The Importance of Depth-Varying Fields for MAMR Switching-Field Reduction

We present a micromagnetic modeling study of microwave assisted magnetic recording (MAMR) on a typical exchange-coupled four-layer media. With the help of an alternating current (ac) magnetic field, the switching field can be reduced, especially at the media resonant frequency. Typical hysteresis loop measurements employ a depth-invariant ac field; however, the field generated from a spin-torque oscillator (STO) varies with spacing. Considering this depth variance, the switching field reduction becomes less than observed with a depth-invariant ac field. Furthermore, we find an even smaller switching field reduction when using a realistic magnetic writer and STO fields to record a dc track on randomly magnetized grains. Consequently, we conclude that MAMR hysteresis loops employing a depth-invariant ac field significantly overestimate the ac field's contribution to grain switching in an actual recording situation.