Optical observation of evanescent surface magnons in thin magnetic films

A new type of nonpropagating surface‐wave‐like magnon has been detected in yttrium iron garnet films (YIG) in a magnetostatic wave (MSW) device structure using Brillouin light scattering. The measurements were carried out on a YIG film stripline device operated at 2–4 GHz. With an applied magnetic field parallel to the stripline, magnon signals were observed at fields above the surface wave band edge. Theory shows that highly localized, evanescent surface modes can exist at such fields, but only when a ground plane is present; the bandwidth in field or frequency is inversely proportional to the separation between the film and the ground plane. The observed evanescent wave band limits are in good agreement with the theoretical values.