Detrimental Effects of the STO Vertical Field Component in MAMR

Microwave-assisted magnetic recording (MAMR) employs a spin-torque oscillator (STO) to assist a standard magnetic writer field in switching grains of a magnetic recording medium. The STO field has different polarization angles at different positions, resulting in different mixes of in-media-plane and out-of-media-plane (vertical) field components. We find that the vertical component of the STO field degrades the writing process, increasing jitter, decreasing low-frequency signal-to-noise ratio, decreasing magnetic write width, and decreasing areal density capability. This is not due to postwrite erasure, but rather is part of the write process itself. Simple vector considerations illuminate the cause for this degradation. Consequently, experimental and modeling approaches that estimate MAMR switching-field reduction using only in-media-plane oscillating fields will erroneously overestimate the impact of the ac fields in the magnetic recording process.