  Heat-assisted magnetic recording (HAMR) is expected to be a key technology to
significantly increase the areal storage density of magnetic recording devices.
At high temperatures thermally induced noise becomes a major problem, which
must be overcome in order to reliably write magnetic bits with narrow
transitions. We propose an elementary model based on the effective recording
time window (ERTW) to compute the switching probability of bits during HARM of
bit-patterned media. With few assumptions this analytical model allows to gain
deeper insights into the basic noise mechanisms like AC and DC noise. Finally,
we discuss strategies to reduce noise and to increase the areal storage density
of both bit-patterned as well as granular media.
