Nonuniform and sequential magnetization reversal via domain structure formation formultilayered system with grain size induced enhanced exchangebias

We report on the magnetization reversal in series ofexchange-biased multilayers NiFe(10.0 nm)/[ Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)] N studied by specularreflection and off-specular scattering of polarized neutrons. Allspecimens are sputtered and post-annealed at 530 K (i.e. above theIrMn Néel temperature of 520 K) in Ar atmosphere before coolingto room temperature in the presence of a field of 130 Oe whichinduces the unidirectional anisotropy. We find HEB isdependent upon the number of bilayers N as it graduallyincreases from 0.33 kOe for N=1 to a considerably higher valueof upto ≈0.9 kOe for N=10. X-ray specular and diffusescattering data reveal no significant variation of the lateralcorrelation length and only a weak dependence of the vertical rmsinterface roughness on N. Atomic and magnetic force microscopy,however, show a strong reduction of the grain size accompanied bydistinct changes of the ferromagnetic domain structure. Theenhancement of the exchange bias effect is presumably related tothe shrinking of the related domain size in the antiferromagnetdue to the structural evolution in the multilayers. Polarizedneutron reflectometry (PNR) measurements are done at differentapplied fields sweeping both branches of the hysteresis loop. Thespin-flip (SF) cross section of both the N=10 and 3 samplesshow diffusely scattered intensity appears gradually as the fieldapproaches HEB and is most intense where the netmagnetization vanishes. The disappearance of diffuse scattering insaturation indicates that the off-specular intensity is related tothe reversal process. The reversal proceeds sequentially startingwith the bottom (top) CoFe layer for decreasing (increasing) fieldand is related to the evolution of the grain size along the stack.The reversal of each CoFe layer is for both field branches due todomain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal inexchange-biased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specularspin-flip scattering indicate a more complex reversal mechanismthan hitherto considered. Moreover, although the grain sizedecreases from N=3 to 10 by a factor of about four the reversalmechanism remains similar.