  A broken interfacial inversion symmetry in ultrathin ferromagnet/heavy metal
(FM/HM) bilayers is generally believed to be a prerequisite for accommodating
the Dzyaloshinskii-Moriya interaction (DMI) and for stabilizing chiral spin
textures. In these bilayers, the strength of the DMI decays as the thickness of
the FM layer increases and vanishes around a few nanometers. In the present
study, through synthesizing relatively thick films of compositions CoPt or
FePt, CoCu or FeCu, FeGd and FeNi, contributions to DMI from the composition
gradient induced bulk magnetic asymmetry (BMA) and spin-orbit coupling (SOC)
are systematically examined. Using Brillouin light scattering spectroscopy,
both the sign and amplitude of DMI in films with controllable direction and
strength of BMA, in the presence and absence of SOC are experimentally studied.
In particular, we show that a sizable amplitude of DMI (0.15 mJ/m^2) can be
realized in CoPt or FePt films with BMA and strong SOC, whereas negligible DMI
strengths are observed in other thick films with BMA but without significant
SOC. The pivotal roles of BMA and SOC are further examined based on the
three-site Fert-Levy model and first-principles calculations. It is expected
that our findings may help to further understand the origin of chiral magnetism
and to design novel non-collinear spin textures.
