  We use micromagnetic simulation to demonstrate layer-selective detection of
magnetization directions from magnetic dots having two recording layers by
using a spin-torque oscillator (STO) as a read device. This method is based on
ferromagnetic resonance (FMR) excitation of recording-layer magnetizations by
the microwave field from the STO. The FMR excitation affects the oscillation of
the STO, which is utilized to sense the magnetization states in a recording
layer. The recording layers are designed to have different FMR frequencies so
that the FMR excitation is selectively induced by tuning the oscillation
frequency of the STO. Since all magnetic layers interact with each other
through dipolar fields, unnecessary interlayer interferences can occur, which
are suppressed by designing magnetic properties of the layers. We move the STO
over the magnetic dots, which models a read head moving over recording media,
and show that changes in the STO oscillation occur on the one-nanosecond
timescale.
