable or meta-stable magnetization states occurring during a magnetic field cycle exhibit a uniform magnetization for fully epitaxial samples, non-uniform states appear for samples with sufficiently high grain orientation disorder. Simultaneously with the occurrence of stable domain states during the magnetization reversal, we observe a qualitative change of the applied field angle dependence of the coercive field. Upon increasing the grain orientation disorder, we observe a disappearance of transient domain wall propagation as the dominating reversal process, which is characterized by an increase of the coercive field for applied field angles away from the easy axis for well-ordered epitaxial samples. Upon reaching a certain disorder threshold level, we also find an anomalous magnetization reversal, which is characterized by a non-monotonic behavior of the remanent magnetization and coercive field as a function of the applied field angle in the vicinity of the nominal hard axis. This anomaly is a collective reversal mode that is caused by disorder-induced frustration and it can be qualitatively and even quantitatively explained by means of a two Stoner-Wohlfarth particle model. Its predictions are furthermore corroborated by Kerr microscopy and by Brillouin light scattering measurements.

Abstract. The spatial profiles and the dissipation characteristics of spin-wave quasieigenmodes are investigated in small magnetic Ni 81 Fe 19 ring structures using Brillouin light scattering microscopy. It is found, that the decay constant of a mode decreases with increasing mode frequency. Indications for a contribution of three-magnon processes to the dissipation of higher-order spin-wave quasi-eigenmodes are found.arXiv:0804.2200v1 [cond-mat.mtrl-sci]