  In this study we investigate the exchange coupling between the hard magnetic
compound MnBi and the soft magnetic alloy FeCo including the interface
structure between the two phases. Exchange spring MnBi-Fe$_x$Co$_{1-x}$ (x =
0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer
were deposited onto quartz glass substrates in a DC magnetron sputtering unit
from alloy targets. Magnetic measurements and density functional theory (DFT)
calculations reveal that a Co-rich FeCo layer leads to more coherent exchange
coupling. The optimum soft layer thickness is about 1 nm. In order to take into
account the effect of incoherent interfaces with finite roughness, we have
combined a cross-sectional High Resolution Transmission Electron Microscopy
(HR-TEM) analysis with DFT calculations and micromagnetic simulations. The
experimental results can be consistently described by modeling assuming a
polycrystalline FeCo layer consisting of crystalline (110) and amorphous grains
as confirmed by HR-TEM. The micromagnetic simulations show in general how the
thickness of the FeCo layer and the interface roughness between the hard and
soft magnetic phases both control the effectiveness of exchange coupling in an
exchange spring system.
