  Magnetic properties of materials ranging from conventional ferromagnetic
metals to strongly correlated materials such as cuprates originate from Coulomb
exchange interactions. The existence of alternate mechanisms for magnetism that
could naturally facilitate electrical control have been discussed theoretically
but an experimental demonstration in an extended system has been missing. Here,
we investigate MoSe$_2$/WS$_2$ van der Waals heterostructures in the vicinity
of Mott insulator states of electrons forming a frustrated triangular lattice
and observe direct evidence for magnetic correlations originating from a
kinetic mechanism. By directly measuring electronic magnetization through the
strength of the polarization-selective attractive polaron resonance, we find
that when the Mott state is electron doped the system exhibits ferromagnetic
correlations in agreement with the Nagaoka mechanism.
