  Micrometer diameter bilayers of NiFe (permalloy, Py) and cobalt oxide (CoO)
deposited on nanomechanical resonators were used to investigate exchange bias
effects. The mechanical compliances of two resonator axes were enhanced by
severing one torsion arm, resulting in a unique three-axis resonator that
responds resonantly to torques generated by a three-axis RF field. Our
technique permits simultaneous measurement of three orthogonal torque
components. Measurements of the anisotropies associated with interfacial
exchange coupling effects have been made. At cryogenic temperatures,
observations of shifted linear hysteresis loops confirmed the presence of
exchange bias from the Py/CoO interface. An in-plane rotating DC bias field was
used to probe in-plane anisotropies through the out-of-plane torque. Training
effects in the rotational hysteresis data were observed and showed that
features due to interfacial coupling did not diminish irrespective of
substantial training of the unidirectional anisotropy. The data from the
rotational hysteresis loops were fit with parameters from a macrospin solution
to the Landau-Lifshitz-Gilbert equation. Each parameter of the exchange bias
model accounts for specific features of the rotational loop.
