Spin wave dispersion in perpendicularly magnetized synthetic antiferromagnets

The spin wave is a promising candidate medium to be used as an information carrier and for processing. Knowledge of the spin wave dispersion relation is a prerequisite for its application and the design of devices. Here, we report on spin wave dispersion in perpendicularly magnetized synthetic antiferromagnets (p-SAFs), consisting of two CoFeB layers and a thin W spacer layer. The Brillouin light scattering technique is employed to measure the spin wave dispersion relations as a function of the in-plane magnetic field and wave vector. Two resonance modes are observed in the finite range of the magnetic field, in which the configuration of the magnetic moments in two CoFeB layers can be divided into antiferromagnetic and ferromagnetic states. In addition, we observe a T-type region, where the magnetization in one CoFeB layer lies in the direction of the in-plane magnetic field and the other stays in the out-of-plane direction. The dependences of the frequency on the wave vector demonstrate the spin wave propagation in this case is reciprocal. We also theoretically derive the spin wave dispersion relation for p-SAFs based on the Landau-Lifshitz equation, which agree well with the experimental results.