Exchange Bias in Fe$_{x}$Ni$_{1-x}$F$_{2}$/Co Bilayers

FeF$_{2}$ is an ideal antiferromagnet with a tetragonal rutile crystal structure and a strong uniaxial magnetic anisotropy along the $c$-axis ([001] direction). NiF$_{2}$ also shares the rutile crystal structure with similar lattice parameters, but its magnetic anisotropy causes spins to be in the $a-b$ plane, resulting in a weak ferromagnetism due to a small tilting of the antiferromagnetic sublattices. Because several theories predict that exchange bias should be a strong function of the magnetic anisotropy in the antiferromagnet, the Fe$_{x}$Ni$_{1-x}$F$_{2}$/Co bilayer system provides a framework whereby this can be studied experimentally. We have grown several single-crystalline Fe$_{x}$Ni$_{1-x}$F$_{2 }$50 nm thick films on MgF$_{2}$ (110) substrates via molecular beam epitaxy with $x$ varying between 0 and 1.0. We will report on the exchange bias as a function of Fe concentration in the alloy.