Microwave-assisted magnetic recording (MAMR) has recently emerged as a candidate approach to solving the trilemma in magnetic recording. In MAMR, the magnetic recording medium can be switched with a much lower magnetic field by magnetic resonance, which requires an oscillator with a higher frequency than 20 GHz. This study, describes a spin-torque oscillator (STO) using a material with a large negative anisotropy that is capable of generating a higher frequency and a stable magnetic field, and shows that it is preferable to a conventional oscillator using soft magnetic materials.