Interaction of Exchange Spin Waves with Transversal and Longitudinal Hypersound in Ferrite Films with Magnetic Inhomogeneity

Abstract

In [1,2] we reported the possibility of effective excitation of hypersound with the use of yttrium iron garnet (YIG) films having smooth variation in the anisotropy field across the film thickness. Effective excitation of the transverse acoustic wave has been demonstrated at frequencies from 2.5 to 16 GHz. An idea of such an acoustic transducer is based on the fact that inhomogeneity of the magnetic medium allows to excite an exchange spin wave (ESW) with a short wavelength. Magnetoelastic interaction between the ESW and an acoustic wave (AW), is most strong when spin and acoustic waves are synchronous, that is, the wave number q of ESW equals to the wave number of AW. At microwave frequencies (ω/2π = 1–20 GHz), the typical value of q at the point of synchronism is of the order of 105 cm-1. A direct linear excitation of short-wavelength spin waves in a homogeneous medium is difficult because of big difference in wave numbers between the ESW (q ∼ 105 cm-1) and the electromagnetic wave (qEMW ∼ 10 cm-1). If the magnetic medium is inhomogeneous, then q depends on coordinate, thus, one can efficiently excite a spin wave in a region of quasi-uniform ferromagnetic resonance, where q ∼ 0 and, at the same time, provide the condition of synchronism in a region where q is equal to the wave number of acoustic wave. The forward and reverse conversion of spin and acoustic waves at the point of synchronism was observed previously in experiments with YIG rods and studied theoretically in [3]. The origin of nonuniformity was the demagnetisation field existing at the end of a magnetised YIG rod.