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Epitaxial thin (110) films of the weak ferromagnet NiF2 were deposited on single-crystal MgF2 (110) substrates via molecular-beam epitaxy. Subsequently polycrystalline Co was grown on the NiF2 film. The antiferromagnetic ordering of the NiF2 was monitored as a function of temperature via neutron diffraction and the exchange bias HE of the Co layers was measured via standard magnetometry measurements. Because in NiF2 the spins lie in the (001) plane, the maximum HE is observed after cooling the sample with a cooling field in the film plane perpendicular to the [001] direction of the NiF2. In 60 nm, 49 nm, and 38 nm thick NiF2 samples, the Néel temperature is TN≈81 K, which is significantly larger than the bulk value of TN=73.2 K. This enhancement also occurs in films without Co overlayers and thus is not due to a proximity effect. For the 38 nm sample with a Co overlayer cooled in a 50 kOe field, HE>0 and vanishes at a blocking temperature (TB) which coincides with the TN of the films. When the sample is cooled in 2.0 kOe, HE<0, disappearing at T=55 K, reappearing at T=65 K, and finally disappearing once again at T=81 K. For the 12 nm thick NiF2 sample, TB⪡TN. Strain-induced enhancement of ferromagnetic exchange interactions between the nearest-neighbor Ni2+ ions along the c axis may be responsible for the TN enhancement. These results also demonstrate that in general, a diminished TB is not necessarily due to a lower TN.

Source Citation

Shi, Hongtao., Lederman, D., O’Donovan, K. V., & Borchers, J. A. (2004). Exchange Bias And Enhancement Of The Néel Temperature In Thin Nif 2 Films. Physical Review B - Condensed Matter and Materials Physics, 69(21).