  We investigate the ground states of antiferromagnetic Mn nanochains on
Ni(110) by spin-polarized scanning tunneling microscopy in combination with
theory. While the ferrimagnetic linear trimer experimentally shows the
predicted collinear classical ground state, no magnetic contrast was observed
for dimers and tetramers where non-collinear structures were expected based on
ab-initio theory. This striking observation can be explained by zero-point
energy motion for even numbered chains derived within a classical equation of
motion leading to non classical ground states. Thus, depending on the parity of
the chain length, the system shows a classical or a quantum behavior.
