  Hysteretic magneto-resistance of point contacts formed between non-magnetic
tips and single ferromagnetic films exchange-pinned by antiferromagnetic films
is investigated. The analysis of the measured current driven and field driven
hysteresis agrees with the recently proposed model of the surface spin-valve,
where the spin orientation at the interface can be different from that in the
bulk of the film. The switching in magneto-resistance at low fields is observed
to depend significantly on the direction of the exchange pinning, which allows
identifying this transition as a reversal of interior spins of the pinned
ferromagnetic films. The switching at higher fields is thus due to a spin
reversal in the point contact core, at the top surface of the ferromagnet, and
does not exhibit any clear field offset when the exchange-pinning direction or
the magnetic field direction is varied. This magnitude of the switching field
of the surface spins varies substantially from contact to contact and sometimes
from sweep to sweep, which suggests that the surface coercivity can change
under very high current densities and/or due to the particular microstructure
of the point contact. In contrast, no changes in the effect of the exchange
biasing on the interior spins are observed at high currents, possibly due to
the rapid drop in the current density away from nanometer sized point contact
cores.
