  In the context of wave propagation, caustics are usually defined as the
envelope of a finite-extent wavefront; folds and cusps in a caustic result in
enhanced wave amplitudes. Here, we tackle a related phenomenon, namely the
existence of well-defined beams originating solely from the geometric
properties of the corresponding dispersion relation. This directional emission,
termed caustic beam, is enabled by a stationary group velocity direction, and
has been observed first in the case of phonons. We propose an overview of this
"focusing" effect in the context of spin waves excited in soft, thin
ferromagnetic films. Based on an analytical dispersion relation, we provide
tools for a systematic survey of caustic spin wave beams. Our theoretical
approach is validated by time-resolved microscopy experiments using the
magneto-optical Kerr effect. Then, we identify two cases of particular interest
both from fundamental and applicative perspectives. Indeed, both of them enable
broadband excitations (in terms of wave vectors) to result in narrowband beams
of low divergence.
