  The structural and magnetic properties of sputter-deposited
Fe$_{100-x}$Ru$_x$ films were studied for $x \leq 50$. The crystal structure of
Fe$_{100-x}$Ru$_x$ is shown to be predominantly body-centered cubic for $x<13$
and to undergo a gradual transition to hexagonal close-packed in the
concentration range $13 \lesssim x \lesssim 20$. Magnetic measurements indicate
that the addition of Ru induces a noncollinear magnetic order in the
body-centered cubic FeRu alloys, while the hexagonal close-packed FeRu alloys
exhibit paramagnetic behavior. Increasing the Ru concentration in body-centered
cubic FeRu alloys decreases the size of magnetic domains, approaching the size
of magnetic grains. A simple atomistic model was used to show that
antiferromagnetic coupling of Fe atoms across Ru atoms can be responsible for
inducing noncollinear order in the FeRu cubic structures. Magnetic multilayer
structures used in thin-film magnetic devices make extensive use of both Fe and
Ru layers. Our results reveal that the presence of even a small amount of Ru in
Fe influences the magnetic order of Fe, which could impact the performance of
these devices.
