  In the development of the first generation of sensors and memory chips based
on spin-dependent tunneling through a thin trilayer, it has become clear that
pinhole defects can have a deleterious effect on magnetoresistance. However,
current diagnostic protocols based on Andreev reflection and the temperature
dependence of junction resistance may not be suitable for production quality
control. We show that the current density in a tunnel junction in the
cross-strip geometry becomes very inhomogeneous in the presence of a single
pinhole, yielding a four-terminal resistance that depends on the location of
the pinhole in the junction. Taking advantage of this position dependence, we
propose a simple protocol of four four-terminal measurements. Solving an
inverse problem, we can diagnose the presence of a pinhole and estimate its
position and resistance.
