Experimental mapping of magnetostatic mode structures of a ferrimagnet spheroid

Summary form only given. The exploration of the interaction of light with spin waves in ferromagnets within an optical cavity might lead to new chiral photonic devices and be a stepping stone towards the coherent optical manipulation of magnons in the quantum regime [1]. The developments made so far in cavity optomagnonics have been focused on the fundamental magnetostatic mode of an yttrium iron garnet (YIG) sphere, so-called `Kittel mode' [2, 3] and lead to the observation of a magnon-induced Brillouin scattering between optical whispering gallery modes following selection rules dictated by the conservation of the orbital momentum. Higher-order magnetostatic modes, with reduced mode volume and different orbital angular momentum, could couple more efficiently with the optical whispering gallery modes hosted by the YIG sphere, lying in the vicinity of the sphere surface equatorial plane. Though the resonance frequencies of these modes can be predicted theoretically [4], small deviations due to the actual sample properties or environment, as well as possible hybridization of the modes, could cause strong misinterpretations. Hence, unambiguously experimentally identifying higher-order magnon modes in a spheroid is required to properly scrutinize their interaction with light.We demonstrate a scheme to map the magnon mode structures of a ferrimagnetic spheroid. Using two small loop coils in close proximity with a millimeter-sized YIG sphere, we perform microwave transmission measurements at various latitude-longitude coordinates. These signals are found to be very sensitive to the magnon mode structures. This makes possible the identification of the modes, supported by the predicted dependence of their resonance frequencies in the dc magnetic field. This work paves the way to the systematic investigation of the optomagnonic interaction.