  The topological Hall effect (THE), induced by the Berry curvature, which
originates from non-zero scalar spin chirality, is an important feature for
mesoscopic topological structures, such as skyrmions. However, the THE might
also arise from other microscopic non-coplanar spin structures in the lattice.
Thus, the origin of the THE inevitably needs to be determined to fully
understand skyrmions and find new host materials. Here, we examine the Hall
effect in both bulk- and micron-sized lamellar samples of MnBi. The sample size
affects the temperature and field range in which the THE is detectable.
Although bulk sample exhibits the THE only upon exposure to weak fields in the
easy-cone state, in thin lamella the THE exists across a wide temperature range
and occurs at fields near saturation. Our results show that both the
non-coplanar spin structure in the lattice and topologically non-trivial
skyrmion bubbles are responsible for the THE, and that the dominant mechanism
depends on the sample size. Hence, the magnetic phase diagram for MnBi is
strongly size-dependent. Our study provides an example in which the THE is
simultaneously induced by two mechanisms, and builds a bridge between
mesoscopic and microscopic magnetic structures.
