  We present a versatile method for detecting the presence and motion of a
trapped domain wall in a narrow ferromagnetic layer using
current-perpendicular-to-plane (CPP) giant magnetoresistance (MR). The CPP-MR
response to small motions of the trapped domain wall is enhanced because the
CPP current is restricted to the region of wall trapping. We use a
Permalloy/Cu/Permalloy spin valve in the shape of a long, ~500-nm-wide wire
with a constriction (notch) near its middle that acts as a trapping site for a
head-to-head domain wall. Two different notch shapes were studied, mostly at
4.2 K but also at 295K.
