  We consider a spin-fermion model consisting of free electrons coupled to
classical spins, where the latter are embedded in a quasi one-dimensional
superlattice structure consisting of spin blocks separated by spinless buffers.
Using a spiral ansatz for the spins, we study the effect of the electron
mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction on the $T=0$ ground
state of the system. We find that the RKKY interaction can lead to
ferromagnetic, antiferromagnetic, or intermediate spiral phases for different
system parameters. When the width is much larger than the length of the
individual blocks, the spiral phases are suppressed, and the ground state
oscillates between ferromagnetic and antiferromagnetic order as the size of the
buffer regions is varied. This is accompanied by a corresponding oscillation in
the Drude weight reflecting an increased conductivity in the ferromagnetic
state compared to the antiferromagnetic one. These results are reminiscent of
classic giant magnetoresistance phenomena observed in a similar geometry of
thin, sandwiched magnetic and non-magnetic layers. Our analysis provides a
robust framework for understanding the role of the RKKY interaction on the
ground state order and corresponding transport properties of such systems,
extending beyond the conventional perturbative regime.
