The electron energy-loss magnetic chiral dichroism (EMCD) technique has been generally applied to single-phase magnetic crystals while rarely used for composite structures. It is mainly due to the lack of in-depth understanding of EMCD in the latter case where an additional phase may present under or above the investigated magnetic phase in the electron beam path. Here, we report EMCD signals acquired on a 15-nm-thick magnetic iron film with different thicknesses of the MgO substrate underlayer. By comparison, for areas with total thicknesses of t ¼ 0:59k and t ¼ 1:02k expressed with the mean free inelastic path of electron k, the relative dichroic signals at the Fe-L 3 edge are 3:8%61:0% and 3:5%61:6%, respectively, demonstrating no significant difference within the error range. However, the dichroic signal intensity at the Fe-L 2 edge peak is 77.6% larger in the thinner area of t ¼ 0:59k. Accordingly, the extracted m L =m s ratio of Fe 3d moments is 63% smaller in the thinner area even after the plural scattering is removed. Then, we confirm that the presence of an additional nonmagnetic phase under a magnetic iron crystal can noticeably affect the quantified value of the m L =m s ratio of iron moment determined from the EMCD measurements. Furthermore, the larger thickness of the underlayer may result in relatively higher valuation of the m L =m s ratio of the upper layer. A correction method, considering the different influence of the underlayer on the Fe-L 3 and L 2 edges, is in demand for developing potential applications of the EMCD technique to such composite nanomaterial systems. Published under license by AIP Publishing. https://doi.org/10.1063/1.5100245 Electron energy-loss magnetic chiral dichroism (EMCD) is an electron energy-loss spectroscopy (EELS)-based technique to locally characterize element-specific spin and orbital magnetic moments in crystalline materials. 1-3 The magnetic information is obtained from the EELS spectra recorded at two different positions on the diffraction plane, where different combinations of electron scattering vectors give rise to left-and right-handed polarized virtual photons in analogy with circular polarized X-rays in the synchrotron-based X-ray magnetic circular dichroism (XMCD) technique. 1 Since EMCD was first demonstrated by Schattschneider et al., 1 much effort has been made to develop it into a routine magnetic characterization technique in the transmission electron microscope (TEM). 4-14 Various EMCD experimental setups, for instance the energy spectrum imaging, 4,5 double aperture q À E mode, 6 spatially resolved EELS mode, 7 and scanning TEM, 8,9 have been proposed to improve the signal to noise ratio and the spatial resolution. In addition, the signal interpretation regarding the magnetic moment quantification has been widely concerned. The EMCD signal is not only simply affected by the intrinsic magnetism but also modulated by the dynamic electron diffraction condition which is related to the crystal structure, crystal orientation, and specimen thickness. 3,10,11 With a deep understanding of the dynamic diffraction, the quantification of element-specific and even site-specific orbital and spin moments has been experimentally realized. 11,12 Moreover, the theoretical simulation of the relative EMCD signal in various diffraction conditions has been widely applied for optimizing the experimental conditions 13,14 of EMCD or broadening its applications. 15 For the moment, the interpretation of quantitative results extracted from the EMCD signal is essentially based on an assumption that only one single-phase crystal is present in the electron beam path for each acquisition. Some EMCD experiments in the literature are actually in accordance with the assumed condition. For instance, experiments were performed on a single crystal in polycrystalline materials of Fe, 9 Co, 16 Ni, 16 or their oxides 7,17 or on epitaxially grown thin layers of single-crystalline Fe, Mn, and their compounds observed from