Coercivity enhancement above the Neel temperature of an antiferromagnet/ferromagnet bilayer

Single-crystal thin films of the antiferromagnet FeF2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Neel temperature of the FeF2. This coercivity reaches a peak value of over 600 Oe close to the Neel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Neel temperature. (C) 2002 American Institute of Physics.