Transition from ferromagnetic to noncollinear to paramagnetic state with increasing Ru concentration in FeRu films

The structural and magnetic properties of sputter-deposited Fe100-xRux films were studied for x <= 50. The crystal structure of Fe100-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 Ru concentration range 13<x <20. Magnetic measurements indicate that the addition of Ru to Fe gives rise to a noncollinear magnetic alignment between Fe atoms in the body-centered cubic FeRu alloys, while the hexagonal close-packed FeRu alloys exhibit paramagnetic behavior. A simple atomistic model was used to show that the competition between ferromagnetic coupling of neighboring Fe atoms and antiferromagnetic coupling of Fe atoms across Ru atoms in cubic FeRu structures can induce noncollinear magnetic order. 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.