  Heterostructures composed of ferromagnetic layers that are mutually
interacting through a nonmagnetic spacer are at the core of magnetic sensor and
memory devices. In the present study, layer-resolved ferromagnetic resonance
was used to investigate the coupling between the magnetic layers of a
Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were
observed for both magnetic layers, as probed at the Co and Ni L3 x-ray
absorption edges, showing a strong interlayer interaction through the
insulating MgO barrier. A theoretical model based on the
Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange
coupling and spin pumping between the magnetic layers. Fits to the experimental
data were carried out, both with and without a spin pumping term, and the
goodness of the fit was compared using a likelihood ratio test. This rigorous
statistical approach provides an unambiguous proof of the existence of
interlayer coupling mediated by spin pumping.
