  Transition metal oxides are well known for their complex magnetic and
electrical properties. When brought together in heterostructure geometries,
they show particular promise for spintronics and colossal magnetoresistance
applications. In this letter, we propose a new mechanism for the coupling
between layers of itinerant ferromagnetic materials in heterostructures. The
coupling is mediated by charge carriers that strive to maximally delocalize
through the heterostructure to gain kinetic energy. In doing so, they force a
ferromagnetic or antiferromagnetic coupling between the constituent layers. To
illustrate this, we focus on heterostructures composed of SrRuO$_3$ and
La$_{1-x}$A$_{x}$MnO$_3$ (A=Ca/Sr). Our mechanism is consistent with
antiferromagnetic alignment that is known to occur in multilayers of
SrRuO$_3$-La$_{1-x}$A$_{x}$MnO$_3$. To support our assertion, we present a
minimal Kondo-lattice model which reproduces the known magnetization properties
of such multilayers. In addition, we discuss a quantum well model for
heterostructures and argue that the spin-dependent density of states determines
the nature of the coupling. As a smoking gun signature, we propose that
bilayers with the same constituents will oscillate between ferromagnetic and
antiferromagnetic coupling upon tuning the relative thicknesses of the layers.
