Exchange bias using the random antiferromagnet with competing anisotropies FexNi1-xF2

FexNi1-xF2 is a random antiferromagnetic alloy with competing magnetic anisotropies.  Because both FeF2 and NiF2 share the same tetragonal rutile structure, but have magnetic anisotropy axes perpendicular to each other, the entire phase diagram of this random anisotropy antiferromagnet can be studied experimentally.  I will demonstrate that antiferromagnetic films of this type with ferromagnetic thin film overlayers have a reversible exchange bias, even at low temperatures.  A search of the literature reveals that the magnetic phase diagram of this compound has not been studied, and therefore remains a mystery.  In order to study this system, we also measured the properties of a series of epitaxial (110) FexNi1-xF2 films deposited on (110) MgF2 via molecular beam epitaxy. Enhancement of the Néel temperature in alloys as well as evidence of spontaneous magnetization along the tetragonal c-axis after field-cooling were observed for samples with x>0.1; for samples with x<0.1 the magnetization was perpendicular to the c-axis.  Two phase transitions were observed for alloy samples with x>0.1.  The phase diagram of the upper transition was consistent with mean field theory of a system with competing anisotropies.  The transition at lower temperatures was unaffected by the application of a magnetic field, whereas the upper temperature transition was broadened by the application of fields as small as 50 Oe.  This suggests the presence of a spin-glass phase at lower temperatures, followed by melting of the spin glass phase prior to the main transition to paramagnetic behavior as the temperature is raised.  This second phase may consist of a weak ferromagnet with sublattices slightly tilted with respect to the anisotropy axis.  These results will be compared with those of other random anisotropy antiferromagnets, such as FexCo1-xCl2 and FexCo1-xBr2.

This work was performed in collaboration with F. A. Perez, T. Johnson, and T. Stanescu at West Virginia University, Michael Fitzsimmons at Los Alamos National Laboratory, and was supported in part by the U. S. National Science Foundation.