  A new mechanism of spin structure-driven anomalous Hall effect (AHE) in
tilted ferromagnetic metals is proposed by taking account of the d-orbital
degree of freedom. We find that a conduction electron acquires a Berry phase
due to the complex d-orbital wavefunction, in the presence of non-collinear
spin structure and the spin orbit interaction. The AHE driven by this
orbital-derived Berry phase is much larger than the AHE induced by spin
chirality, and it naturally explains the salient features of spin
structure-driven AHE in pyrochlore Nd$_2$Mo$_2$O$_7$. Since the proposed AHE
can occur even for coplanar spin orders ($M_z=0$), it is expected to emerge in
other interesting geometrically frustrated systems.
