Abstract
Magnetic exchange coupling in pinned, asymmetric CoPt18(50)∕Ru(x)∕CoFe16(38) trilayers with 0⩽x⩽25Å has been investigated with magnetometry and ferromagnetic resonance. We found the parameters associated with coupling (remanence, coerctivity, and resonance position) to be oscillatory as a function of Ru thickness with extrema at x=7Å [antiparallel (AP)], 14Å [parallel(P)], and 20Å (AP), consistent with observations for Ru spacer material in unpinned, more symmetric systems. Utilizing analysis methods unique to pinned systems with resonance arising from the soft layer only, we were able to extract coupling strengths of 0.55, −0.29, and 0.27erg∕cm2 at Ru thicknesses of 7, 14, and 20Å, respectively. Noteworthy in the analysis method is the ability to extract P coupling strength of both signs from magnetization data. The resonance linewidth correlates with coupling, where minimum relaxation rates occur at low coupling strengths. Variable temperature magnetization loops revealed that the exchange coupling monotonically increases with decreasing temperatures.
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