  Assessing and controlling magnetic interactions in magnetic nanostructures
are critical to nanomagnetic and spintronic explorations, such as magnetic
recording media, permanent magnets, magnetic memory and logic devices, etc.
Here we demonstrate an extremely sensitive magnetic yoking effect and tunable
interactions in FePt based hard/soft bilayers mediated by the soft layer. Below
the exchange length, a thin soft layer strongly exchange couples to the
perpendicular moments of the hard layer; above the exchange length, just a few
nanometers thicker, the soft layer moments turn in-plane and act to yoke the
dipolar fields from the adjacent hard layer perpendicular domains. The
evolution from exchange to dipolar-dominated interactions is experimentally
captured by first-order reversal curves, the delta-M method, and polarized
neutron reflectometry, and confirmed by micromagnetic simulations. These
findings demonstrate an effective yoking approach to design and control
magnetic interactions in wide varieties of magnetic nanostructures and devices.
