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We investigate the exchange bias fields and compositional depth profiles of the NiFe (bottom)/FeMn∕CoFe (top) trilayers after a thermal treatment at different annealing temperatures. Interestingly, the magnetic hysteresis measurement revealed that the NiFe∕FeMn∕CoFe trilayers exhibit a contrasting variation of the exchange bias fields at the two interfaces in a completely different way to each other. High angle x-ray diffraction indicates that there is no distinguishable effect of a thermal treatment on the NiFe (111) and FeMn (111) peaks. The Ni 2p and Mn 2p x-ray photoelectron spectroscopy (XPS) spectrums near these two interfaces along with the XPS compositional depth profiles are measured. We find the asymmetric depth profiles of the Fe and Mn atoms throughout the FeMn layer and the preferential Mn diffusion into the NiFe layer compared to the CoFe layer. We believe that in situ applied fields during sample growth and ex situ cooling fields after sample growth have a different effect on the exchange bias fields of both top and bottom interfaces.

With a combination of vector magneto-optical Kerr effect magnetometry and polarized neutron reflectometry, the detailed magnetization reversal mechanism of the exchange-biased Py(30-nm)/FeMn[t AFM = (0-30)-nm]/CoFe(30-nm) trilayers was studied. We found that Py/FeMn(15-nm) and FeMn(15-nm)/CoFe bilayers show completely different magnetization reversal modes, whereas they become very similar to each other in the corresponding Py/FeMn/CoFe trilayers. This is convincing evidence that the 15-nm FeMn layer mediates the magnetization reversal behaviors of both Py and CoFe layers through interlayer exchange bias coupling. Furthermore, magnetization reversal of Py and CoFe layers are decoupled for t AFM = 30 nm, indicating that the exchange length the magnetization reversal between two adjacent ferromagnetic layers is correlated over is less than 30 nm. This is in reasonable agreement with the theoretically predicted domain-wall width such as 28 nm for the polycrystalline FeMn/Co bilayer and 50 nm for the perfect Fe 50 Mn 50 crystal.

Magneto-optical Kerr effect (MOKE) magnetometry was used to investigate magnetization reversal dynamics in 30-nm NiFe/15-nm FeMn, 15-nm FeMn/30-nm CoFe bilayers, and 30-nm NiFe/(2,10)-nm FeMn/30-nm CoFe trilayers. The in-plane magnetization components of each ferromagnetic layer, both parallel and perpendicular to the applied field, were separately determined by measuring the longitudinal and transverse MOKE hysteresis loops from both the front and back sides of the film for an oblique incident s-polarized beam. The magnetization of the FeMn/CoFe bilayer was reversed abruptly and symmetrically through nucleation and domain wall propagation, while that of the NiFe/FeMn bilayer was reversed asymmetrically with a dominant rotation. In the NiFe/FeMn/CoFe trilayers, the magnetic reversal of the two ferromagnetic layers proceeded via nucleation and domain wall propagation for 2-nm FeMn, but via asymmetric rotation for 10-nm FeMn. The exchange-biased ferromagnetic layers showed the magnetization reversal along the same path in the film plane for the decreasing and increasing field branches from transverse MOKE hysteresis loops, which can be qualitatively explained by the theoretical model of the exchange-biased ferromagnetic/antiferromagnetic systems.