Grain size effect on ferro-antiferromagnetic coupling of NiFe/FeMn systems

In order to investigate grain size effects on ferro-antiferromagnetic coupling in NiFe/FeMn systems, samples of glass/Cu zÅ/NiFe 75 Å/FeMn 150 Å/Ta 30 Å were made and the grain size of FeMn was changed by changing the Cu thickness z. Increasing z from 0 to 315 Å, the mean grain size increases from 18 to 50 Å. The blocking temperature, at which the coupling field NiFe/FeMn disappears, increases from 330 to 430 K with the increase of the mean grain size. The curve shape of the temperature dependence of the coupling field changes from a concave type to a convex type with the increase of the mean grain size. Temperature dependence of the coupling field were calculated using a thermal fluctuation model of antiferromagnet. The calculated curve of the temperature dependence of the coupling field agrees with the experiment qualitatively. However, the calculated curves of the temperature dependence of coercivity does not agree with the experiment.