  More than 30 years ago, Malozemoff (Phys. Rev. B 35, 3679 (1987))
hypothesized that exchange interaction at the interface between a ferromagnet
(F) and an antiferromagnet (AF) can act as an effective random field, which can
profoundly affect the magnetic properties of the system. However, until now
this hypothesis has not been directly experimentally tested. We utilize
magnetoelectronic measurements to analyze the effective exchange fields at
Permalloy/CoO interface. Our results cannot be explained in terms of
quasi-uniform effective exchange fields, but are in agreement with the
random-field hypothesis of Malozemoff. The presented approach opens a new route
for the quantitative analysis of effective exchange fields and anisotropies in
magnetic heterostructures for memory, sensing and computing applications.
