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Abstract:
Although high-tunnelling spin polarization has been observed in soft,
ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials,
there has been no experimental observation to date of high-tunnelling
magnetoresistance in the latter. Here we report the preparation of
highly textured, polycrystalline Mn3Ge films on amorphous substrates,
with very high magnetic anisotropy fields exceeding 7 T, making them
technologically relevant. However, the small and negative tunnelling
magnetoresistance that we find is attributed to predominant tunnelling
from the lower moment Mn-Ge termination layers that are oppositely
magnetized to the higher moment Mn-Mn layers. The net spin polarization
of the current reflects the different proportions of the two distinct
termination layers and their associated tunnelling matrix elements that
result from inevitable atomic scale roughness. We show that by
engineering the spin polarization of the two termination layers to be of
the same sign, even though these layers are oppositely magnetized,
high-tunnelling magnetoresistance is possible.
