  Frustration introduces a nontrivial dispersion relation of spinwave even in a
ferromagnetic phase in a spin system. We study the reflection and refraction
process of spinwaves in the ferromagnet/frustrated ferromagnet junction system
by using the Holstein-Primakoff spinwave expansion and taking the large-S
limit. We discuss the relation between the incident angle and refraction angle
of spinwave, namely, the Snell's low of spinwave. As concrete examples of
frustrated ferromagnets, we study the fully polarized ferromagnet phases in the
J1-J2 chains and the J1-J2 model on the square lattice. The interesting
refraction processes, such as the splitting of the incident spinwave and the
negative refraction, are discussed. We also study the transmittance and
reflectance in these concrete models.
