Enhanced exchange anisotropy of Ni–Fe/Mn–Ni bilayers fabricated under the extremely clean sputtering process

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Abstract
In order to clarify the influence of the impurities in the sputtering atmosphere on the exchange anisotropy of ferromagnet/antiferromagnet bilayers, Ni–Fe/Mn–Ni films were prepared under different purities of the sputtering atmosphere by changing the base pressure from 10−11 Torr [extremely clean (XC) process] to 10−7 Torr [lower grade (LG) process]. The correlation between the exchange anisotropy and the microstructure of the films is discussed. As a result, we found that: (1) The exchange anisotropy was enhanced in the XC processed films comparing to the LG processed ones, especially when the thicknesses of both the ferromagnetic and antiferromagnetic layers were very thin. (2) The critical thicknesses of the antiferromagnetic layers were 110 and 150 Å for the XC and the LG processed films, respectively. (3) In the XC processed films, the fcc-[111] direction of the Ni–Fe grains were highly oriented perpendicularly to the film plane and an enlargement of antiferromagnetic grains was observed. We conclude that the enhancement of exchange anisotropy is caused by the enlargement of antiferromagnetic grains in the XC processed films.